The use of RCDs in various network systems. Scheme, description

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Residual current devices. The use of RCDs in various network systems. Encyclopedia of radio electronics and electrical engineering

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RCD application in modern electrical installations with grounding systems - TN-C, TN-S, TN-CS, TT, IT - has its own characteristics.

On fig. 14.16 -14.20 examples of the inclusion of RCDs in various network systems are given. On fig. 14.16 shows an example of the use of RCDs in an electrical installation of the TN-S system. The TN-S mode, according to experts, provides the best electrical safety conditions for the operation of electrical installations and is most favorable for the successful operation of the RCD.

The use of RCDs in various network systems
Rice. 14.16. The use of RCDs in the TN-S system: 1 - grounding of the power source (at the substation); 2 - protective grounding of the electrical installation of the building (in the input shield); 3 - exposed conductive parts

The use of RCDs in various network systems
Rice. 14.17. The use of RCDs in the TT system: 1 - grounding of the power source; 2 - protective grounding of the electrical installation of the building; 3 - exposed conductive parts

In a TT system, all exposed conductive parts of an electrical installation are connected to an earth that is electrically independent of the power supply neutral earth.

Until now, the EMPs have prohibited the use of the TT system in the electrical installations of buildings.

The standard (PUE, 7th edition, clause 7.1.84) prescribes the use of the TT system as the main one in the case of connecting these electrical installations to the input and distribution devices of a neighboring (capital) building.

The PUE, clause 413.1.4, states that in the TT system, overcurrent protection devices can be used to protect against indirect contact only in electrical installations; having grounding devices with very low resistance. At the same time, a guaranteed power off of the electrical installation should be carried out when a voltage of no more than 50 V appears on the open conductive parts of the electrical installation. 14.17 shows the use of RCDs in an electrical installation of a TT system.

In real conditions, it is very problematic to automatically turn off the power supply of the electrical installation of the TT system using circuit breakers for a number of reasons (the need to provide a large multiplicity of short-circuit current, low resistance of the grounding device, etc.).

An effective solution to the problem of automatic power off is the use of sensitive RCDs.

The PUE, clause 1.7.59, contains a requirement to apply an RCD to ensure electrical safety conditions in the TT system. In this case, the value of the setting (nominal breaking differential current) must be less than the value of the fault current to grounded open conductive parts at a voltage of 50 V relative to the zero potential zone.

This means that in the electrical installations of individual residential buildings, cottages, country (garden) houses and other private structures, where it is not always possible to make a grounding conductor with standard parameters, it is necessary to use a TT system with the mandatory installation of an RCD. In this case, the requirements for the value of the resistance of the earth electrode are significantly reduced.

In an IT system, the value of the earth fault current is determined by the state of the insulation of the network with respect to earth. When the insulation condition is good (high resistance to earth), the earth fault current is very low. In the case of direct contact of a person with the current-carrying parts of an electrical installation, the amount of current passing through the human body is also determined by the insulation resistance and, if the insulation resistance is above a certain value, it does not pose a danger to life.

Thus, the level of insulation resistance in IT networks is a factor that determines both the reliability and electrical safety of their operation. Since it is very important to keep the insulation resistance high in IT networks, continuous automatic insulation monitoring is essential for electrical protection.

The use of RCDs in IT networks is regulated (PUE, clause 1.7.58) as follows:

"... In such electrical installations, for protection against indirect contact at the first earth fault, protective grounding must be performed in combination with network insulation monitoring or RCDs with a rated residual breaking current of not more than 30 mA must be used."

In IT installations, insulation monitors give a signal at the first earth fault. If a second ground fault occurs before the first fault is eliminated, then the RCD is triggered (Fig. 14.18).

On fig. 14.19 shows the use of RCDs in the electrical installation of a building of the TN-CS system. Here, the PEN conductor is divided into N- and PE-conductors not for the entire electrical installation of the building, but only for part of it. The first electrical receiver is installed in that part of the electrical installation of the building in which there is a PEN conductor. The second electrical receiver is used in the part of the electrical installation of the building, where a zero protective conductor is used.

The standard (PUE, notes to clause 413.1.3.8) has restrictions on the use of RCDs as a protective device in the TN system.

Restriction 1. The TN-C system must not use residual current protective devices.

Restriction 2. When a residual current protective device is used for automatic tripping in a TN-CS system, a PEN conductor must not be used on the load side. The connection of the protective conductor to the PEN conductor must be carried out on the power supply side of the residual current protective device.

At the same time, according to the specified standard, it is permissible to use RCDs in those parts of the electrical installation of the building where electrical circuits with PEN conductors are located before the RCD input terminals.

In clause 1.7.80 of the EIC of the 7th edition there is an indication:

"It is not allowed to use an RCD that reacts to differential current in four-wire three-phase circuits (TN-C system). If it is necessary to use an RCD to protect individual electrical receivers powered by the TN-C system, the protective PE conductor of the electrical receiver must be connected to PEN - the conductor of the circuit supplying the electrical receiver to the protective switching device.

This means that, as an exception for the protection of individual electrical receivers, the PUE allow the use of RCDs in the TN-C system, subject to certain conditions - connecting open conductive parts of electrical receivers to the PEN conductor from the power source in relation to the RCD. On fig. 14.20 shows an example of the use of RCDs in the electrical installation of the TN-C system.

The use of RCDs in various network systems
Rice. 14.18. The use of RCDs in the IT system: 1 - protective grounding of the electrical installation of the building; 2 - exposed conductive parts

The use of RCDs in various network systems
Rice. 14.19. The use of RCDs in the TN-CS system: 1 - grounding of the power source; 2 - protective grounding of the electrical installation of the building; 3 - exposed conductive parts

The use of RCDs in various network systems
Rice. 14.20. The use of RCDs in the TN-C system: 1 - grounding of the power source; 2 - protective grounding of the electrical installation of the building; 3 - exposed conductive parts

Until now, most electrical installations in our country work with a grounding system similar to TN-C (without a protective conductor PE).

It is necessary to consider in more detail the functioning of the RCD in such electrical installations. In such an electrical installation, in the event of a breakdown of the insulation on the body of the electrical receiver, if this case is not grounded (for example, a refrigerator or a washing machine on an insulating base), the RCD included in the power supply circuit of the electrical receiver will not work, since there is no leakage current circuit - there is no difference ( differential) current.

In this case, a dangerous potential with respect to earth will appear on the body of the electrical receiver. In this case, when a person touches the body of the electrical receiver and a current flows through his body to the ground that exceeds the rated residual current of the RCD (setting current), the RCD will react and disconnect the electrical installation from the network, as a result, human life will be saved.

In the considered case, this means that from the moment the insulation is broken and the electrical potential appears on the body of the power receiver until the moment the defective circuit is disconnected from the network, there is a period of potential danger of human injury.

From the foregoing, it follows that in electrical installations with a TN-C grounding system, the use of RCDs is also justified, since it provides effective protection against electrical shock.

Author: Koryakin-Chernyak S.L.

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