MV Switchgear rating and IEC909

[radug]

Hello,

I am trying to validate some MV switchgear, and when looking at the specs, it says: "short time 1s-3s, short-circuit rating of 20 kA".
If the calculations are done with a software for IEC909, which short-circuit current is the one I must compare the equipment rating with?

Thanks.

 

[KJvR]

(a) Ik (steady state short circuit current) or 30 cycle Ib (symm break current) for short time current rating of the breaker and switchboard - Thermal
(b) Ib (3 or 5 (for older MV breakers) cycle symm break current) should be use for the breaker's fault current breaking capability
(c) ip (peak short circuit current)for peak withstand current of the breaker and switchboard - Mechanical Forces
(d) X/R should be smaller than 14 else derating of circuit breaker's breaking capability is required.

 

[Alex68]

Dear Radug,
to size a switchgear you need two parameters:

Ith = thermal equivalent short circuit current, described at point 4.8 of the IEC 60909-0 / 2001

and

ip = peak short circuit current, described at point 4.3 of the same standard.

Please don't use the Ik as Ith because they are very different!!!!
Actually Ik is the steady state short circuit current, that is the permanent s.c. current after the subtransient and transient periods.
While Ith is a constant current producing the same thermal effects as the actual s.c. current and is calculated by an integral of the fault current over a time period of 1s or 3s.
So Ith takes into account the sub-transient, transient and steady state currents.

Regards
Alex

 

[radug]

Dear Alex68,

I am using Ith for cable validation, as in cable datasheets I have the short circuit withstand capability in 0.5s or 1s. And the software I use, PSS Sincal, obtains the equivalent short circuit withstand time of cables.

However, with MV switchgear, I draw them as a busbar with each circuit breaker connected to that busbar, but as PSS Sincal treats the busbar as a node, it does not calculate the Ith value, and so I have no value apart from ip or ik''.

Best regards, Radug.