The principle of selectivity for choosing automatic circuit breakers and RCD

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When laying electrical wiring in apartments are created wiring diagrams that always address safety issues operation. Electric current can cause great harm. To this did not happen, install protection devices: fuses, circuit breakers, residual current devices, and other means. They all have specific, specific opportunities, but can not be universal. Therefore, when choosing devices should clearly take into account their individual characteristics. Only in this case they will work correctly, not create unnecessary problems in the future.


  1. Selectivity principle for automatic selection circuit breakers and RCD
  2. Types of selectivity
  3. Selection of circuit breakers for response time
  4. Selection of circuit breakers and RCDs for current triggering

Selectivity principle for automatic selection circuit breakers and RCD

This property is also called selectivity. Selectivity allows you to reliably operate the electrical industry thanks to proper selection of protective devices. For any electrical schemes applied hierarchy of circuit breakers sharing electrical wiring with consumers to certain areas – electrical circuits even when current flows from source to consumer directly, bypassing the intermediate links. Malfunction in this very A simple circuit may arise inside:

  • generator;
  • receiver
  • or connecting wires.

Each of these cases requires its own technical solution, which will allow quick ways to reliably identify and localize damaged area.

Selectivity defines installation and compatibility rules. protections To do this, the entire power supply system is divided into separate component sections, divided into zones with inclusion in them disconnecting devices that respond to malfunctions.

Types of selectivity

Selectivity happens:

  • absolute;
  • relative.

The principle of absolute selectivity means disabling damage arising exclusively in its area. Protection performed on a relative basis, respond to malfunctions own and neighboring plots. They can work on any launcher. factor. Therefore, to exclude false shutdowns, they are given additional functions:

  • the value of the delay time for the response;
  • settings for current, voltage, frequency, electrical resistance, power direction or other parameters network.

Selection of circuit breakers for response time

This principle can be demonstrated by the circuit.

time selectivitytime selectivitytime selectivity

To explain its operation, all machines have one set point. cut-off current of 25 amperes, but disconnect the damaged area with different time. If a malfunction occurs in the circuit of any consumer, for example, powered by a circuit breaker No. 3, the short circuit current will be felt by the machines:

  • faulty plot No. 3;
  • switchboard No. 2;
  • Main switchboard No. 3.

Response time delay 0.1 sec smallest machine number 3. It will work first, localizing the malfunction. Current damage will be interrupted, and circuit breakers No. 2 and No. 1 will remain on to continue powering consumers zones No. 4 and No. 5.

In this situation, breakdown of machine number 3 is possible, then it does not will work. Short-circuit current after passing the time of 0.1 seconds will remain in scheme. After a delay of 0.5 seconds, it will turn off the protection switchboard – circuit breaker No. 2.

He reserves the work of the defenses of the plot No. 3, but additionally disconnects consumers of chains No. 4 and 5 on which short-circuit current was absent.

If for some reason this circuit breaker is also turns out to be faulty, then the function of eliminating fault currents performs protection of the main switchboard (MDB) automatically No. 1. It should be imagined that after 1 sec it will deenergize not only sections of zones No. 3, 4 and 5, powered by the switch of the switchboard No. 2, but also other consumers who are connected to additional distribution boards of main switchboard No. 1.

About the types of RCDs and its connection is described in detail articles:

  • What types of RCDs exist and what is their difference?
  • How to connect an RCD? Wiring diagram

Selection of circuit breakers and RCDs for current triggering

current selectivitycurrent selectivitytripping current selectivity

The presented diagram shows the principle of choosing automatic circuit breakers and RCDs according to the operation current. Here the same The principle, as in the previous scheme: first, the protection should work, Damage closest to the site, and reserve it similar devices of the next, second stage.

In case of short circuit in consumer circuits No. 3, 4, or 5, they are disconnected at the beginning automatic switch of the damaged area, and automatic machine No. 2 reserves his work. In turn, serviceability of protection switchboard is insured by switch No. 1 of the main switchboard.

The residual current device monitors the status of the circuit on lack of leakage currents. 300 mA maximum setpoint assigned to the protection of the main switchboard No. 1. The smallest settings 30 mA put up on the RCD of the final connections. In the RC head RCD No. 2 Configurable to trigger intermediate values ​​of 100 mA.

In practice, the settings for the protection are set according to the combined method taking into account the combination of the principles of selectivity in time, current and other parameters that complement the reliability of the work circuit.

Read also about the choice of automatic breakers article: “Rules for installing automatic circuit breaker “” Automatic infrared switch “” What is opening circuit breaker? “” How is the remote switch?”

Tasks to be Solved

The principle of selectivity allows you to provide:

  • electrical safety of equipment and people;
  • automatic detection of the fault zone and its localization
  • supplying electricity to serviceable areas adjacent to damaged;
  • maintaining the quality of electricity for all consumers.

For these reasons, the selectivity of protective devices should be always consider in practice when choosing equipment when laying electrical wiring for reliable operation electrical equipment.

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