Tripping Time of a Circuit Breaker. Definition & Meaning

Tripping time: the time interval from the instant at which the associated tripping current begins to flow in the main circuit to the instant when this current is interrupted (in all poles) [defined in the IEC 60050-442].

Note 1 to entry: For circuit-breakers “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 when the opening command becomes irreversible.

Note 2 to entry:

  • in the case of a directly operated circuit-breaker, the instant of initiation of the opening time is the instant of initiation of a current large enough to cause the circuit-breaker to operate;
  • in the case of a circuit-breaker operated by any form of auxiliary power, the instant of initiation of the opening time is the instant of application or removal of the auxiliary power to the opening release.

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 device trips.

The moment in time when the trip command becomes irreversible is the moment when the overcurrent release actuates the circuit breaker’s holding device and induces 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.

Tripping Time Setting of Overcurrent Releases

The tripping time shall be stated as follows, depending on the type of over-current release:

1) Definite time-delay overcurrent releases.

The time-delay of such releases is independent of the over-current. The tripping time setting shall be stated as the duration in seconds of the opening time of the circuit-breaker, if the time-delay is not adjustable, or the extreme values of the opening time, if the timedelay is adjustable.

2) Inverse time-delay overcurrent releases.

The time-delay of such releases is dependent on the overcurrent.

The time/current characteristics shall be given in the form of curves supplied by the manufacturer. These shall indicate how the opening time, starting from the cold state, varies with current within the range of operation of the release. The manufacturer shall indicate, by suitable means, the tolerances applicable to these curves.

These curves shall be given for each extreme value of the current setting and, if the time setting for a given current setting is adjustable, it is recommended that they be given in addition for each extreme value of the time setting.

It is recommended that the current be plotted as abscissa and the time as ordinate, using logarithmic scales. Furthermore, in order to facilitate the study of co-ordination of different types of overcurrent protection, it is recommended that the current be plotted as multiples of the setting current and the time in seconds on the standard graph sheets detailed in 5.6.1 of IEC 60269-1:2006.

Standard Time-Current Zone

The tripping characteristic of circuit-breakers shall be such that they ensure adequate protection of the circuit, without premature operation.

The zone of the time-current characteristic (tripping characteristic) of a circuit-breaker is defined by the conditions and the values stated in Table 1.

This table refers to a circuit-breaker mounted in accordance with the reference conditions operating at the reference calibration temperature of 30 °C, with a tolerance of +5 °C.

The test may be made at any convenient temperature, the results being referred to 30 °C, using the information given by the manufacturer.

In any case the variation from the test current of Table 1 shall not exceed 1,2 %/ per K of calibration temperature variation.
If the circuit-breakers are marked for a calibration temperature different from 30 °C, they are tested for that different temperature.
The manufacturer shall be prepared to give information on the variation of the tripping characteristic for calibration temperatures differing from the reference value.

A current equal to 1,13 In (conventional non-tripping current) is passed for the conventional time through all poles, starting from cold (see Table 1).

Moreover the circuit-breakers shall trip within the time corresponding to the test c of Table 1 when a current equal to 2,8 In is passed through all poles, the lower time limit being 0,1 s instead of 1 s.

TestTypeTest currentInitial conditionLimits of tripping or non-tripping timeResult to be obtainedRemarks
aB, C, D1,13 InCold at ≤ 1 h
(for In ≤ 63 A)
t ≤ 2 h
(for In > 63 A)
No tripping
bB, C, D1,45 InImmediately following test at < 1 h (for In ≤ 63 A) t < 2 h (for In > 63 A)TrippingCurrent steadily increased
within 5 s
cB, C, D2,55 InCold a1 s < t < 60 s (for In ≤ 32 A) 1 s < t < 120 s (for In > 32 A)Tripping
dB
C
D
3 In
5 In
10 In
Cold at ≤ 0,1 sNo trippingCurrent established by closing an auxiliary switch
eB
C
D
5 In
10 In
20 In b
Cold at < 0,1 sTrippingCurrent established by closing an auxiliary switch
Table 1 – Time-current operating characteristics

NOTE. An additional test, intermediate between c and d, is under consideration for circuit-breakers of type D.

a The term “cold” means without previous loading.
b 50 In for special cases.