Short circuit withstand (Icw) and Peak Withstand rating (Ipk)

SHORT CIRCUIT WITHSTAND is the ability of your switchboard to handle a short circuit fault for a specified duration. In the event of a fault, the busbar and cable supports must hold the conductors in position, and not sustain any damage, until the fault is cleared. The rating is determined without an upstream short circuit protective device being in circuit.

When it comes to the short circuit rating of your switchboard, the issue is not so much the duration (i.e. 1 second or 3 seconds) but the rated “Peak Withstand Current (Ipk)”. The damage occurs in the first cycle of the fault where the highest asymmetrical component exists.

In the following video you will see the amount of force present during a Short Circuit Withstand test. In the preliminary test, the cable broke free of the cable ties upon initiation of the fault when most of the force is present. This constituted a failed test.

In conclusion: It is important to ensure that the prospective r.m.s. value of the short circuit current is less than the rated short time withstand of your switchboard, but it’s also important to check that the switchboard rated peak withstand current is greater than the peak value of the short circuit current at that location.

JPR has successfully tested cable bus systems to withstand a 20kA for 1 second fault and bus bar systems to 63kA for 1 second.

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Short Circuit Withstand Strength

TABLE D.1 CHARACTERISTIC 11
Characteristic	Definition	Reference	Helpful points for designers
Short Circuit Withstand Strength	Ability of a switchboard to handle a fault current for a specific duration, in r.m.s.	Refer AS/NZS 61439.1 Section 9.3.2 and section 10.11	The switchboard rated short time withstand must be greater than the prospective r.m.s value of the fault current. ie Icw > Icp The test is undertaken by bypassing any incoming Short circuit protective device (SCPD). Tests the strength of the bus system. Refer 10.11.5.5 for pass/ fail criteria.
Peak withstand current	The associated peak current that the switchboard sees during the fault (based on power system typical impedance, X/R, ratios)	Refer AS/NZS 61439.1 Section 9.3.3, and Table 7	This is the most destructive component of the short circuit current, and is what causes the most damage, generally in the first cycle.