As professional electricians know, it is not possible to guarantee selective short-circuit protection with household circuit-breakers.
With household circuit-breakers, only partial selectivity can be provided for small overloads and small short-circuit currents. I will explain why this is the case later in this article.
What Is Selectivity?
According to IEC 60898-1-2020 , let us define what total selectivity is:
Total selectivity: overcurrent selectivity where, in the presence of two overcurrent protective devices in series, the protective device on the load side effects the protection without causing the other protective device to operate.
In the case of short-circuits and overloads, selective operation of the circuit breakers connected in series should be sought. The second circuit breaker closest to the point of overcurrent occurrence must trip earlier, so that the first circuit breaker closest to the power supply in front of it does not trip in time. The conditions for selective overcurrent operation of the circuit breakers connected in series can be formalised by means of terminology.
The following terms are defined in IEC 60898-1 :
Opening time: time measured from the instant at which, the circuit-breaker being in the closed position, the current in the main circuit reaches the operating value of the overcurrent release to the instant when the arcing contacts have separated in all poles.
Note 1 to entry: The opening time is commonly referred to as tripping time, although, strictly speaking, tripping time applies to the time between the instant of initiation of the opening time and the instant at which the opening command becomes irreversible.
Arcing time of a pole: interval of time between the instant of initiation of the arc in a pole and the instant of final arc extinction in that pole.
Arcing time of a multipole circuit-breaker: interval of time between the instant of first initiation of an arc and the instant of final extinction in all poles.
Break time: interval of time between the beginning of the opening time of a circuit-breaker and the end of the arcing time.
The opening time is the time interval between the moment when the electric current in the main circuit of the circuit breaker reaches the tripping level of the overcurrent release and the moment when the arc contacts in all poles of the circuit breaker open.
The tripping time is equal to the time interval between the moment of opening time initiation and the moment when the opening command of the circuit breaker becomes irreversible. The moment of tripping time initiation is the moment in time when the electric current in the main circuit of the circuit breaker reaches the value at which its overcurrent release is tripped.
The moment in time when the trip command becomes irreversible represents the moment the overcurrent release actuates, acting on the circuit breaker’s holding device and inducing the opening of its main contacts. Therefore, the tripping time is actually the time interval between the moment when the electric current in the circuit breaker’s main circuit reaches the tripping level of its overcurrent release and the moment when the overcurrent release trips.
Arcing Time of a Pole
When the all-pole circuit breaker switches off the overcurrent on the main contacts which are open, the electric arcs are ignited. After ignition, the electric arc in each pole is drawn into the arc-quenching chamber where it is extensively extinguished. The ignition and extinguishing of the arcs in the poles of a multi-pole circuit-breaker usually occurs in different time intervals. The arcing time of a multi-pole circuit breaker is counted from the time of initiation of the first arc in one of the poles of the circuit breaker until the last arc is extinguished in one of the poles.
The break time is the time interval between the start of the circuit breaker opening time and the end of the arcing time. The start of the trip time is the moment when the electric current in the main circuit of the circuit breaker reaches the tripping level of its overcurrent release. The end of the arc time is the moment the electric arcs in all poles of the circuit-breaker are extinguished. Therefore, the break time of a single-pole circuit breaker is approximately equal to the sum of the opening time and the arcing time of the pole, and of a multi-pole circuit breaker to the sum of the opening time and the arcing time of the multi-pole circuit breaker.
The overcurrent will flow through the circuit breakers in series during the tripping time of the second circuit breaker. This will initiate automatic operation of the first circuit breaker, the overcurrent release of which may trip. The main contacts of the first circuit breaker will begin to open under the influence of the energy stored in its mechanism when it closes. Therefore, to ensure selective operation, the tripping time of the first circuit breaker Tt1 must be longer than the break time of the second circuit breaker Tb2:
Tt1 > Tb2.
In other words, the full selectivity of the operation of the circuit breakers can be ensured if the tripping time of any overcurrent of the first circuit breaker located closer to the power supply is greater than the break time of the second circuit breaker located closer to the short-circuit or overload point, which is located behind it.
That is, as conventionally shown in Figure 1, the time-current characteristic of the first circuit-breaker must be “above” the time-current characteristic of the second circuit-breaker. To meet this condition, the first circuit-breaker must be of selectivity category B and the second circuit-breaker must be of selectivity category A or household use.
Figure 1 shows:
1 – QF1 circuit-breaker of selectivity category B;
2 – QF2 circuit breaker of selectivity category A or household circuit breaker.
Circuit-breakers of utilization categories A and B are manufactured in accordance with the international requirements of IEC 60947-2 , which is applied in conjunction with IEC standard 60947-1. IEC standard 60947-2 classifies circuit-breakers into the following selectivity categories:
- circuit-breakers of selectivity category A, which include circuit-breakers not specifically designed for short-circuit selectivity in relation to other short-circuit protection devices connected in series to them on the load side. These circuit-breakers do not have the specified short-time delay necessary to ensure selectivity under short-circuit conditions. The “rated short-time withstand current” characteristic is not specified for them;
- circuit-breakers of the selectivity category B, which includes circuit-breakers specifically designed for short-circuit selectivity in relation to other short-circuit protection devices connected in series to them on the load side. Such circuit-breakers have a specified short-time delay necessary for short-circuit selectivity and a “rated short-time withstand current” characteristic.
Circuit-breakers of the selectivity category B are specifically designed to provide selective short-circuit operation with circuit-breakers of selectivity category A or household circuit-breakers switched after them. Short-circuit selectivity is assured through the existence of a short-time delay in the tripping time of circuit-breakers of selectivity category B, whose preferred values are set in IEC 60947-2 to 0.05 (minimum value); 0.10; 0.25; 0.50 and 1.00 s.
During this interval, the circuit-breakers installed after the circuit-breakers of selectivity category B closer to the short-circuit location trip the short-circuit currents.
A circuit-breaker of selectivity category B ensures selective operation up to the value of its rated short-time withstand current Icw. For circuit-breakers with rated current In up to and including 2500 A, the value of the rated short-time withstand current must not be lower than the following values: either Icw = 12 In or Icw = 5000 A (the higher value must be selected). For circuit-breakers with rated currents above 2500 A, the minimum value of the rated short-time withstand current is specified in the standards as 30000 A.
However, IEC 60947-2 compliant circuit-breakers cannot ordinary persons can normally use them in places accessible to the general public, for example in low-voltage switchgear in electrical installations of individual houses and flats.
In such cases, household circuit-breakers conforming to IEC 60898-1 and IEC 60898-2 are used. However, these circuit-breakers can only provide partial selectivity in the area of small overcurrents, which are overload currents.
About Partial Selectivity
According to , let us define what partial selectivity is:
Partial selectivity: over-current selectivity where, in the presence of two over-current protective devices in series, the protective device on the load side effects the protection up to a given level of over-current, without causing the other protective device to operate.
Selective operation of household circuit breakers in series with overcurrents is relatively easy to achieve. To achieve this, the circuit breaker closest to the power supply must have a rated current greater than that of the second circuit breaker behind it. It is more difficult to ensure their selective operation for short-circuit currents. The higher rated current of the first circuit-breaker is not a sufficient condition to ensure selective short-circuit operation.
Consider the common case of two household circuit-breakers connected in series, with the time-current characteristics shown conventionally in Figure 2, which comply with IEC 60898-1 and IEC 60898-2.
Figure 2 shows:
1, 2 – circuit breakers QF1 and QF2 respectively.
If an overcurrent or short-circuit current, smaller than the instantaneous tripping current1 IIT1 of circuit breaker QF1, occurs in the electric circuit (downstream of circuit breaker QF2), the circuit breakers trip differently. The tripping of the QF2 circuit breaker in the overcurrent area limited by its instantaneous trip current IIT2 is initiated by the thermal overcurrent release, which is an integral part of the circuit breaker’s overcurrent release.
In the area of the overcurrent above the instantaneous tripping current IIT2, but less than the instantaneous tripping current IIT1, the tripping of the QF2 is initiated by its electromagnetic short-circuit breaker. The tripping of QF1 in the overcurrent area is initiated by its thermal overload release.
Note 1: The instantaneous tripping current is the minimum electrical current that causes the circuit breaker to trip automatically without time delay.
The tripping time of the first circuit breaker is therefore longer than the tripping time of the second circuit breaker. Moreover, in the region of small overcurrents, the first circuit breaker will not trip because these currents are less than its notional no-release current of 1.13 In, and in some cases less than its rated current. This means that when any overcurrent appears in the circuit, which is less than the instantaneous tripping current of the QF1 IIT1, the QF1 and QF2 circuit breakers can be operated selectively.
If the overcurrent in the circuit exceeds the instantaneous trip current of circuit breaker QF1, both circuit breakers operate in the same way. Their tripping is initiated by the electromagnetic short-circuit breakers, inducing the circuit breakers to trip within less than 0.1 s. Modern overcurrent circuit breakers are usually tripped simultaneously, because their actual tripping time usually does not exceed 0.01 s, i.e. if any overcurrent exceeds the instantaneous tripping current of QF1 is present in the circuit, the selective operation of QF1 and QF2 cannot be ensured.
Consequently, only partial selectivity can be ensured when circuit breakers for household appliances are connected in series.
Based on the parameters of the standard time-current zones, it is possible to outline the areas of overcurrent in which household circuit breakers will operate both selectively and non-selectively. Fig. 3 shows conventionally the standard time-current zones of series-connected circuit-breakers. An estimate of the overcurrent zone in which the circuit-breakers will operate selectively can be made on the basis of the standard instantaneous tripping current range of the first circuit-breaker.
The minimum possible instantaneous trip current IIT1min of the first circuit-breaker is slightly higher than the lower limit of the standard range of instantaneous trip currents ISR IT1min, which is 3 In, 5 In or 10 In for instantaneous trip types B, C or D respectively. The maximum permissible instantaneous release current IIT1 max of QF1 is equal to the upper limit of the standard instantaneous release current range ISR IT1 max, i.e. 5 In, 10 In and 20 In for instantaneous trip types B, C and D. The actual instantaneous trip current IIT1 of the first circuit breaker lies between these limits of the standard range of instantaneous trip currents:
Figure 3 shows:
1, 2 – circuit breakers QF1 and QF2 respectively.
The area in which overcurrents cannot be made selective for circuit breakers starts at the upper limit of the standard instantaneous trip current range set for the QF1 circuit breaker. Any good-quality circuit-breaker exposed to an overcurrent occurring in this area will trip in less than 0.1 s (actually 0.01 s or less), i.e. at any overcurrent equal to or greater than the upper limit of the standard range of instantaneous trip currents of the first breaker ISR IT1 max, selective operation of the circuit-breakers in question cannot be ensured. They will normally trip simultaneously.
The overcurrent region in which it is possible to ensure selective operation of the above circuit breakers ends at the lower limit of the standard instantaneous trip current range set for circuit breaker QF1. In this overcurrent range, any circuit breaker QF1 will have a tripping time higher than that of any circuit breaker QF2, i.e. at an overcurrent that does not exceed the lower limit of the standard instantaneous tripping current range of the first circuit breaker ISR IT1 min, it is always possible to ensure selective operation of the circuit breakers.
If an overcurrent exceeding the lower limit of the standard instantaneous trip current range of circuit breaker QF1 but lower than the upper limit of its standard instantaneous trip current range (ISR IT1 max > I > ISR IT1 min ) appears in the circuit breakers, the circuit breakers can operate both selectively and non-selectively, i.e. the above overcurrent area is an overcurrent area for which the possibility of selective operation cannot be given unambiguously. In fact, they will operate selectively at any overcurrent below the instantaneous trip current IIT1 of breaker QF1. However, at the design stage of an electrical installation of a building its value is not known.
About overcurrent selectivity
According to , let us define what overcurrent selectivity is:
Overcurrent selectivity: coordination of the operating characteristics of two or more overcurrent protective devices in series such that, on the incidence of overcurrents within stated limits, the device intended to operate within these limits does so, while the other(s) does (do) not.
When circuit breakers are connected in series, it must be ensured that they operate selectively in the event of overloads, short-circuits and earth faults. The circuit breaker closest to the point of overload, short-circuit or earth fault, usually in the final circuit, should operate first. The second should be operated by the circuit-breaker closest to the source of supply, e.g. the one installed at the entrance to the electrical installation of a building, or the circuit-breaker protecting some kind of electrical distribution circuit.
Otherwise, if the main circuit breaker or the circuit breaker installed in the distribution circuit is tripped first, the entire electrical installation of a building or a part of it consisting of several final electric circuits will be disconnected instead of one final electric circuit in which an overload, short-circuit or earth fault has occurred. A similar unwanted tripping will also occur if both circuit breakers are tripped simultaneously. For this reason, due consideration must be given to the selective operation of circuit-breakers when planning the electrical installation of buildings.
Regulatory documentation distinguishes between full selectivity and partial selectivity. With full selectivity, series-connected circuit breakers operate selectively over the entire range of overcurrents. With partial selectivity their selective operation is possible within a limited range of overcurrents, usually representing overcurrents.
It is also advisable to provide partial selectivity for earth faults in electrical installations of buildings, corresponding to the TN-S and TN-C-S system earthing types, between circuit-breakers and residual current devices (RCDs) without built-in overcurrent protection. Overcurrent earth faults must be tripped by residual current devices. Otherwise, if circuit-breakers and residual current devices trip simultaneously, it is difficult to determine the cause of their operation because RCDs without built-in overcurrent protection may trip at overload and short-circuit currents of more than 6 times their rated currents.
Applying the terminology further we can move on to a simple real-life example. So we have 2 circuit breakers connected in series.
Let us assume, for example, that the first circuit-breaker QF1, installed on the main entry of the switchgear of an electrical installation of an individual house, has a rated current of 50 A and instantaneous trip type C, and the second circuit-breaker QF2, installed in the switchgear and protecting the final electrical circuit of the socket-outlets against overcurrent, has a rated current of 16 A and instantaneous trip type B.
Our task is to ensure proper coordination (selectivity) between these 2 overcurrent protection devices connected in series.
This should be done so that in the event of an overload or short-circuit, the AB closest to the location of the overcurrent (our 16 A circuit breaker) will trip before the circuit breaker closest to the power supply (QF1 at the entrance to the switchgear). So, QF1 must not trip and the electrical installation of a building will continue to operate, except for one of the electrical circuits of the socket-outlets, which has been de-energised as a result of the selective operation of QF2. This is what we would like to do. Now read on under which conditions this is possible.
IEC 60898-1  clause 5.3.5 specifies the following standard ranges of instantaneous release currents for each type of instantaneous release (for simplicity let’s call it IIT):
|B||3In < IIT ≤ 5In|
|C||5In < IIT ≤ 10In|
|D||10In < IIT ≤ 20In|
The overcurrent range, in which selective operation of these circuit breakers cannot be ensured, starts from the upper limit of the standard range of instantaneous trip currents equal to 10 In or 500 A. The area of overcurrents in which selective operation can be assured is located up to the lower limit of the standard instantaneous trip current range of 5 In or 250 A.
This means that our QF1 breaker, which is installed in the input of the switchboard, can trip instantaneously at overcurrents greater than 250 A (e.g. 251 A), and must trip instantaneously at overcurrents greater than or equal to 500 A.
Below I have prepared a graph showing the areas of overcurrent in which selective tripping of two series-connected circuit breakers with instantaneous trip types C (first) and B (second) is ensured (zone 1), possible (zone 2) or impossible (zone 3). The figure 4 clearly illustrates the fact that, for household circuit-breakers, selective tripping can only be ensured at low overcurrent values.
Important: this figure is valid when a circuit breaker of the instantaneous trip type C (QF1) and a circuit breaker of the magnetic trip type B (QF2) are connected in series. In this way QF1 is closer to the power supply, and QF2 is closer to the potential overcurrent point. In addition, the requirement for the rated currents of the circuit breakers is fulfilled: In1 > In2, where
- In1 – rated current of the first circuit breaker;
- In2 – rated current of the second circuit breaker.
Thus, between QF1 and QF2 it is possible to ensure selective operation at overcurrents up to 250 A, because in this overcurrent range the tripping time of QF1 (Tt1) will always be longer than the break time of QF2 (Tb2), i.e. Tt1 > Tb2. In other words, in this overcurrent range, QF2 will trip first and QF1 will not trip at all, i.e. coordination is assured.
In the overcurrent range from 251 to 499 A selective tripping is possible (no definite answer here!). For overcurrents from 500 A selective tripping is not possible, because in this case both QF1 and QF2 will trip almost simultaneously (in less than 0.1 second).
As a conclusion, using household circuit-breakers can only provide partial selective protection for low overloads and low short-circuit currents.