11kV Vacuum Contactor Short circuit withstand capability

[kssschsekhar]

Dear Folks,

In market, 11kV vacuum contactors are available with 8 to 10kA short circuit withstand capability for one second.
If 11kV Switchgear is required to design for 25kA, 3sec, how can we use the contactors (8kA, 1sec) with fuses for the 25kA, 3sec switchgear?

 

[defreitas]

That's only going to depend on your system.

As far as I know accourding to British Standard, they like 3 seconds of withstand, but when you go across the sea (Europe continent) a lot of the gear is only designed for 1s.

Here in Australia, it only depends on your design.

Also as far as fault current goes if you have a system that has been designed in a way that the maximum fault current is 5kA, it's OK for the fuses to be 25kA/3s, therefore you can you use the contactor mentioned.

An other thing as well, depending on where your contactor is on the system, because of impedances and everything else, the fault current won't be 25kA anymore. So it'll also be OK to be used.

I hope I have helped.

Dan de Freitas
Field Services Technician
AUSTRALIA

 

[tulum]

Have the contactor manufacturer size the fuses for the rating you are looking for.... most manufacturers have tested combinations they can supply. I have dealt with toshiba on this same issue many times.

Regards,
TULUM

 

[kssschsekhar]

Thanks for replies.
Tulum: " manufactueres give the combination" - Is it mean fuse will clear the fault before thermal withstand capability of the contactor reaches?
The same question, I am giving below with ratings.
The switchgear is designed for 25kA, 3sec.
short time withstand current of
motor feeder's fuse -25kA,1sec
Motor feeder's contactor-8kA, 1sec
Calculated fault current - 20kA on bus (or motor feeder contactor output terminal points)
If fuse is failed/delayed to clear the fault at contactor output terminals in worst case , back-up system (switchgear incoming breaker) suppose to clear. How motor feeder contactor can withstand thermal withstand currents of 20kA during this time?

 

[rcw retired EE]

The fuse is to protect the contactor from faults in excess of its rating. If the fuse fails to clear the fault, the contactor is destroyed. If you need to have double contingency as you suggest (first contingency = fault, 2nd contingency = fuse failure) use a fully rated circuit breaker.

The fuse is selected to clear any fault above the contactor's rating and do it before the contactor can open. It will protect the contactor.

You could set the contactor's overcurrent relay to trip the upstream breaker for any fault above the contactor rating with a short time delay to let the fuse clear.

But if you need the full 3 second fault withstand, I think you are stuck with a breaker.