Real currents circulating inside circuit-breakers

Evaluation of the real currents circulating inside circuit-breakers.

When comparing the time-current curves of two circuit-breakers, you are likely to consider the trip times of the two devices as if they were passed through by the same current.

This consideration is true only when, between the two circuit-breakers in series, there are no other shunts, i.e. there is a single incoming and a single outgoing feeder insisting on the same node. When, on the other hand, there are several supply-side circuit-breakers that insist on the same busbar or several outgoing feeders on the load side, the currents flowing through the devices can be significantly different.

Three cases are considered:

A single circuit-breaker on the supply side of a single downstream circuit-breaker
(passed through by the same current).
A single circuit-breaker on the supply side of several downstream circuit-breakers
(supply-side circuit- breaker passed through by a current value higher than that of the load-side circuit-breakers).
Several circuit-breakers on the supply side and several circuit-breakers downstream

Examples

Here are some examples to show how an incorrect calculation of the real currents circulating in the circuit-breakers can lead to lack of selectivity in the overload zone or to oversizing the circuit-breakers to obtain selectivity in the short-circuit zone.

Example nr. 1

Example nr. 2

Example nr. 3

A single circuit-breaker on the supply side of a single downstream circuit-breaker

The two circuit-breakers are passed through by the same current both under normal conditions and in the case of overcurrent. Therefore, to verify time-current selectivity in the overload and short-circuit zone, it is sufficient to check that the trip curves of the two circuit-breaker have no intersections.

A single circuit-breaker on the supply side of several downstream circuit-breaker

This installation is the most frequent in practice.
Having more than one circuit-breaker on the load side, there will be different current values between the supply-side circuit-breaker and the load-side circuit-breaker towards which selectivity is required.
Therefore, the trip time (due to an overcurrent) of the downstream circuit-breaker must be compared with the trip time of the supply-side circuit-breaker, in correspondence with the sum of all the currents which pass through it.

Example
In the installation in the figure, under normal conditions, the supply-side circuit-breaker is passed through by a current of 360A whereas any outgoing feeder is passed through by 90 A.
Possible settings of the circuit-breakers based on the currents which pass through the devices are:

CB A	I₁ = 0.92 x 400 = 368 A	t₁= 3 s
CB B1	I₁ = 0.90 x 100 = 90 A

The curves of the circuit-breakers with the settings indicated above are shown in the figure. From an initial analysis, time-current selectivity seems to be ensured between the two circuit-breakers.

Let us now suppose that there are overload conditions and load L1 is absorbing a current of 200A.
Circuit-breaker B1 will therefore be passed through by 200A, whereas circuit-breaker A will be passed through by 470A (200+ 90+ 90+90).
With the settings hypothesized above, there are the conditions shown in the figure, where both the circuit-breakers trip in a time of about 50s.
Therefore, with the hypothesized settings, in case of overload, there will not be selectivity between the couple of circuit-breakers considered.

By modifying the settings of the supply-side circuit-breaker, for example by raising the trip time of protection L against overload, it results:

CB A	I₁ = 0.92 x 400 = 368 A	t₁ = 12 s

Selectivity can be obtained in the overload zone since the load-side circuit-breaker B trips in about 50s, whereas the supply-side circuit-breaker A trips in about 200s.

In most cases, even not carrying out this analysis, the size and distribution of the overload between the circuit-breakers ensures a difference in the trip times such as to realize time-current selectivity.

Several circuit-breakers on the supply side and several circuit-breakers downstream

To carry out a simplified analysis, the circuit is assumed to be perfectly symmetrical. Therefore, the total current recalled by the loads is assumed to be divided equally in the three supply-side circuit-breakers

Example
Under normal conditions, in the installation in the figure, each supply-side circuit-breaker is passed through by a current of 1000 A, whereas the two outgoing feeders are passed through by 1000 A and by 2000 A respectively.
In the following analysis, selectivity between a supply-side circuit-breaker A and the largest outgoing feeder B1 is verified.
Possible settings of the circuit-breakers based on the currents which pass through the devices are:

CB A	I₁ = 0.925 x 1250 = 1156 A I₂ = 8 x 1250 = 10000 A I₃= OFF	t₁= 12 s t₂= 0.4 s
CB B1	I₁ = 0.80 x 2500 = 2000 A I₂ = 3 x 2500 = 7500 A I₃= OFF	t₁= 3 s t₂= 0.2 s

The curves of the two circuit-breakers being examined with the settings above are shown in the figure.
At first glance, there would not seem to be time-current selectivity between the two devices.
Being circuit-breakers equipped with electronic trip units, the trip times of the two devices at the significant currents are verified.

1.05 x I₁ of the supply-side circuit-breaker

I_A = 1156 x 1.05 = 1214 A	t_A = 700 s

which corresponds to a current on A of:

I_B = ( 1214 x 3 ) - 1000 = 2642 A	t_B = 450 s

1.2 x I₃ of the load-side circuit-breaker

I_B= 7500x1.1 = 8250 A	t_A= 45 s

which corresponds to a current on B of:

I_A= (8250+1000)/3= 3083 A	t_B = 174 s

As can be seen, even if the curves overlap, there is time-current selectivity in the overload zone.
Also the selection of the Icw must take into account the real currents circulating in the circuit-breaker. The circuit-breakers A are passed through by a maximum of:

36 kA due to a fault between the circuit-breaker and the transformer
18 kA due to a fault on the busbar.

Therefore, these circuit-breakers shall be selected with:

Icu > 36 kA as the breaking capacity must be higher than the maximum short-circuit current value
Icw > 18 kA as time selectivity is only required towards the load-side circuit-breakers.

To realize selectivity towards other downstream devices, circuit-breaker B1 must have: Icw > 55kA.

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