Verification of the short-circuit making capacity of switching devices
Since 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 result in the destruction of the device.
The value of the current to be taken into account for this verification is the maximum possible instantaneous value of the prospective short-circuit current at the time of closing, in other words the peak value of the short-circuit current ip.
Calculation of ip according to standard IEC 60909-0
The standard IEC 60909-0 provides the formula for calculating the current ip:
ip = k √2 Ik
Ik being the r.m.s. value of the maximum short-circuit current at the short-circuit point and a coefficient depending on the R / X ratio of the short-circuit impedance (seen from the short-circuit point). The maximum value of k is 2.
Calculation of ip according to standard IEC 60439-1
A simplified approach to the calculation of ip in low voltage networks is given in standard IEC 60439 1:
ip = n Ik
n taking the following values:
|Ik ≤ 5||1,5|
|5 < Ik ≤10||1,7|
|10 < Ik ≤ 20||2|
|20 < Ik ≤ 50||2,1|
|50 < Ik||2,2|
Case of circuit-breakers
The manufacturing standards for circuit-breakers, IEC 60947-2 for LV and IEC 62271-100 for HV, impose a minimum ratio between the short-circuit making capacity and the short-circuit breaking capacity. This ratio is compatible with the ip / Ik ratio defined above. Consequently, there is no need for a specific verification: any circuit-breaker having an adequate short-circuit breaking capacity will also have an appropriate short-circuit making capacity.
Case of switches
The manufacturing standards for switches, IEC 60947-3 for LV and IEC 62271-103 for HV, require manufacturers to provide the short-circuit making the capacity of devices (Icm or Ima). Like the short-circuit breaking capacity of the circuit-breakers, this value may be different depending on the operating voltage of the switch. Note that the combination of a switch with an upstream short-circuit protection device (SCPD) can increase the Icm of the switch. It is then necessary to refer to the coordination tables provided by the manufacturers.
It is, therefore, necessary, when choosing a device reference, to verify that the short-circuit making the capacity of the device is greater than the peak value of the short-circuit current ip.
Case of contactors
The manufacturing standards of the contactors do not define an intrinsic value of short-circuit making capacity. On the other hand, they define test procedures to ensure that a contactor / DPCC / thermal relay combination is capable of supporting the closing of the contactor on short-circuit to a given ip level.
It is, therefore, necessary, when choosing a device reference, to verify that the short-circuit making the capacity of the contactor / DPCC / thermal relay combination is greater than the peak value of the short-circuit current ip. There is no choice but to refer to the coordination tables provided by the manufacturers.
elec calc™ software calculates the peak value of the short-circuit currents (ip) at any point in the installation. The calculation method depends on the chosen installation standard and the voltage level. When choosing a switch reference, elec calc™ checks that the intrinsic Icm of the component is greater than ip. Otherwise, an error is displayed on the component.
Article written by Philippe Aupetit- Product Manager of elec calc™ software
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