Selectivity of household circuit breakers using an example in simple language

  • Dec 14, 2020
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As professional electricians know, using circuit breakers for household use, it is impossible to provide guaranteed selective short circuit protection.

It is possible to provide, with the help of household circuit breakers (hereinafter referred to as AB), only partial selectivity at small overloads and small short-circuit currents. Why so and not otherwise - I will tell later in this article.

Important! To begin with, let's agree that our AVs comply with the requirements of modern standards GOST R 50345–2010 and IEC 60898-1: 2015.

Photo illustration of household circuit breakers from my working collection. For reference: these ABs also corresponded to GOST R 50345-99, which was eventually replaced by GOST R 50345-2010
Photo illustration of household circuit breakers from my working collection. For reference: these ABs also corresponded to GOST R 50345-99, which was eventually replaced by GOST R 50345-2010

Next, let's imagine that we have 2 ABs connected in series. Take, for example, that the first QF1, with a rated current of 50A and type C instantaneous release, stands at the input to the apartment shield (KShch), and the second QF2, with a rated current of 16 A and type B instantaneous release, protects the electrical circuit of the plug sockets.

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Our task is to ensure proper coordination (selectivity) between these 2 series-connected overcurrent protection devices.

This must be done in such a way that in the event of an overload or short circuit, AB, which is closer to the place the appearance of an overcurrent (our AB at 16 A), triggered earlier than AB, which is closer to the power source (QF1 at the input to KSC). That is, QF1 as a result should not work and the electrical installation of the building will continue to work with the exception of one from the electrical circuits of the sockets, which was de-energized as a result of the selective operation of QF2. This is what we would like. Now read on under what conditions this is possible.

IN P. 5.3.5 GOST R 50345–2010 for each type of instantaneous tripping, the following standard ranges of instantaneous tripping currents are set (for simplicity, we will call it Im.r):

  • Type B: 3In
  • Type C: 5In
  • Type D: 10In

Then our AB QF1, which is installed at the input to the KSH, can instantly work at an overcurrent greater than 250 A (for example, 251 A) and must instantly uncouple at an overcurrent greater than or equal to 500 A.

Below I have prepared a graphical figure showing the areas of overcurrents in which the selective actuation of two series-connected AB, ensured (zone 1), possible (zone 2) or impossible (zone 3).

Important: this graph is valid for the case where we connect in series a circuit breaker with instantaneous trip type C (first) and AB with magnetic trip type B (second). In this case, the first AB is closer to the power source, and the second closer to the potential location of the overcurrent. And besides, the requirement for rated currents AB is fulfilled: In1> In2, where

  • In1 - rated current of the first AB
  • In2 - rated current of the second AB

Thus, between QF1 and QF2, it is possible to ensure selective operation at overcurrents up to 250 A, since in this overcurrent range, the tripping time QF1 ( Tt1) will always be longer than the tripping time QF2 (Tb2), that is Tt1> Tb2. In other words, in this overcurrent range, QF2 will "trip" first, and QF1 will not trip at all, that is, selectivity will be ensured.

In the range of overcurrents from 251 to 499, selective operation is possible (there is no definite answer here!). With an overcurrent of 500 A or more, selective tripping is not possible, since then both QF1 and QF2 will trip almost simultaneously (in less than 0.1 second).

As a result, using circuit breakers for household use, it is possible to provide selective protection only with insignificant overloads and small short-circuit currents.

I also want to note that you should not expect selectivity from ABs, which are not intended for this. If you want guaranteed selectivity, then buy and install special selective AB application categories B, which correspond to GOST R 50030.2-2010.

I recommend reading:

  1. You can find an explanation of the terms used in this article in the constantly updated dictionary of electrician terms on our website: https://www.asutpp.ru/slovar-terminov-elektrika
  2. How to color-code conductors 100% correctly? I put an end to the disputes:
  3. How to use UDT (RCD) in electrical installations of private houses and apartments?