Short circuit withstand time

[jasl]

This may be very basic for the expert members of this forum.On what basis do you select the short circuit withstand time on a switchgear (say HV switchgear)?
For example 40kA,1sec or 3sec ? Low voltage switchgears are generally found to have 3sec withstand time.

 

[RalphChristie]

I think the different time-ratings depends a lot on the test standard being used. There are a lot of different codes and standards being used in the world today.

The rated withstand time is always being used in conjunction with the rated short circuit withstand current.
Basically it is the max rms current a breaker (switchgear) can withstand without any damage (electrical, thermal, or mechanical damage or permanent deformation) linked to a certain time.

But, if you have to specify a certain withstand-time, you have to look how long a fault is going to stay on the system and how fast the protection is going to clear it.

Hope it answer your question.

Regards
Ralph

 

[RalphChristie]

Just an addition:

Your withstand-time for a breaker must be longer than the time the protection is going to take to operate the breaker. (Under heavy fault condition)

Ralph