As for the requirements for grounding electrical products, which include automation panels (cabinets), it is necessary to additionally familiarize yourself with the following RTDs:
1) GOST R 12.1.019-2009 "System of labor safety standards. Electrical safety. General requirements and nomenclature of types of protection" clause 4.2.2 (note - for the Russian Federation), which lists methods for ensuring protection against electric shock when touching metal non-current-carrying parts that may become energized as a result of damage to the insulation, which is very important for shields (cabinets).
2) GOST 12.2.007.0-75 "System of labor safety standards. Electrotechnical products. General safety requirements" with amendments to clause 3.3. Protective grounding requirements, incl. p.3.3.7, p.3.3.8, which indicates the need for equipment with elements for grounding shells, cases, cabinets, etc..
3) RM 4-249-91 "Technological process automation systems. Installation of grounding networks. Manual", and there everything is about grounding, incl. item 2.12, item 3.15, . There is clause 2.25, which provides a link to the requirements of RM3-82-90 "Shields and consoles for process automation systems. Design. Application features."
4) PM3-54-90 "Shields and consoles of automation systems. Installation of electrical wiring. Manual" clause 1.4 Requirements for grounding (grounding) with examples of connections of the elements of the shield (cabinet) inside the shield (cabinet).
5) RM 4-6-92 Part 3 "Technological process automation systems. Design of electrical and pipe wiring. Guidelines for the implementation of documentation. Manual" clause 3.6 Protective grounding and grounding and clause 3.7.1 regarding the implementation of instructions for protective grounding and grounding of electrical installations with examples in the appendices.
6)etc. etc.
7) GOST 21.408-2013 "SPDS. Rules for the execution of working documentation for automation of technological processes" p.
I draw your attention, there is a concept to familiarize yourself with and check for existing NTD, the main thing is where to get useful information and be able to filter and apply it.
And in complex design, usually a cable for connecting an electrical receiver, which is the automation shield (cabinet), to the switchgear of the power supply system and the arrangement of ground loops and ground nodes in control rooms and operator rooms, as well as the connection of these nodes to ground loops, are taken into account in the power supply kit. parts (note - mark "ES"), but the very disconnection of this cable is already given on the drawings of the corresponding circuits in the automation kit, the automation kit indicates (taken into account) and requirements and (or) is shown on the drawings (approx.-usually these are external connection diagrams or external wiring connection tables) connecting grounding conductors to nodes and ground loops from instrument cases and shields, etc..
Grounding techniques in industrial automation systems are very different for galvanically coupled and galvanically isolated circuits. Most of the methods described in the literature relate to galvanically coupled circuits, the share of which has recently decreased significantly due to the sharp drop in prices for isolating DC-DC converters.
3.5.1. Galvanically coupled circuits
An example of a galvanically coupled circuit is the connection of the source and receiver of a standard 0 ... 5 V signal (Fig. 3.95, Fig. 3.96). To explain how to properly ground, consider the option of incorrect (Fig. 3.95) and correct (Fig. 3.96, installation. The following errors were made in Fig. 3.95:
The listed errors lead to the fact that the voltage at the receiver input is equal to the sum of the signal voltage and the interference voltage . To eliminate this disadvantage, a large copper bar can be used as a grounding conductor, but it is better to perform grounding as shown in fig. 3.96, namely:
The general rule for weakening communication through a common ground wire is to divide the lands into analog, digital, power and protective, and then connect them at only one point. When separating the groundings of galvanically coupled circuits, the general principle is used: grounding circuits with a high level of interference should be carried out separately from circuits with a low level of interference, and they should only be connected at one common point. There can be several grounding points if the topology of such a circuit does not lead to the appearance of "dirty" ground areas in the circuit, including the source and receiver of the signal, and also if closed loops are not formed in the grounding circuit, through which the current induced by electromagnetic interference circulates.
The disadvantage of the method of separating ground conductors is low efficiency at high frequencies, when the mutual inductance between adjacent ground conductors plays an important role, which only replaces galvanic couplings with inductive ones, without solving the problem as a whole.
Long conductor lengths also increase the ground resistance, which is important at high frequencies. Therefore, grounding at one point is used at frequencies up to 1 MHz, it is better to ground at several points above 10 MHz, in the intermediate range from 1 to 10 MHz, a single-point circuit should be used if the longest conductor in the ground circuit is less than 1/20 of the interference wavelength. Otherwise, the multipoint scheme [Barnes] is used.
Single point grounding is often used in military and space applications [Barnes].
3.5.2. Shielding of signal cables
Consider the grounding of screens when transmitting a signal over a twisted shielded pair, since this case is most typical for industrial automation systems.
If the interference frequency does not exceed 1 MHz, then the cable must be grounded on one side. If it is grounded from both sides (Fig. 3.97), then a closed circuit is formed, which will work as an antenna, receiving electromagnetic interference (in Fig. 3.97, the interference current path is shown by a dashed line). The current flowing through the screen is a source of inductive interference on adjacent wires and wires inside the screen. Although the magnetic field of the braid current inside the shield is theoretically equal to zero, but due to the technological spread in the manufacture of the cable, as well as the non-zero resistance of the braid, the pickup on the wires inside the shield can be significant. Therefore, the screen must be grounded only on one side, and on the side of the signal source.
The cable sheath must be grounded at the signal source side. If grounding is done from the side of the receiver (Fig. 3.98), then the interference current will flow along the path shown in fig. 3.98 dashed line, i.e. through the capacitance between the cable cores, creating interference voltage on it and, consequently, between the differential inputs. Therefore, it is necessary to ground the braid from the side of the signal source (Fig. 3.99). In this case, there is no path for the interference current to pass. Please note that these diagrams show a differential signal receiver, i.e. both of its inputs have infinite resistance to ground.
If the signal source is not grounded (for example, a thermocouple), then the shield can be grounded from either side, because in this case, a closed loop for the interference current is not formed.
At frequencies above 1 MHz, the inductive reactance of the screen increases and the capacitive pickup currents create a large voltage drop on it, which can be transmitted to the internal conductors through the capacitance between the braid and the conductors. In addition, with a cable length comparable to the interference wavelength (the interference wavelength at a frequency of 1 MHz is 300 m, at a frequency of 10 MHz - 30 m), the resistance of the braid increases (see the "Ground Model" section), which sharply increases the interference voltage on the braid. Therefore, at high frequencies, the cable sheath must be grounded not only on both sides, but also at several points between them (Fig. 3.100). These points are chosen at a distance of 1/10 of the interference wavelength from one another. In this case, part of the current will flow through the cable braid, transmitting interference to the central core through mutual inductance. The capacitive current will also flow along the path shown in fig. 3.98, however, the high frequency component of the interference will be attenuated. The choice of the number of cable grounding points depends on the difference in interference voltages at the ends of the screen, the frequency of the interference, the requirements for protection against lightning strikes, or the magnitude of the currents flowing through the screen if it is grounded.
As an intermediate option, you can use the second screen grounding through the capacitance (Fig. 3.99). At the same time, at high frequency, the screen turns out to be grounded from two sides, at low frequency - from one side. This makes sense in the case when the interference frequency exceeds 1 MHz, and the cable length is 10 ... 20 times less than the interference wavelength, i.e. when it is not yet necessary to carry out grounding at several intermediate points. The capacitance value can be calculated by the formula 
, where is the upper frequency of the boundary of the interference spectrum, is the capacitance of the grounding capacitor (fractions of Ohm). For example, at a frequency of 1 MHz, a 0.1 uF capacitor has a resistance of 1.6 ohms. The capacitor must be high-frequency, with low self-inductance.
For high-quality shielding in a wide frequency range, a double screen is used (Fig. 3.101) [Zipse]. The inner shield is grounded on one side, the signal source side, to prevent capacitive interference from passing through the mechanism shown in fig. 3.98, and the outer shield reduces high-frequency interference.
In all cases, the screen must be insulated to prevent accidental contact with metal objects and the ground.
Recall that the interference frequency is the frequency that can be perceived by sensitive inputs of automation equipment. In particular, if there is a filter at the input of the analog module, then the maximum noise frequency that must be taken into account when shielding and grounding is determined by the upper cutoff frequency of the filter's passband.
Since even with proper grounding, but with a long cable, the interference still passes through the screen, it is better to transmit the signal in digital form or through an optical cable to transmit a signal over a long distance or with increased requirements for measurement accuracy. For this, you can use, for example, analog input modules RealLab! series with a digital RS-485 interface or fiber optic converters of the RS-485 interface, for example, type SN-OFC-ST-62.5/125 from RealLab! .
We have carried out an experimental comparison of different ways of connecting a signal source (a thermistor with a resistance of 20 kOhm) through a shielded twisted pair (0.5 turns per centimeter) 3.5 m long. An instrumental amplifier RL-4DA200 with a data acquisition system RL-40AI from RealLab! was used. The amplification channel gain was 390, the bandwidth was 1 kHz. Type of interference for the circuit of Fig. 3.102-a is shown in fig. 3.103.
3.5.4. Cable screens in electrical substations
At electrical substations, on the braid (screen) of the automation signal cable, laid under high-voltage wires at ground level and grounded on one side, a voltage of hundreds of volts can be induced during the switching of the current by the switch. Therefore, for the purpose of electrical safety, the cable braid is grounded on both sides.
To protect against electromagnetic fields with a frequency of 50 Hz, the cable screen is also grounded on both sides. This is justified in cases where it is known that the electromagnetic pickup with a frequency of 50 Hz is greater than the pickup caused by the flow of equalizing current through the braid.
3.5.5. Cable shields for lightning protection
To protect against the magnetic field of lightning, signal cables of automation systems passing through open areas must be laid in metal pipes made of a ferromagnetic material, such as steel. The pipes play the role of a magnetic screen [Vijayaraghavan]. Stainless steel cannot be used as this material is not ferromagnetic. Pipes are laid underground, and when above ground, they must be grounded approximately every 3 meters [Zipse]. The cable must be shielded and the shield must be grounded. The grounding of the screen must be very high quality with minimal resistance to ground.
Inside the building, the magnetic field is weakened in reinforced concrete buildings and is not weakened in brick buildings.
A radical solution to lightning protection problems is the use of a fiber optic cable, which is already quite cheap and easily connected to the RS-485 interface, for example, through converters of the SN-OFC-ST-62.5/125 type.
3.5.6. Grounding for differential measurements
If the signal source has no resistance to ground, then a "floating input" is formed during differential measurement (Fig. 3.105). The floating input can be statically charged by atmospheric electricity (see also the Ground Types section) or by the input leakage current of an op amp. To drain charge and current to ground, the potential inputs of analog input modules typically contain 1 MΩ to 20 MΩ resistors internally to connect the analog inputs to ground. However, with a high level of interference or a high resistance of the signal source, the resistance of 20 MΩ may not be sufficient, and then it is necessary to additionally use external resistors with resistance from tens of kΩ to 1 MΩ or capacitors with the same resistance at the interference frequency (Fig. 3.105).
3.5.7. Smart Sensors
Recently, so-called smart sensors, which contain a microcontroller for linearizing the conversion characteristic of the sensor, have been rapidly spread and developed (see, for example, "Temperature, pressure, humidity sensors"). Smart sensors provide a signal in digital or analog form [Caruso ]. Due to the fact that the digital part of the sensor is combined with the analog part, the output signal has an increased noise level if the ground is incorrect.
Some sensors, such as those from Honeywell, have a DAC with a current output and therefore require an external load resistance (on the order of 20 kΩ [Caruso]), so the useful signal is obtained in the form of a voltage drop across the load resistor when the sensor output current flows.
cabinets are interconnected, which creates a closed loop in the ground circuit, see fig. 3.69, section "Protective grounding of buildings", "Grounding conductors", "Electromagnetic interference";
analog and digital ground conductors in the left cabinet run in parallel over a large area, so inductive and capacitive pickups from digital ground may appear on analog ground;
the power supply (more precisely, its negative terminal) is connected to the cabinet body at the nearest point, and not at the ground terminal, therefore, interference current flows through the cabinet body, penetrating through the power supply transformer (see Fig. 3.62,);
one power supply is used for two cabinets, which increases the length and inductance of the ground conductor;
in the right cabinet, the ground terminals are not connected to the ground terminal, but directly to the cabinet body. In this case, the cabinet body becomes a source of inductive pickup on all wires passing along its walls;
in the right cabinet, in the middle row, analog and digital grounds are connected directly at the output of the blocks, which is wrong, see fig. 3.95, fig. 3.104.
These shortcomings are eliminated in Fig. 3.108. An additional wiring improvement in this example would be to use a separate ground conductor for the most sensitive analog input modules.
Within a cabinet (rack), it is desirable to group analog modules separately, and digital modules separately, so that when laying wires in a cable duct, the length of the sections of parallel passage of digital and analog ground circuits is reduced.
3.5.9. Distributed control systems
In control systems distributed over a certain area with characteristic dimensions of tens and hundreds of meters, it is impossible to use input modules without galvanic isolation. Only galvanic isolation allows you to connect circuits grounded at points with different potentials.
Cables passing through open areas must be protected from magnetic impulses during a thunderstorm (see the section "Lightning and atmospheric electricity", "Cable screens for lightning protection") and magnetic fields when switching powerful loads (see the section "Cable screens at electrical substations). Pay special attention to the grounding of the cable screen (see section "Shielding of signal cables"). A radical solution for a geographically distributed control system is the transmission of information over an optical fiber or radio channel.
Good results can be obtained by refusing to transmit information by analog standards in favor of digital ones. To do this, you can use the modules of a distributed control system RealLab! NL series from Reallab! . The essence of this approach lies in the fact that the input module is located near the sensor, thereby reducing the length of wires with analog signals, and the signal is transmitted to the PLC via a digital channel. A variation of this approach is the use of sensors with built-in ADCs and digital interfaces (for example, sensors of the NL-1S series).
3.5.10. Sensitive measuring circuits
For highly sensitive measuring circuits in poor electromagnetic environments, the best results are obtained by using a "floating" ground (see section "Types of grounding") in combination with battery supply [Floating ] and fiber optic transmission.
3.5.11. Actuating equipment and drives
The power supply circuits of pulse-controlled motors, servo motors, PWM-controlled actuators must be made with a twisted pair to reduce the magnetic field, and also shielded to reduce the electrical component of the radiated interference. The cable screen must be earthed on one side. The circuits for connecting the sensors of such systems should be placed in a separate screen and, if possible, spatially distant from the actuating devices.
Grounding in industrial networks
An industrial network based on the RS-485 interface is performed by a shielded twisted pair cable with the obligatory use of galvanic isolation modules fig. 3.110). For short distances (about 10 m), in the absence of sources of interference nearby, the screen can not be used. At large distances (the standard allows a cable length of up to 1.2 km), the difference in ground potentials at remote points can reach several units and even tens of volts (see the "Shielding of signal cables" section). Therefore, in order to prevent the current flowing through the shield, equalizing these potentials, the cable shield must be grounded. only one point(no matter which one). It will also prevent a large area closed loop in the ground circuit, in which high currents can be induced by electromagnetic induction during lightning strikes or the switching of powerful loads. This current, through mutual inductance, induces on the central pair of wires e. d.c., which can damage the port driver chips.
When using an unshielded cable, a large static charge (several kilovolts) can be induced on it due to atmospheric electricity, which can destroy the galvanic isolation elements. To prevent this effect, the isolated part of the galvanic isolation device should be grounded through a resistance, for example, 0.1 ... 1 MΩ (shown in Fig. 3.110 by a dashed line).
The effects described above are especially pronounced in Ethernet networks with a coaxial cable, when several Ethernet network cards fail during a thunderstorm when grounding at several points (or lack of grounding) during a thunderstorm.
On low bandwidth Ethernet networks (10 Mbps), the shield should only be grounded at one point. On Fast Ethernet (100 Mbps) and Gigabit Ethernet (1 Gbps), shield grounding must be done at several points, following the recommendations in the "Shielding Signal Cables" section
When laying the cable in an open area, you must use all the rules described in the section "Shielding of signal cables"
3.5.12. Grounding at explosive objects
At explosive industrial facilities (see the "Hazardous Facilities Automation" section), when installing ground circuits with a stranded wire, it is not allowed to use soldering for soldering the cores together, since due to the cold flow of the solder, weakening of the contact pressure points in the screw terminals is possible.
The shield of the RS-485 interface cable is grounded at one point, outside the hazardous area. Within the hazardous area, it must be protected from accidental contact with earthed conductors. Intrinsically safe circuits must not be grounded unless the operating conditions of the electrical equipment require it (GOST R 51330.10, section "Shielding of signal cables").
3.6. Galvanic isolation
Galvanic isolation(isolation) of circuits is a radical solution to most of the problems associated with grounding, and its use has become a de facto standard in industrial automation systems.
To implement galvanic isolation, it is necessary to supply energy to the isolated part of the circuit and exchange signals with it. Energy is supplied using an isolation transformer (in DC-DC or AC-DC converters) or using autonomous power sources: galvanic batteries and accumulators. Signal transmission is carried out through optocouplers and transformers, elements with magnetic coupling, capacitors or optical fiber.
The basic idea of galvanic isolation is that the path through which conductive interference can be transmitted is completely eliminated in the electrical circuit.
Galvanic isolation solves the following problems:
reduces the common-mode noise voltage at the input of the differential analog signal receiver to almost zero (for example, in Fig. 3.73, the common-mode voltage on the thermocouple relative to the ground does not affect the differential signal at the input of the input module);
protects the input and output circuits of the input and output modules from breakdown by a large common-mode voltage (for example, in Fig. 3.73, the common-mode voltage on a thermocouple relative to Earth can be arbitrarily large if it does not exceed the insulation breakdown voltage).
To use galvanic isolation, the automation system is divided into autonomous isolated subsystems, the exchange of information between which is carried out using galvanic isolation elements. Each subsystem has its own local ground and local power supply. Subsystems are grounded only for electrical safety and local protection against interference.
The main disadvantage of circuits with galvanic isolation is the increased level of interference from the DC-DC converter, which, however, can be made sufficiently small for low-frequency circuits using digital and analog filtering. At high frequencies, the capacitance of the subsystem to ground, as well as the throughput capacitance of the galvanic isolation elements, is a limiting factor in the merits of galvanically isolated systems. The capacitance to ground can be reduced by using optical cable and reducing the geometric dimensions of the isolated system.
When using galvanically isolated circuits, the concept " insulation voltage" is often misunderstood. In particular, if the insulation voltage of an input module is 3 kV, this does not mean that its inputs can be under such a high voltage under working conditions. In foreign literature, three standards are used to describe the insulation characteristics: UL1577, VDE0884 and IEC61010 -01, but descriptions of galvanic isolation devices do not always refer to them.Therefore, the concept of "insulation voltage" is interpreted in domestic descriptions of foreign devices ambiguously.The main difference is that in some cases we are talking about a voltage that can be applied to isolation indefinitely (working insulation voltage) , in other cases it is probationary tension (insulation voltage), which is applied to the sample for 1 min. up to several microseconds. The test voltage can be up to 10 times the operating voltage and is intended for accelerated testing during production, since the voltage at which breakdown occurs depends on the duration of the test pulse.
tab. 3.26 shows the relationship between operating and test (test) voltage according to the IEC61010-01 standard. As you can see from the table, concepts such as operating voltage, constant, rms or peak value of the test voltage can vary greatly.
The electrical insulation strength of domestic automation equipment is tested in accordance with GOST 51350 or GOST R IEC 60950-2002 with a sinusoidal voltage with a frequency of 50 Hz for 60 seconds at a voltage indicated in the instruction manual as "insulation voltage". For example, with a test insulation voltage of 2300 V, the operating voltage of the insulation is only 300 V (Table 3.26 RMS, 50/60 Hz,
1 min.
Today we'll talk about grounding in transformer substation and industrial, the main goals of which are maintenance personnel and stable operation. Many people misunderstand the topic of grounding in industrial systems, and its incorrect connection leads to bad consequences, accidents and even costly downtime due to violation and breakdown. Interference is a random variable, which is very difficult to detect without special equipment.
Interference sources on the ground bus
Sources and causes of interference can be lightning, static electricity, electromagnetic radiation, "noisy" equipment, 220 V power supply network with a frequency of 50 Hz, switched network loads, triboelectricity, galvanic couples, thermoelectric effect, electrolytic, conductor movement in a magnetic field, etc. In industry, there is a lot of interference associated with malfunctions or the use of non-certified equipment. In Russia, interference is regulated by standards - R 51318.14.1, GOST R 51318.14.2, GOST R 51317.3.2, GOST R 51317.3.3, GOST R 51317.4.2, GOST 51317.4.4, GOST R 51317.4.11, GOST R 51522, GOST R 50648. For the design of industrial equipment, in order to reduce the level of interference, they use a low-power element base with a minimum speed and try to reduce the length of the conductors and shielding.
Basic definitions on the topic "Common grounding"
Protective earth- connection of the conductive parts of the equipment with the ground of the Earth through a grounding device in order to protect a person from electric shock.
Grounding device- a set of grounding conductors (that is, a conductor in contact with the ground) and grounding conductors.
Common wire - a conductor in the system, relative to which the potentials are measured, for example, the common wire of the PSU and the device.
Signal ground- connection to the ground of the common wire of the signal transmission circuits.
The signal ground is divided into digital land and analog. Signal analog ground is sometimes divided into analog input ground and analog output ground.
force ground- a common wire in the system, connected to protective earth, through which a large current flows.
Solidly grounded neutral b - the neutral of the transformer or generator, connected to the ground electrode directly or through low resistance.
Zero wire- a wire connected to a solidly grounded neutral.
Insulated neutral b - neutral of the transformer or generator, not connected to the grounding device.
Zeroing- connection of equipment with a solidly grounded neutral of a transformer or generator in three-phase current networks or with a solidly grounded output of a single-phase current source.
APCS grounding is usually subdivided into:
- Protective grounding.
- Working ground, or FE.
Grounding purposes
Protective earthing is required to protect people from electric shock for equipment with a supply voltage of 42 V AC or 110 V DC, except for hazardous areas. But at the same time, protective grounding often leads to an increase in the level of interference in the process control system.
Electrical networks with an isolated neutral are used to avoid interruptions in the consumer's power supply with a single insulation fault, since in the event of an insulation breakdown to the ground in networks with a dead-earthed neutral, protection is triggered and the power to the network is interrupted.
The signal ground serves to simplify the electrical circuit and reduce the cost of industrial devices and systems.
Depending on the purpose of the application, signal grounds can be divided into basic and screen ones. The reference ground is used for reference and signal transmission in the electronic circuit, and the shield ground is used for grounding the shields. Screen earth is used for grounding cable screens, shielding, instrument cases, as well as for removing static charges from rubbing parts of conveyor belts, electric drive belts.
Types of grounding
One of the ways to mitigate the harmful effects of ground circuits on automation systems is the separate implementation of grounding systems for devices that have different sensitivity to interference or are sources of interference of different power. The separate design of the grounding conductors allows their connection to the protective earth at one point. At the same time, different earth systems represent the rays of a star, the center of which is the contact to the protective grounding bus of the building. Due to this topology, dirty ground noise does not flow through the clean ground conductors. Thus, although the ground systems are separate and have different names, ultimately they are all connected to the Earth through a protective earth system. The only exception is "floating" land.
Power ground
Automation systems can use electromagnetic relays, micropower servomotors, electromagnetic valves and other devices, the current consumption of which significantly exceeds the current consumption of I/O modules and controllers. The power circuits of such devices are made with a separate pair of twisted wires (to reduce radiated interference), one of which is connected to the protective ground bus. The common wire of such a system (usually the wire connected to the negative terminal of the power supply) is the power ground.
Analog and digital ground
Industrial automation systems are analog-digital. Therefore, one of the sources of the analog part is the interference created by the digital part of the system. To prevent the passage of interference through the ground circuits, digital and analog ground are made in the form of unconnected conductors connected together at only one common point. I/O modules and industrial controllers have separate outputs for this. analog ground(A.GND) and digital(D.GND).
"Floating" land
A "floating" ground occurs when the common wire of a small part of the system is not electrically connected to the protective earth bus (that is, to Earth). Typical examples of such systems are battery meters, car automation, aircraft or spacecraft on-board systems. Floating ground is used more often in small signal measurement technology and less often in industrial automation systems.
Galvanic isolation
Galvanic isolation solves many grounding problems, and its use has actually become in process control systems. To implement galvanic isolation (isolation), it is necessary to supply energy by an isolation transformer and transmit a signal to an isolated part of the circuit through optocouplers and transformers, elements with magnetic coupling, capacitors or optical fiber. In the electrical circuit, the path through which the transmission of conducted interference is possible is completely eliminated.
Grounding Methods
The grounding for galvanically coupled circuits is very different from the grounding for decoupled circuits.
Grounding of galvanically coupled circuits
We recommend avoiding the use of galvanically coupled circuits, and if there is no other option, it is desirable that these circuits be sized to
opportunities small and that they are located within the same cabinet.
An example of incorrect grounding of the source and receiver of a standard signal 0 ... 5 V
Here are the following errors:
- heavy load (DC motor) current flows on the same ground bus as the signal, creating a ground voltage drop;
- used unipolar inclusion of the signal receiver, and not differential;
- an input module without galvanic isolation of the digital and analog parts was used, so the power supply current of the digital part, containing interference, flows through the output AGND and creates an additional interference voltage drop across the resistance R1
These errors lead to the fact that the voltage at the input of the receiver Vin equal to the sum of the signal voltage Vout and interference voltage VGrounds = R1 (Ipit + IM)
To overcome this shortcoming, a large copper bar can be used as the ground conductor, but it is better to perform grounding as shown below.
Need to do:
- connect all ground circuits at one point (in this case, the interference current IM R1);
- connect the ground conductor of the signal receiver to the same common point (in this case, the current Ipit no longer flows through resistance R1, a
voltage drop across conductor resistance R2 does not add to the output voltage of the signal source Vout)
Example of correct grounding of the source and receiver of a standard signal 0…5 V
The general rule for weakening the connection through a common ground wire is to divide the lands into analog, digital, power and protective followed by their connection at only one point.
When separating the groundings of galvanically coupled circuits, the general principle is used: grounding circuits with a high noise level should be carried out separately from circuits with a low noise level, and they should only be connected at one common point. There may be several grounding points if the topology of such a circuit does not lead to the appearance of "dirty" ground areas in the circuit, including the source and receiver of the signal, and also if closed loops that receive electromagnetic interference do not form in the ground circuit.
Grounding of galvanically isolated circuits
A radical solution to the described problems is the use of galvanic isolation with separate grounding of the digital, analog and power parts of the system.
The power section is usually grounded via a protective earth bus. The use of galvanic isolation allows you to separate the analog and digital ground, and this, in turn, eliminates the flow of noise currents through the analog ground from the power and digital ground. Analog ground can be connected to protective earth through a resistor. RAGND.
Grounding the screens of signal cables in process control systems
An example of an incorrect ( on both sides) grounding the cable shield at low frequencies, if the interference frequency does not exceed 1 MHz, then the cable must be grounded on one side, otherwise a closed loop is formed that will work as an antenna.
An example of incorrect (on the signal receiver side) grounding of the cable screen. The cable sheath must be grounded at the signal source side. If grounding is done on the receiver side, then the interference current will flow through the capacitance between the cable cores, creating an interference voltage on it and, therefore, between the differential inputs.
Therefore, it is necessary to ground the braid from the side of the signal source, in this case there is no path for the passage of the interference current.
Correct shield grounding (additional grounding on the right is used for high frequency signal). If the signal source is not grounded (for example, a thermocouple), then the shield can be grounded from either side, since in this case a closed loop for the interference current is not formed.
At frequencies above 1 MHz, the inductive resistance of the screen increases, and capacitive pickup currents create a large voltage drop on it, which can be transmitted to the internal conductors through the capacitance between the braid and the conductors. In addition, with a cable length comparable to the interference wavelength (the interference wavelength at a frequency of 1 MHz is 300 m, at a frequency of 10 MHz - 30 m), the braid resistance increases, which sharply increases the interference voltage on the braid. Therefore, at high frequencies, the cable braid must be grounded not only on both sides, but also at several points between them.
These points are chosen at a distance of 1/10 of the interference wavelength from one another. In this case, part of the current will flow through the cable braid I Earth, which transmits interference to the central core through mutual inductance.
The capacitive current will also flow along the path shown in Fig. 21, however, the high frequency component of the interference will be attenuated. The choice of the number of cable grounding points depends on the difference in interference voltages at the ends of the screen, the frequency of the interference, the requirements for protection against lightning strikes, or the magnitude of the currents flowing through the screen if it is grounded.
As an intermediate option, you can use second grounding of the screen through the capacitance. At the same time, at high frequency, the screen turns out to be grounded from two sides, at low frequency - from one side. This makes sense in the case when the interference frequency exceeds 1 MHz, and the cable length is 10 ... 20 times less than the interference wavelength, that is, when it is not yet necessary to ground at several intermediate points.
The inner shield is grounded on one side - the signal source side, in order to exclude the passage of capacitive interference along the path shown, and the outer shield reduces high-frequency interference. In all cases, the screen must be insulated to prevent accidental contact with metal objects and the ground. For signal transmission over long distances or with increased requirements for measurement accuracy, it is necessary to transmit the signal in digital form or, even better, via an optical cable.
Grounding of cable screens of automation systems in electrical substations
At electrical substations, on the braid (screen) of the signal cable of the automation system, laid under high-voltage wires at ground level and grounded on one side, a voltage of hundreds of volts can be induced during the switching of the current by the switch. Therefore, for the purpose of electrical safety, the cable braid is grounded on both sides. To protect against electromagnetic fields with a frequency of 50 Hz, the cable screen is also grounded on both sides. This is justified in cases where it is known that the electromagnetic pickup with a frequency of 50 Hz is greater than the pickup caused by the flow of equalizing current through the braid.
Grounding cable shields for lightning protection
To protect against the magnetic field of lightning, the signal cables (with a grounded shield) of the process control system passing through the open area must be laid in metal pipes made of steel, the so-called magnetic shield. Better underground, otherwise ground every 3 meters. The magnetic field has little effect inside a reinforced concrete building, unlike other materials.
Grounding for differential measurements
If the signal source has no resistance to ground, the differential measurement produces a floating input. The floating input can be statically charged by atmospheric electricity or the input leakage current of an op amp. To drain charge and current to ground, the potential inputs of analog input modules typically contain 1 to 20 MΩ resistors internally that connect the analog inputs to ground. However, with a high level of interference or a large signal source, even a resistance of 20 MΩ may not be sufficient, and then it is necessary to additionally use external resistors with a value of tens of kΩ to 1 MΩ or capacitors with the same resistance at the interference frequency.
Grounding Smart Sensors
Nowadays, the so-called smart sensors with a microcontroller inside to linearize the output from the sensor, giving a signal in digital or analog form. Due to the fact that the digital part of the sensor is combined with the analog part, the output signal has an increased noise level if the ground is incorrect. Some sensors have a DAC with a current output and therefore require an external load resistance of the order of 20 kΩ, so the useful signal in them is obtained in the form of a voltage drop across the load resistor when the sensor output current flows.
The load voltage is:
Vload = Vout – Iload R1+ I2 R2,
i.e. it depends on the current I2, which includes the digital ground current. The digital ground current contains noise and affects the voltage across the load. To eliminate this effect, ground circuits must be made as shown below. Here the digital ground current does not go through the resistance R21 and does not introduce noise into the signal at the load.
Proper grounding of smart sensors:
Grounding of cabinets with equipment of automation systems
Installation of APCS cabinets must take into account all the previously stated information. The following examples of grounding control cabinets are divided conditionally on the correct, giving a lower noise level, and erroneous.
Here is an example (incorrect connections are highlighted in red; GND is a pin for connecting a grounded power pin), in which each difference from the following figure worsens digital failures and increases analog error. The following "wrong" connections are made here:
- the cabinets are grounded at different points, so the potentials of their grounds are different;
- the cabinets are interconnected, which creates a closed circuit in the ground circuit;
- conductors of analog and digital grounds in the left cabinet run in parallel over a large area, so inductive and capacitive pickups from digital ground may appear on analog ground;
- conclusion GND the power supply unit is connected to the cabinet body at the nearest point, and not at the ground terminal, therefore, interference current flows through the cabinet body, penetrating through the power supply transformer;
- one power supply is used for two cabinets, which increases the length and inductance of the ground conductor;
- in the right cabinet, the ground terminals are connected not to the ground terminal, but directly to the cabinet body, while the cabinet body becomes a source of inductive interference to all wires running along its walls;
- in the right cabinet in the middle row, analog and digital grounds are connected directly at the output of the blocks.
The listed shortcomings are eliminated by the example of proper grounding of industrial automation system cabinets:
Add. The benefit of the wiring in this example would be to use a separate ground conductor for the most sensitive analog input modules. Within a cabinet (rack), it is desirable to group analog modules separately, and digital modules separately, so that when laying wires in a cable duct, the length of the sections of parallel passage of digital and analog ground circuits is reduced.
Grounding in remote control systems
In systems distributed over a certain territory with characteristic dimensions of tens and hundreds of meters, it is impossible to use input modules without galvanic isolation. Only galvanic isolation allows you to connect circuits grounded at points with different potentials. The best solution for signal transmission is optical fiber and the use of sensors with built-in ADC and digital interface.
Grounding of actuating equipment and APCS drives
Power supply circuits for pulse-controlled motors, servo motors, and PWM-controlled actuators must be twisted pair to reduce the magnetic field, and shielded to reduce the electrical component of the radiated interference. The cable screen must be earthed on one side. The circuits for connecting the sensors of such systems should be placed in a separate screen and, if possible, spatially distant from the actuating devices.
Grounding in industrial networks RS-485, Modbus
Interface-based industrial network is shielded twisted pair with mandatory use galvanic isolation modules.
For short distances (about 15 m) and in the absence of nearby noise sources, the screen can not be used. At large lengths of the order of up to 1.2 km, the difference in ground potentials at points remote from each other can reach several tens of volts. To prevent current from flowing through the shield, the cable shield should only be grounded at ANY one point. When using an unshielded cable, a large static charge (several kilovolts) can be induced on it due to atmospheric electricity, which can disable the galvanic isolation elements. To prevent this effect, the isolated part of the galvanic isolation device should be grounded through a resistance, for example 0.1 ... 1 MΩ. The resistance shown by the dashed line also reduces the possibility of breakdown due to ground faults or high galvanic isolation resistance in the case of shielded cable. On low bandwidth Ethernet networks (10 Mbps), the shield should only be grounded at one point. For Fast Ethernet (100 Mbps) and Gigabit Ethernet (1 Gbps), the shield must be grounded at multiple points.
Grounding at explosive industrial facilities
At explosive objects, when installing grounding with a stranded wire, it is not allowed to use soldering for soldering the cores together, since due to the cold flow of the solder, the places of contact pressure in the screw terminals may be weakened.
The screen of the interface cable is grounded at one point outside the hazardous area. Within the hazardous area, it must be protected from accidental contact with earthed conductors. intrinsically safe circuits must not be grounded unless required by the operating conditions of the electrical equipment ( GOST R 51330.10, p6.3.5.2). And they must be installed in such a way that interference from external electromagnetic fields (for example, from a radio transmitter located on the roof of a building, from overhead power lines or nearby high power cables) does not create voltage or current in intrinsically safe circuits. This can be achieved by shielding or removing intrinsically safe circuits from the source of electromagnetic interference.
When laying in a common bundle or channel, cables with intrinsically safe and intrinsically safe circuits must be separated by an intermediate layer of insulating material or grounded metal. No separation is required if metal-sheathed or shielded cables are used. Grounded metal structures should not have gaps and poor contacts between themselves, which can spark during a thunderstorm or when switching powerful equipment. In explosive industrial facilities, electrical distribution networks with isolated neutral are predominantly used to eliminate the possibility of a spark during a phase-to-earth short circuit and tripping of protection fuses in case of insulation damage. For protection against static electricity use the grounding described in the appropriate section. Static electricity can ignite an explosive mixture.
electrical installations above 1 kV in networks with an effectively grounded neutral (with high earth fault currents);
electrical installations above 1 kV in networks with isolated neutral (with low earth fault currents);
electrical installations up to 1 kV with dead-earthed neutral;
electrical installations up to 1 kV with isolated neutral.
1.7.3. An electrical network with an effectively grounded neutral is a three-phase electrical network above 1 kV, in which the earth fault factor does not exceed 1.4.
The earth fault ratio in a three-phase electrical network is the ratio of the potential difference between an undamaged phase and earth at the earth fault point of another or two other phases to the potential difference between the phase and earth at this point before the fault.
1.7.4. A dead-earthed neutral is a transformer or generator neutral connected to a grounding device directly or through low resistance (for example, through current transformers).
1.7.5. An isolated neutral is a transformer or generator neutral that is not connected to a grounding device or connected to it through signaling, measuring, protection devices, grounding arc-suppressing reactors and similar devices with high resistance.
1.7.6. The grounding of any part of an electrical installation or other installation is the intentional electrical connection of this part with a grounding device.
1.7.7. Protective grounding is the grounding of parts of an electrical installation in order to ensure electrical safety.
1.7.8. Working grounding is the grounding of any point of the current-carrying parts of the electrical installation, which is necessary to ensure the operation of the electrical installation.
1.7.9. Zeroing in electrical installations with voltages up to 1 kV is the deliberate connection of parts of an electrical installation that are not normally energized with a dead-earthed neutral of a generator or transformer in three-phase current networks, with a dead-earthed output of a single-phase current source, with a dead-earthed midpoint source in DC networks.
1.7.10. An earth fault is an accidental connection of energized parts of an electrical installation to structural parts that are not isolated from earth, or directly to earth. A ground fault is an accidental connection of energized parts of an electrical installation with their structural parts that are not normally energized.
1.7.11. A grounding device is a combination of a grounding conductor and grounding conductors.
1.7.12. A grounding conductor is a conductor (electrode) or a set of metal-connected conductors (electrodes) that are in contact with the ground.
1.7.13. An artificial grounding conductor is a grounding conductor specially made for grounding purposes.
1.7.14. The natural grounding conductor is the electrically conductive parts of communications, buildings and structures for industrial or other purposes that are in contact with the ground and are used for grounding purposes.
1.7.15. The grounding or grounding line is called, respectively, a grounding or zero protective conductor with two or more branches.
1.7.16. A grounding conductor is a conductor connecting the grounded parts to the ground electrode.
1.7.17. A protective conductor (PE) in electrical installations is a conductor used to protect against electric shock to people and animals. In electrical installations up to 1 kV, a protective conductor connected to a dead-earthed neutral of a generator or transformer is called a zero protective conductor.
1.7.18. Zero working conductor (N) in electrical installations up to 1 kV is a conductor used to power electrical receivers, connected to a solidly grounded neutral of a generator or transformer in three-phase current networks, with a solidly grounded output of a single-phase current source, with a solidly grounded source point in three-wire DC networks.
A combined zero protective and zero working conductor (PEN) in electrical installations up to 1 kV is a conductor that combines the functions of a zero protective and zero working conductor.
In electrical installations up to 1 kV with a solidly grounded neutral, the zero working conductor can perform the functions of a zero protective conductor.
1.7.19. The spreading zone is the area of the earth, within which a noticeable potential gradient occurs when the current drains from the ground electrode.
1.7.20. The zone of zero potential is the zone of the earth outside the spreading zone.
1.7.21. The voltage on the grounding device is the voltage that occurs when current drains from the ground electrode into the ground between the current input point in the grounding device and the zone of zero potential.
1.7.22. The voltage relative to earth when shorting to the case is the voltage between this case and the zone of zero potential.
1.7.23. The touch voltage is the voltage between two points of the earth fault current circuit (to the case) while a person touches them at the same time.
1.7.24. The step voltage is the voltage between two points of the earth, due to the spreading of the fault current to the ground, while simultaneously touching them with the feet of a person.
1.7.25. The ground fault current is the current flowing into the ground through the fault.
1.7.26. The resistance of the grounding device is the ratio of the voltage on the grounding device to the current flowing from the grounding electrode to the ground.
1.7.27. The equivalent resistivity of the earth with a heterogeneous structure is such a resistivity of the earth with a homogeneous structure, in which the resistance of the grounding device has the same value as in the earth with a heterogeneous structure.
The term "resistivity" used in these Regulations for non-homogeneous earth should be understood as "equivalent resistivity".
1.7.28. Protective shutdown in electrical installations up to 1 kV is the automatic shutdown of all phases (poles) of a network section, which provides combinations of current and its passage time that are safe for humans in case of short circuits to the case or a decrease in the insulation level below a certain value.
1.7.29. Double insulation of an electrical receiver is a combination of working and protective (additional) insulation, in which the accessible parts of the electrical receiver do not acquire dangerous voltage if only the working or only protective (additional) insulation is damaged.
1.7.30. Low voltage is a rated voltage of not more than 42 V between phases and with respect to earth, used in electrical installations to ensure electrical safety.
1.7.31. An isolation transformer is a transformer designed to separate the network supplying the electrical receiver from the primary electrical network, as well as from the grounding or zeroing network.
GENERAL REQUIREMENTS
1.7.32. To protect people from electric shock in case of insulation damage, at least one of the following protective measures must be applied: grounding, neutralizing, protective shutdown, isolating transformer, low voltage, double insulation, potential equalization.
1.7.33. Grounding or grounding of electrical installations should be carried out:
1) at a voltage of 380 V and above alternating current and 440 V and above direct current - in all electrical installations (see also 1.7.44 and 1.7.48);
2) at rated voltages above 42 V, but below 380 V AC and above 110 V, but below 440 V DC - only in rooms with increased danger, especially dangerous and in outdoor installations.
Grounding or grounding of electrical installations is not required at rated voltages up to 42 V AC and up to 110 V DC in all cases, except for those specified in 1.7.46, clause 6, and in Ch. 7.3 and 7.6.
1.7.34. Grounding or grounding of electrical equipment installed on overhead lines (power and instrument transformers, disconnectors, fuses, capacitors and other devices) must be carried out in compliance with the requirements given in the relevant chapters of the PUE, as well as in this chapter.
The resistance of the grounding device of the overhead line support on which the electrical equipment is installed must meet the requirements:
1) 1.7.57-1.7.59 - in electrical installations above 1 kV network with isolated neutral;
2) 1.7.62 - in electrical installations up to 1 kV with dead-earthed neutral;
3) 1.7.65 - in electrical installations up to 1 kV with isolated neutral;
4) 2.5.76 - in networks of 110 kV and above.
In three-phase networks up to 1 kV with a dead-earthed neutral and in single-phase networks with a grounded output of a single-phase current source, the electrical equipment installed on the overhead line support must be zeroed (see 1.7.63).
1.7.35. For grounding electrical installations, natural grounding conductors should be used in the first place. If, at the same time, the resistance of the grounding devices or the contact voltage has acceptable values, and the normalized voltage values \u200b\u200bof the grounding device are provided, then artificial ground electrodes should be used only if it is necessary to reduce the density of currents flowing through natural ground electrodes or flowing from them.
1.7.36. For grounding electrical installations of various purposes and different voltages, geographically close to each other, it is recommended to use one common grounding device.
To combine the grounding devices of various electrical installations into one common grounding device, all available natural, especially long, grounding conductors should be used.
A grounding device used for grounding electrical installations of the same or different purposes and voltages must meet all the requirements for grounding these electrical installations: protecting people from electric shock if the insulation is damaged, operating conditions of networks, protecting electrical equipment from overvoltage, etc.
1.7.37. The resistance of grounding devices and contact voltage required by this chapter must be provided under the most unfavorable conditions.
The specific earth resistance should be determined, taking as a calculated value corresponding to that season of the year when the resistance of the grounding device or the contact voltage takes on the highest values.
1.7.38. Electrical installations up to 1 kV AC can be with a solidly grounded or insulated neutral, DC electrical installations with a solidly grounded or isolated midpoint, and electrical installations with single-phase current sources with one solidly grounded or with both insulated terminals.
In four-wire networks of three-phase current and three-wire networks of direct current, dead earthing of the neutral or midpoint of current sources is mandatory (see also 1.7.105).
1.7.39. In electrical installations up to 1 kV with a solidly grounded neutral or a solidly grounded output of a single-phase current source, as well as with a solidly grounded midpoint in three-wire DC networks, zeroing must be performed. The use in such electrical installations of grounding the housings of electrical receivers without their grounding is not allowed.
1.7.40. Electrical installations up to 1 kV AC with an isolated neutral or an isolated output of a single-phase current source, as well as DC electrical installations with an isolated midpoint should be used with increased safety requirements (for mobile installations, peat excavations, mines). For such installations, as a protective measure, earthing must be carried out in combination with network insulation monitoring or protective disconnection.
1.7.41. In electrical installations above 1 kV with an isolated neutral, grounding must be performed.
In such electrical installations, it should be possible to quickly find ground faults (see 1.6.12). Earth fault protection should be installed with a tripping action (throughout the entire electrically connected network) in cases where it is necessary for safety reasons (for lines supplying mobile substations and mechanisms, peat mines, etc.).
1.7.42. Protective disconnection is recommended as a primary or additional protection measure if safety cannot be ensured by a grounding or neutralizing device, or if a grounding or neutralizing device causes difficulties due to implementation conditions or for economic reasons. Protective shutdown must be carried out by devices (devices) that meet special technical conditions in terms of reliability of operation.
1.7.43. A three-phase network up to 1 kV with an isolated neutral or a single-phase network up to 1 kV with an insulated output, connected through a transformer to a network above 1 kV, must be protected by a breakdown fuse from the danger that occurs when the insulation is damaged between the high and low voltage windings of the transformer. A blowout fuse must be installed in the neutral or phase on the low voltage side of each transformer. In this case, control over the integrity of the breakdown fuse must be provided.
1.7.44. In electrical installations up to 1 kV in places where isolation or step-down transformers are used as a protective measure, the secondary voltage of the transformers must be: for isolating transformers - no more than 380 V, for step-down transformers - no more than 42 V.
When using these transformers, the following must be observed:
1) isolating transformers must meet special specifications for increased design reliability and increased test voltages;
2) from an isolation transformer, it is allowed to supply only one electrical receiver with a rated current of a fusible link or a circuit breaker release on the primary side of not more than 15 A;
3) grounding of the secondary winding of the isolating transformer is not allowed. The transformer housing, depending on the neutral mode of the network supplying the primary winding, must be grounded or zeroed. Grounding of the housing of the electrical receiver connected to such a transformer is not required;
4) step-down transformers with a secondary voltage of 42 V and below can be used as isolation transformers if they meet the requirements given in clauses 1 and 2 of this paragraph. If the step-down transformers are not isolation, then, depending on the neutral mode of the network supplying the primary winding, the transformer case should be grounded or grounded, as well as one of the terminals (one of the phases) or the neutral (middle point) of the secondary winding.
1.7.45. If it is impossible to perform grounding, grounding and protective shutdown that meet the requirements of this chapter, or if this presents significant difficulties for technological reasons, maintenance of electrical equipment from insulating platforms is allowed.
Insulating platforms must be designed so that the ungrounded (non-zeroed) parts representing a danger can only be touched from the platforms. In this case, the possibility of simultaneous contact with electrical equipment and parts of other equipment and parts of the building should be excluded.
PARTS SUBJECT TO EARTHING OR GROUNDING 1.7.46. Parts subject to zeroing or grounding in accordance with 1.7.33 include:
1) cases of electrical machines, transformers, devices, lamps, etc. (see also 1.7.44);
2) drives of electrical apparatus;
3) secondary windings of instrument transformers (see also 3.4.23 and 3.4.24);
4) frames of switchboards, control panels, shields and cabinets, as well as removable or opening parts, if the latter are equipped with electrical equipment with a voltage above 42 V AC or more than 110 V DC;
5) metal structures of switchgears, metal cable structures, metal cable couplings, metal sheaths and armor of control and power cables, metal sheaths of wires, metal sleeves and pipes of electrical wiring, casings and supporting structures of busbars, trays, boxes, strings, cables and steel strips on which cables and wires are fixed (except for strings, cables and strips along which cables with a grounded or zeroed metal sheath or armor are laid), as well as other metal structures on which electrical equipment is installed;
6) metal sheaths and armor of control and power cables and wires with voltage up to 42 V AC and up to 110 V DC, laid on common metal structures, including common pipes, boxes, trays, etc. Together with cables and wires, metal sheaths and armor of which are subject to grounding or grounding;
7) metal cases of mobile and portable power receivers;
8) electrical equipment placed on the moving parts of machine tools, machines and mechanisms.
1.7.47. In order to equalize the potentials in those premises and outdoor installations in which grounding or grounding is used, building and industrial structures, permanently laid pipelines for all purposes, metal cases of process equipment, crane and railway rail tracks, etc. must be connected to the ground network or nulls. In this case, natural contacts in the joints are sufficient.
1.7.48. It is not required to deliberately ground or neutralize:
1) cases of electrical equipment, apparatus and electrical installation structures installed on grounded (zeroed) metal structures, switchgear, on shields, cabinets, shields, beds of machines, machines and mechanisms, provided that reliable electrical contact with grounded or zeroed bases is ensured (exception - see chapter 7.3);
2) the structures listed in 1.7.46, clause 5, provided that there is reliable electrical contact between these structures and the grounded or grounded electrical equipment installed on them. At the same time, these structures cannot be used for grounding or grounding of other electrical equipment installed on them;
3) fittings of insulators of all types, guys, brackets and lighting fittings when installed on wooden poles of overhead lines or on wooden structures of open substations, if this is not required by the conditions of protection against atmospheric surges.
When laying a cable with a metal grounded sheath or an uninsulated grounding conductor on a wooden support, the listed parts located on this support must be grounded or zeroed;
4) removable or opening parts of the metal frames of switchgear chambers, cabinets, fences, etc., if no electrical equipment is installed on the removable (opening) parts or if the voltage of the installed electrical equipment does not exceed 42 V AC or 110 V DC (exception - see chapter 7.3);
5) cases of electrical receivers with double insulation;
6) metal brackets, fasteners, pipe sections of mechanical protection of cables in places where they pass through walls and ceilings and other similar parts, including pull and branch boxes up to 100 cm² in size, electrical wiring performed by cables or insulated wires laid along walls, ceilings and other building elements.
ELECTRICAL INSTALLATIONS OVER 1 kV NETWORKS WITH EFFICIENTLY EARTHED NEUTRAL
1.7.49. Grounding devices of electrical installations above 1 kV with an effectively grounded neutral should be made in compliance with the requirements either for their resistance (see 1.7.51) or for touch voltage (see 1.7.52), as well as in compliance with the design requirements (see . 1.7.53 and 1.7.54) and to limit the voltage on the grounding device (see 1.7.50). Requirements 1.7.49 - 1.7.54 do not apply to grounding devices of overhead lines.
1.7.50. The voltage on the grounding device when the earth fault current drains from it should not exceed 10 kV. Voltage above 10 kV is allowed on grounding devices, from which the removal of potentials outside the buildings and external fences of the electrical installation is excluded. At voltages on the grounding device of more than 5 kV and up to 10 kV, measures must be taken to protect the insulation of outgoing communication and telemechanics cables and to prevent the removal of dangerous potentials outside the electrical installation.
1.7.51. The grounding device, which is carried out in compliance with the requirements for its resistance, must have a resistance of not more than 0.5 Ohm at any time of the year, including the resistance of natural grounding conductors.
In order to equalize the electrical potential and ensure the connection of electrical equipment to the ground electrode in the territory occupied by the equipment, longitudinal and transverse horizontal ground electrodes should be laid and connected to each other in a ground grid.
Longitudinal grounding conductors should be laid along the axes of electrical equipment from the service side at a depth of 0.5-0.7 m from the ground surface and at a distance of 0.8-1.0 m from foundations or equipment foundations. It is allowed to increase the distances from the foundations or bases of the equipment up to 1.5 m with the laying of one ground electrode for two rows of equipment, if the service sides face one another, and the distance between the foundations or bases of the two rows does not exceed 3.0 m.
Transverse ground electrodes should be laid in convenient places between equipment at a depth of 0.5-0.7 m from the ground. The distance between them is recommended to be taken as increasing from the periphery to the center of the grounding grid. In this case, the first and subsequent distances, starting from the periphery, should not exceed 4.0, respectively; 5.0; 6.0; 7.5; 9.0; 11.0; 13.5; 16.0 and 20.0 m. The dimensions of the cells of the grounding grid adjacent to the places of connection of the neutrals of power transformers and short circuits to the grounding device should not exceed 6x6 m².
Horizontal grounding conductors should be laid along the edge of the territory occupied by the grounding device, so that they together form a closed loop.
If the circuit of the grounding device is located within the external fence of the electrical installation, then at the entrances and entrances to its territory, the potential should be equalized by installing two vertical ground electrodes at the external horizontal ground electrode opposite the entrances and entrances. Vertical earthing should be 3-5 m long, and the distance between them should be equal to the width of the entrance or entrance.
1.7.52. The grounding device, which is carried out in compliance with the requirements for the contact voltage, must provide at any time of the year when the ground fault current drains from it, the values of the contact voltage that do not exceed the rated ones. In this case, the resistance of the grounding device is determined by the allowable voltage on the grounding device and the ground fault current.
When determining the value of the allowable contact voltage, the sum of the protection action time and the total switch off time should be taken as the estimated exposure time. At the same time, the determination of the permissible values of contact voltage at workplaces where, during the production of operational switching, short circuits can occur on structures that are accessible to touch by the personnel performing the switching, the duration of the backup protection should be taken, and for the rest of the territory - the main protection.
The placement of longitudinal and transverse horizontal grounding conductors should be determined by the requirements for limiting contact voltages to normalized values and the convenience of connecting grounded equipment. The distance between longitudinal and transverse horizontal artificial ground electrodes should not exceed 30 m, and the depth of their laying in the ground should be at least 0.3 m. At workplaces, it is allowed to lay ground electrodes at a shallower depth, if the need for this is confirmed by calculation, and the implementation itself does not reduce ease of maintenance of the electrical installation and the service life of grounding conductors. To reduce the contact voltage at workplaces, in justified cases, crushed stone can be backfilled with a layer of 0.1-0.2 m thick.
1.7.53. When making a grounding device in compliance with the requirements for its resistance or contact voltage, in addition to the requirements of 1.7.51 and 1.7.52, it should be:
grounding conductors connecting equipment or structures to the ground electrode should be laid in the ground at a depth of at least 0.3 m;
near the locations of the grounded neutrals of power transformers, short circuits, lay longitudinal and transverse horizontal ground electrodes (in four directions).
When the grounding device goes beyond the fence of the electrical installation, horizontal ground electrodes located outside the territory of the electrical installation should be laid at a depth of at least 1 m. In this case, the external contour of the grounding device is recommended to be made in the form of a polygon with obtuse or rounded corners.
1.7.54. It is not recommended to connect the external fence of electrical installations to a grounding device. If overhead lines of 110 kV and above depart from the electrical installation, then the fence should be grounded using vertical ground electrodes 2-3 m long installed at the fence posts along its entire perimeter after 20-50 m. The installation of such ground electrodes is not required for a fence with metal posts and with those posts made of reinforced concrete, the reinforcement of which is electrically connected to the metal links of the fence.
To exclude the electrical connection of the external fence with the grounding device, the distance from the fence to the elements of the grounding device located along it on the inside, on the outside or on both sides must be at least 2 m. Horizontal grounding switches, pipes and cables with a metal sheath extending beyond the fence and other metal communications should be laid in the middle between the posts of the fence at a depth of at least 0.5 m. less than 1 m.
Do not install electrical receivers up to 1 kV on the outer fence, which are powered directly from step-down transformers located on the territory of the electrical installation. When placing electrical receivers on an external fence, they should be powered through isolation transformers. These transformers are not allowed to be installed on the fence. The line connecting the secondary winding of the isolating transformer with the power receiver located on the fence must be isolated from the ground by the calculated voltage value at the grounding device.
If it is not possible to perform at least one of the above measures, then the metal parts of the fence should be connected to a grounding device and potential equalization should be performed so that the contact voltage on the outer and inner sides of the fence does not exceed the permissible values. When making a grounding device according to the permissible resistance, for this purpose, a horizontal ground electrode must be laid on the outer side of the fence at a distance of 1 m from it and at a depth of 1 m. This earth electrode must be connected to the earthing device at least at four points.
1.7.55. If the grounding device of an industrial or other electrical installation is connected to the ground electrode of an electrical installation above 1 kV with an effectively grounded neutral cable with a metal sheath or armor or other metal connections, then in order to equalize the potentials around such an electrical installation or around the building in which it is located, one of the following must be observed: following conditions:
1) laying in the ground at a depth of 1 m and at a distance of 1 m from the foundation of the building or from the perimeter of the territory occupied by the equipment, a ground electrode connected to metal structures for construction and industrial purposes and a grounding (grounding) network, and at the entrances and at the entrances to the building - laying conductors at a distance of 1 and 2 m from the ground electrode at a depth of 1 and 1.5 m, respectively, and connecting these conductors to the ground electrode;
2) the use of reinforced concrete foundations as grounding conductors in accordance with 1.7.35 and 1.7.70, if this ensures an acceptable level of potential equalization. The provision of potential equalization conditions with the help of reinforced concrete foundations used as grounding conductors is determined on the basis of the requirements of special directive documents.
It is not required to fulfill the conditions specified in paragraphs 1 and 2 if there are asphalt pavements around the buildings, including at the entrances and entrances. If there is no blind area at any entrance (entrance), potential equalization must be performed at this entrance (entrance) by laying two conductors, as indicated in paragraph 1, or the condition according to paragraph 2 must be met. In this case, in all cases, requirements 1.7.56.
1.7.56. In order to avoid potential carryover, it is not allowed to supply electrical receivers located outside the grounding devices of electrical installations above 1 kV of a network with an effectively grounded neutral, from windings up to 1 kV with a grounded neutral of transformers located within the contour of the grounding device. If necessary, such electrical receivers can be powered from a transformer with an isolated neutral on the side up to 1 kV via a cable line made with a cable without a metal sheath and without armor, or via overhead lines. The power supply of such electrical receivers can also be carried out through an isolating transformer. The isolation transformer and the line from its secondary winding to the power receiver, if it passes through the territory occupied by the grounding device of the electrical installation, must be insulated from the ground by the calculated voltage value at the grounding device. If it is impossible to fulfill the specified conditions in the territory occupied by such electrical receivers, potential equalization must be performed.
ELECTRICAL INSTALLATIONS WITH VOLTAGE ABOVE 1 kV NETWORKS WITH INSULATED NEUTRAL
1.7.57. In electrical installations above 1 kV of a network with an isolated neutral, the resistance of the grounding device R, Ohm, during the passage of the rated earth fault current at any time of the year, taking into account the resistance of natural grounding conductors, there should be no more than:
when using a grounding device simultaneously for electrical installations with voltage up to 1 kV
R = 125 / I, but not more than 10 ohms.
where I- rated earth fault current, A.
At the same time, the requirements for grounding (grounding) of electrical installations up to 1 kV must also be met;
when using a grounding device only for electrical installations above 1 kV
R = 250 / I, but not more than 10 ohms.
1.7.58. The following is taken as the rated current:
1) in networks without compensation of capacitive currents - full earth fault current;
2) in networks with compensation of capacitive currents;
for grounding devices to which compensating devices are connected - a current equal to 125% of the rated current of these devices;
for grounding devices to which compensating devices are not connected, the residual earth fault current passing in this network when the most powerful of the compensating devices or the most branched section of the network is turned off.
As the rated current, the fuse melting current or the tripping current of the relay protection against single-phase earth faults or phase-to-phase faults can be taken, if in the latter case the protection provides disconnection of earth faults. In this case, the earth fault current must be at least one and a half times the current of operation of the relay protection or three times the rated current of the fuses.
The rated earth fault current must be determined for that of the network schemes possible in operation, in which this current has the greatest value.
1.7.59. In open electrical installations above 1 kV of networks with isolated neutral around the area occupied by the equipment, at a depth of at least 0.5 m, a closed horizontal grounding conductor (circuit) must be laid to which the grounded equipment is connected. If the resistance of the grounding device is higher than 10 Ohm (in accordance with 1.7.69 for earth with a specific resistance of more than 500 Ohm m), then horizontal ground electrodes should be additionally laid along the rows of equipment from the service side at a depth of 0.5 m and at a distance of 0.8 -1.0 m from foundations or equipment bases.
ELECTRICAL INSTALLATIONS WITH VOLTAGE UP TO 1 kV WITH A DEEPLY EARTHED NEUTRAL
1.7.60. The neutral of the generator, transformer on the side up to 1 kV must be connected to the grounding conductor using a grounding conductor. The cross section of the grounding conductor must not be less than that indicated in Table. 1.7.1.
The use of a zero working conductor coming from the neutral of the generator or transformer to the switchgear panel as a grounding conductor is not allowed.
The specified grounding conductor must be located in close proximity to the generator or transformer. In some cases, for example, in intrashop substations, it is allowed to build a ground electrode directly near the wall of the building.
1.7.61. The output of the zero working conductor from the neutral of the generator or transformer to the switchgear switchboard must be carried out: when the phases are output by tires - a bus on insulators, when the phases are output by a cable (wire) - a residential cable (wires). In cables with an aluminum sheath, it is allowed to use the sheath as a zero working conductor instead of the fourth core.
The conductivity of the zero working conductor coming from the neutral of the generator or transformer must be at least 50% of the conductivity of the phase output.
1.7.62. The resistance of the grounding device, to which the neutrals of generators or transformers or the outputs of a single-phase current source are connected, at any time of the year should be no more than 2, 4 and 8 ohms, respectively, at line voltages of 660, 380 and 220 V of a three-phase current source or 380, 220 and 127 In a single-phase current source. This resistance must be provided taking into account the use of natural grounding conductors, as well as grounding conductors for repeated grounding of the neutral wire of overhead lines up to 1 kV with the number of outgoing lines of at least two. In this case, the resistance of the ground electrode located in close proximity to the neutral of the generator or transformer or the output of a single-phase current source should be no more than: 15, 30 and 60 Ohms, respectively, at line voltages of 660, 380 and 220 V of a three-phase current source or 380, 220 and 127 In a single-phase current source.
With a specific earth resistance of more than 100 Ohm m, it is allowed to increase the above norms by 0.01 times, but not more than ten times.
1.7.63. On overhead lines, grounding should be carried out with a zero working wire laid on the same supports as the phase wires.
At the ends of overhead lines (or branches from them) longer than 200 m, as well as at the inputs from overhead lines to electrical installations that are subject to grounding, re-grounding of the neutral working wire must be performed. In this case, in the first place, natural grounding should be used, for example, underground parts of supports (see 1.7.70), as well as grounding devices made to protect against lightning surges (see 2.4.26).
The indicated repeated groundings are carried out if more frequent groundings are not required by the conditions of lightning surge protection.
Re-grounding of the neutral wire in DC networks should be carried out using separate artificial grounding conductors, which should not have metal connections with underground pipelines. Grounding devices on DC overhead lines designed to protect against lightning surges (see 2.4.26) are recommended to be used for re-grounding the neutral working wire.
Grounding conductors for re-grounding the neutral wire must be selected from the condition of long-term current flow of at least 25 A. In terms of mechanical strength, these conductors must have dimensions not less than those given in Table. 1.7.1.
1.7.64. The total spreading resistance of ground electrodes (including natural ones) of all re-groundings of the neutral working wire of each overhead line at any time of the year should be no more than 5, 10 and 20 Ohms, respectively, at line voltages of 660, 380 and 220 V of a three-phase current source or 380, 220 and 127 V single-phase current source. In this case, the spreading resistance of the grounding conductor of each of the repeated groundings should be no more than 15, 30 and 60 ohms, respectively, at the same voltages.
With a specific earth resistance of more than 100 Ohm m, it is allowed to increase the indicated norms by 0.01 times, but not more than ten times.
ELECTRICAL INSTALLATIONS WITH VOLTAGE up to 1 kV WITH INSULATED NEUTRAL
1.7.65. The resistance of the grounding device used for grounding electrical equipment must be no more than 4 ohms.
With a power of generators and transformers of 100 kVA and less, grounding devices can have a resistance of not more than 10 ohms. If generators or transformers operate in parallel, then a resistance of 10 ohms is allowed with a total power of not more than 100 kVA.
1.7.66. Grounding devices of electrical installations with voltages above 1 kV with an effectively grounded neutral in areas with high earth resistivity, including permafrost areas, are recommended to be performed in compliance with the requirements for touch voltage (see 1.7.52).
In rocky structures, it is allowed to lay horizontal ground electrodes at a shallower depth than required by 1.7.52 - 1.7.54, but not less than 0.15 m. In addition, it is allowed not to carry out the vertical ground electrodes required by 1.7.51 at entrances and entrances.
1.7.67. When constructing artificial ground electrodes in areas with high earth resistivity, the following measures are recommended:
1) the installation of vertical ground electrodes of increased length, if the resistivity of the earth decreases with depth, and there are no natural recessed ground conductors (for example, wells with metal casing pipes);
2) the installation of remote ground electrode systems, if there are places with a lower earth resistivity near (up to 2 km) from the electrical installation;
3) laying in trenches around horizontal ground electrodes in rocky structures of wet clay soil, followed by tamping and backfilling with crushed stone to the top of the trench;
4) the use of artificial soil treatment in order to reduce its resistivity, if other methods cannot be applied or do not give the desired effect.
1.7.68. In areas of permafrost, in addition to the recommendations given in 1.7.67, one should:
1) place ground electrodes in non-freezing water bodies and thawed zones;
2) use well casing pipes; 3) in addition to deep earthing, use extended earthing at a depth of about 0.5 m, designed to work in the summer when the surface layer of the earth thaws;
4) create artificial thawed zones by covering the soil above the ground electrode with a layer of peat or other heat-insulating material for the winter period and opening them for the summer period.
1.7.69. In electrical installations above 1 kV, as well as in electrical installations up to 1 kV with an isolated neutral for earth with a resistivity of more than 500 ohm m, if the measures provided for in 1.7.66-1.7.68 do not allow obtaining earth electrodes acceptable for economic reasons, it is allowed to increase the resistance values of grounding devices required by this chapter by a factor of 0.002, where is the equivalent earth resistivity, Ohm m. In this case, the increase in the resistance of grounding devices required by this chapter should not be more than tenfold.
EARTHING
1.7.70. It is recommended to use as natural ground conductors: 1) water and other metal pipelines laid in the ground, with the exception of pipelines of flammable liquids, flammable or explosive gases and mixtures;
2) casing pipes of wells;
3) metal and reinforced concrete structures of buildings and structures in contact with the ground;
4) metal shunts of hydraulic structures, conduits, gates, etc.;
5) lead sheaths of cables laid in the ground. Aluminum sheaths of cables are not allowed to be used as natural grounding conductors.
If cable sheaths serve as the only grounding conductors, then in the calculation of grounding devices they must be taken into account when the number of cables is at least two;
6) ground electrodes of the overhead line supports connected to the grounding device of the electrical installation with the help of an overhead line lightning protection cable, if the cable is not isolated from the overhead line supports;
7) neutral wires of overhead lines up to 1 kV with repeated earthing switches with at least two overhead lines;
8) rail tracks of the main non-electrified railways and access roads in the presence of a deliberate arrangement of jumpers between the rails.
1.7.71. Grounding conductors must be connected to the grounding lines with at least two conductors connected to the grounding conductor in different places. This requirement does not apply to overhead lines, re-grounding of the neutral wire and metal sheaths of cables.
1.7.72. For artificial grounding, steel should be used.
Artificial ground electrodes should not be colored.
The smallest dimensions of steel artificial ground electrodes are given below:
The cross section of horizontal grounding conductors for electrical installations with voltages above 1 kV is selected according to thermal resistance (based on the allowable heating temperature of 400 ° C).
Earthing conductors should not be located (used) in places where the earth dries out under the influence of heat from pipelines, etc.
The trenches for horizontal grounding conductors must be filled with homogeneous soil that does not contain crushed stone and construction debris.
In case of danger of corrosion of ground electrodes, one of the following measures should be taken:
increase in the cross-section of grounding conductors, taking into account the estimated period of their service;
use of galvanized ground electrodes;
application of electrical protection.
As artificial grounding conductors, it is allowed to use grounding conductors made of electrically conductive concrete.
GROUNDING AND ZERO PROTECTIVE CONDUCTORS
1.7.73. As zero protective conductors, zero working conductors must be used first of all (see also 1.7.82).
The following can be used as grounding and zero protective conductors (for exceptions, see chapter 7.3):
1) conductors specially provided for this purpose;
2) metal structures of buildings (trusses, columns, etc.);
3) reinforcement of reinforced concrete building structures and foundations;
4) metal structures for industrial purposes (crane tracks, switchgear frames, galleries, platforms, elevator shafts, elevators, elevators, channel framing, etc.);
5) steel pipes for electrical wiring;
6) aluminum cable sheaths;
7) metal casings and supporting structures of busbars, metal boxes and trays of electrical installations;
8) metal stationary openly laid pipelines for all purposes, except for pipelines of combustible and explosive substances and mixtures, sewerage and central heating.
Given in paragraphs. 2-8 conductors, structures and other elements can serve as the only grounding or zero protective conductors if they meet the requirements of this chapter in terms of conductivity and if the continuity of the electrical circuit is ensured throughout the use.
Grounding and zero protective conductors must be protected from corrosion.
1.7.74. The use of metal sheaths of tubular wires, carrying cables for cable wiring, metal sheaths of insulating tubes, metal hoses, as well as armor and lead sheaths of wires and cables as grounding or zero protective conductors is prohibited. The use of lead sheaths of cables for these purposes is allowed only in reconstructed urban electrical networks 220/127 and 380/220 V.
In indoor and outdoor installations that require the use of earthing or earthing, these elements must be earthed or earthed and have reliable connections throughout. Metal couplings and boxes must be attached to the armor and metal shells by soldering or bolting.
1.7.75. Grounding or zeroing mains and branches from them in enclosed spaces and in outdoor installations must be accessible for inspection and have sections not less than those given in 1.7.76 - 1.7.79.
The requirement for accessibility for inspection does not apply to zero cores and cable sheaths, to reinforcement of reinforced concrete structures, as well as to grounding and neutral protective conductors laid in pipes and ducts, as well as directly in the body of building structures (embedded).
Branches from mains to electrical receivers up to 1 kV can be laid hidden directly in the wall, under a clean floor, etc., with their protection from aggressive environments. Such branches should not have connections.
In outdoor installations, grounding and zero protective conductors may be laid in the ground, in the floor or along the edge of sites, foundations of technological installations, etc.
The use of bare aluminum conductors for laying in the ground as grounding or neutral protective conductors is not allowed.
1.7.76. Grounding and zero protective conductors in electrical installations up to 1 kV must have dimensions not less than those given in Table. 1.7.1 (see also 1.7.96 and 1.7.104).
Cross-sections (diameters) of zero protective and zero working conductors of overhead lines must be selected in accordance with the requirements of Ch. 2.4.
Table 1.7.1. The smallest dimensions of grounding and zero protective conductors
| Name | Copper | Aluminum | Steel | ||
| in buildings | in outdoor installations | in the ground | |||
| Bare conductors: | |||||
| section, mm² | 4 | 6 | - | - | - |
| diameter, mm | - | - | 5 | 6 | 10 |
| Insulated wires: | |||||
| section, mm² | 1,5* | 2,5 | - | - | - |
|
* When laying wires in pipes, the cross section of zero protective conductors may be used equal to 1 mm² if the phase conductors have the same cross section. |
|||||
| Grounding and neutral conductors of cables and stranded wires in a common protective sheath with phase conductors: cross-section, mm² | 1 | 2,5 | - | - | - |
| Angle steel: flange thickness, mm | - | - | 2 | 2,5 | 4 |
| Flat steel: | |||||
| section, mm² | - | - | 24 | 48 | 48 |
| thickness, mm | - | - | 3 | 4 | 4 |
| Water and gas pipes (steel): wall thickness, mm | - | - | 2,5 | 2,5 | 3,5 |
| Thin-walled pipes (steel): wall thickness, mm | - | - | 1,5 | 2,5 | Not allowed |
1.7.77. In electrical installations above 1 kV with an effectively grounded neutral, the cross-sections of the grounding conductors must be selected so that when the highest current of a single-phase short circuit flows through them, the temperature of the grounding conductors does not exceed 400 ° C (short-term heating corresponding to the duration of the main protection and the full time of the switch off).
1.7.78. In electrical installations up to 1 kV and higher with an insulated neutral, the conductivity of the grounding conductors must be at least 1/3 of the conductivity of the phase conductors, and the cross section must be at least those given in Table. 1.7.1 (see also 1.7.96 and 1.7.104). It is not required to use copper conductors with a cross section of more than 25 mm², aluminum - 35 mm², steel - 120 mm². In industrial premises with such electrical mains, grounding from a steel strip must have a cross section of at least 100 mm². Use of round steel of the same section is allowed.
1.7.79. In electrical installations up to 1 kV with a dead-earthed neutral, in order to ensure automatic shutdown of the emergency section, the conductivity of the phase and zero protective conductors must be chosen such that when a short circuit occurs on the case or on the neutral protective conductor, a short-circuit current occurs that exceeds at least:
3 times the rated current of the fuse element of the nearest fuse;
3 times the rated current of the non-adjustable release or the current setting of the adjustable release of the circuit breaker, which has a characteristic inversely dependent on current.
When protecting networks with automatic switches that have only an electromagnetic release (cut-off), the conductivity of these conductors must provide a current not lower than the instantaneous operating current setting multiplied by a factor that takes into account the spread (according to factory data) and by a safety factor of 1.1. In the absence of factory data for circuit breakers with a rated current of up to 100 A, the short-circuit current ratio relative to the setting should be taken at least 1.4, and for circuit breakers with a rated current of more than 100 A - at least 1.25.
The total conductivity of the neutral protective conductor in all cases must be at least 50% of the conductivity of the phase conductor.
If the requirements of this paragraph are not met with regard to the value of the fault current to the case or to the neutral protective conductor, then disconnection during these faults must be ensured by means of special protections.
1.7.80. In electrical installations up to 1 kV with a solidly grounded neutral, in order to meet the requirements given in 1.7.79, it is recommended to lay zero protective conductors together with or in close proximity to phase ones.
1.7.81. Zero working conductors must be designed for a long flow of working current.
It is recommended to use conductors with insulation equivalent to the insulation of phase conductors as zero working conductors. Such insulation is mandatory for both zero working and zero protective conductors in those places where the use of bare conductors can lead to the formation of electric pairs or damage to the insulation of phase conductors as a result of sparking between the bare neutral conductor and the shell or structure (for example, when laying wires in pipes, boxes, trays). Such insulation is not required if casings and supporting structures of complete busbars and busbars of complete switchgears (shields, distribution points, assemblies, etc.), as well as aluminum or lead cable sheaths are used as zero working and zero protective conductors (see. 1.7.74 and 2.3.52).
In industrial premises with a normal environment, it is allowed to use the metal structures specified in 1.7.73 as zero working conductors, pipes, casings and supporting structures of busbars to power single single-phase low-power electrical receivers, for example: in networks up to 42 V; when switching on the phase voltage of single coils of magnetic starters or contactors; when switching on the phase voltage of electric lighting and control and signaling circuits on cranes.
1.7.82. It is not allowed to use zero working conductors going to portable power receivers of single-phase and direct current as zero protective conductors. To neutralize such electrical receivers, a separate third conductor must be used, connected in the plug-in connector of the branch box, in the shield, shield, assembly, etc. to the neutral working or neutral protective conductor (see also 6.1.20).
1.7.83. In the circuit of grounding and neutral protective conductors, there should be no disconnecting devices and fuses.
In the circuit of zero working conductors, if they simultaneously serve for grounding purposes, it is allowed to use switches that, simultaneously with disconnecting zero working conductors, disconnect all live wires (see also 1.7.84).
Single-pole switches should be installed in the phase conductors, and not in the zero working conductor.
1.7.84. Zero protective conductors of lines are not allowed to be used for grounding electrical equipment powered by other lines.
It is allowed to use zero working conductors of lighting lines for neutralizing electrical equipment powered by other lines, if all these lines are fed from one transformer, their conductivity meets the requirements of this chapter and it is impossible to disconnect zero working conductors during operation of other lines. In such cases, switches should not be used that disconnect the neutral working conductors together with the phase ones.
1.7.85. In dry rooms, without aggressive environment, grounding and zero protective conductors can be laid directly along the walls.
In humid, damp and especially damp rooms and in rooms with an aggressive environment, grounding and zero protective conductors should be laid at a distance of at least 10 mm from the walls.
1.7.86. Grounding and zero protective conductors must be protected from chemical influences. In places where these conductors cross with cables, pipelines, railway tracks, in places where they enter buildings and in other places where mechanical damage to grounding and neutral protective conductors is possible, these conductors must be protected.
1.7.87. The laying of grounding and zero protective conductors in places of passage through walls and ceilings should be carried out, as a rule, with their direct termination. In these places, the conductors should not have connections and branches.
1.7.88. Identification marks must be provided at the places where grounding conductors enter buildings.
1.7.89. The use of specially laid grounding or zero protective conductors for other purposes is not allowed.
CONNECTIONS AND CONNECTIONS OF GROUNDING AND ZERO PROTECTIVE CONDUCTORS
1.7.90. Connections of grounding and zero protective conductors to each other must ensure reliable contact and be carried out by welding.
It is allowed indoors and in outdoor installations without aggressive environments to connect grounding and neutral protective conductors in other ways that ensure the requirements of GOST 10434-82 "Contact electrical connections. General technical requirements" for the 2nd class of connections. At the same time, measures must be taken to prevent weakening and corrosion of contact connections. Connections of grounding and zero protective conductors of electrical wiring and overhead lines are allowed to be performed by the same methods as for phase conductors.
Connections of grounding and zero protective conductors must be accessible for inspection.
1.7.91. Steel pipes of electrical wiring, boxes, trays and other structures used as grounding or zero protective conductors must have connections that meet the requirements of GOST 10434-82 for the 2nd class of connections. Reliable contact of steel pipes with electrical equipment housings into which pipes are inserted, and with metal junction (branch) boxes must also be ensured.
1.7.92. The places and methods of connecting grounding conductors with extended natural grounding conductors (for example, with pipelines) must be chosen so that when the grounding conductors are disconnected for repair work, the calculated value of the resistance of the grounding device is provided. Water meters, gate valves, etc. must have bypass conductors to ensure continuity of the ground circuit.
1.7.93. The connection of grounding and neutral protective conductors to parts of the equipment to be grounded or grounded must be done by welding or bolting. The connection must be accessible for inspection. For bolted connection, measures must be taken to prevent loosening and corrosion of the contact connection.
Grounding or grounding of equipment that is subject to frequent dismantling or installed on moving parts or parts subject to shock or vibration must be carried out with flexible grounding or zero protective conductors.
1.7.94. Each part of the electrical installation to be grounded or grounded must be connected to the grounding or grounding network using a separate branch. Consistent connection to the grounding or zero protective conductor of the grounded or grounded parts of the electrical installation is not allowed.
PORTABLE ELECTRIC RECEIVERS
1.7.95. Portable electrical receivers should be powered from a mains voltage not exceeding 380/220 V.
Depending on the category of the premises according to the level of danger of electric shock to people (see Chap. 1.1), portable electrical receivers can be powered either directly from the mains, or through isolation or step-down transformers (see 1.7.44).
The metal cases of portable power receivers above 42 V AC and above 110 V DC in high-risk rooms, especially dangerous rooms and in outdoor installations must be grounded or grounded, with the exception of double-insulated electrical receivers or powered by isolation transformers.
1.7.96. Grounding or grounding of portable electrical receivers should be carried out with a special core (the third one - for single-phase and direct current electrical receivers, the fourth - for three-phase current electrical receivers), located in the same sheath with the phase conductors of the portable wire and attached to the body of the electrical receiver and to the special contact of the plug-in connector (see 1.7.97). The cross section of this core must be equal to the cross section of the phase conductors. The use of a zero working conductor for this purpose, including one located in a common shell, is not allowed.
Due to the fact that GOST for some brands of cables provides for a reduced cross section of the fourth core, it is allowed to use such cables for three-phase portable electrical receivers until the corresponding change in GOST.
The cores of wires and cables used for grounding or grounding of portable power receivers must be copper, flexible, with a cross section of at least 1.5 mm² for portable power receivers in industrial installations and at least 0.75 mm² for portable household power receivers.
1.7.97. Portable power receivers of test and experimental installations, the movement of which is not provided for during their operation, is allowed to be grounded using stationary or separate portable grounding conductors. In this case, stationary grounding conductors must meet the requirements of 1.7.73 - 1.7.89, and portable grounding conductors must be flexible, copper, with a cross section not less than the cross section of phase conductors, but not less than that specified in 1.7.96.
In plug-in connectors of portable electrical receivers, extension wires and cables, conductors must be connected to the socket from the side of the power source, and to the plug - from the side of electrical receivers.
Plug-in connectors must have special contacts to which grounding and neutral protective conductors are connected.
The connection between these contacts at switch-on must be established before the contacts of the phase conductors come into contact. The order of disconnection of contacts during disconnection must be reversed.
The design of plug-in connectors must be such that the possibility of connecting the contacts of the phase conductors to the grounding (zeroing) contacts is included.
If the body of the plug-in connector is made of metal, it must be electrically connected to the earth (neutral) contact.
1.7.98. Grounding and zero protective conductors of portable wires and cables must have a distinctive feature.
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