If a switching device can be closed on a pre-existing short-circuit in the electrical installation, it is necessary to check that this operation will not destroy the device.
Calculating and taking into account values
The current value to be taken into account for this check is the maximum possible instantaneous value of the presumed short-circuit current at the moment of closure, i.e. the peak value of the short-circuit current Ip.
Calculation of Ip according to NF EN 60909-0: NF EN 60909-0 provides the formula for calculating the current Ip: Ip = K √2 Ik
Ik is the maximum RMS short-circuit current at the short-circuit point, and K is a coefficient dependent on the ratio R/X of the short-circuit impedance (seen from the short-circuit point). The maximum value of K is 2.
Calculation of Ip according to NF EN 60439-1: A simplified approach to calculating Ip for low voltage is given in NF EN 60439-1: Ip = n.Ik
n taking the following values :
| Ik in kA | n |
|---|---|
| Ik ≤ 5 | 1,5 |
| 5 < Ik ≤10 | 1,7 |
| 10 < Ik ≤ 20 | 2 |
| 20 < Ik ≤ 50 | 2,1 |
| 50 < Ik | 2,2 |
Switching devices
Circuit breakers: The manufacturing standards for circuit breakers, NF EN 60947-2 for LV and EN NF 62271-100 for HV, impose a minimum ratio between short-circuit closing capacity and short-circuit breaking capacity. This ratio is compatible with the Ip/Ik ratio defined above. Consequently, there is no need for a specific check: any circuit-breaker with an adequate short-circuit breaking capacity will also have an adequate short-circuit closing capacity.
Switches: The manufacturing standards for switches, NF EN 60947-3 for LV and EN NF 62271-103 for HV, require manufacturers to supply the short-circuit closing capacity of the devices (Icm orIma). Like the short-circuit rating of circuit-breakers, this value may vary according to the switch’s operating voltage. Note that combining a switch with an upstream short-circuit protection device (DPCC) can increase the switch’s Icm. In this case, refer to the coordination tables supplied by the manufacturers. When selecting an equipment reference, it is therefore necessary to check that the short-circuit closing capacity of the device is higher than the peak value of the short-circuit current Ip.
Contactors: Contactor manufacturing standards do not define an intrinsic value for short-circuit closing capacity. However, they do define test procedures to ensure that a contactor/DPCC/thermal relay combination is capable of withstanding short-circuit closure up to a given Ip level.
elec calc™ software: the choice of manufacturer references
When selecting an equipment reference, it is therefore necessary to check that the short-circuit closing capacity of the contactor/DPCC/thermal relay combination is well above the peak value of the short-circuit current Ip. There is no choice but to refer to the coordination tables supplied by the manufacturers.
The elec calc™ software calculates the peak value of short-circuit currents at any point in the installation. The calculation method depends on the installation standard chosen and the voltage level. When selecting a switch manufacturer reference, elec calc™ checks that the component’s intrinsic Icm is greater than the Ip. If this is not the case, an error is displayed on the component.