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ZSCT Pickup

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COelectrician

Electrical
Nov 23, 2010
7
I'm hoping someone can provide some insight into ground fault pickup settings.
Motor info:
1. 2400 volt;
2. Solidly grounded;
3. Fuse/vacuum contactor starter;
4. 1.15 SF
5. ~8k amps ground fault current
6. Zero sequence 50:5 CT for ground fault protection;
7. 300Hp

I've read through the Buff book as well as many many online resources and can't find a resource which provides an explanation on how to choose the ground fault pickup current setting for a solidly grounded mv motor. With a time delay of 0.1 seconds, the fuse will clear a fault.

Any insight would be appreciated.

Thanks.
 
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Hi There CO,
A zero-sequence CT on a solidly grounded system is not a good idea if you have a fused vacuum contactor (as opposed to a circuit-breaker). This has to do with the IC of a vacuum-contactor. IC of a typical vacuum contactor is approx 5ka, with fuses is approx 50ka. With your 8ka of GF current, you could smoke the contactor if it were called upon to interrupt the full 8ka. However, you might get lucky and have the fuse clear the fault prior to the contactor opening.
To summarize, do use the ZSCT on a vacuum-contactor on a solidly grounded system.
Regards,
GG


"I have not failed. I've just found 10,000 ways that won't work." Thomas Alva Edison (1847-1931)

 
Groovy - Yep, understand. The fuse will clear a ground fault well below the rating of the contactor. I'm looking to find a minimum pickup level.
 
Howdy CO,
Sorry, I made a mistake in the last line of my tirade above. What I meant to say was:
To summarize, do NOT use the ZSCT with a vacuum-contactor on a solidly grounded system.
GG


"I have not failed. I've just found 10,000 ways that won't work." Thomas Alva Edison (1847-1931)

 
Appreciate it, Groovy. Unfortunately this is an existing installation. So rather than try to use the ZSCT for high impedance faults (below contactor rating), I should short/ground the zsct and rely on the fuse for any/all ground faults, correct?
 
Hi CO,
Frankly, i would remove the ZSCT from the starter. That will prevent anyone in the future form trying to hook it up.
GG

"I have not failed. I've just found 10,000 ways that won't work." Thomas Alva Edison (1847-1931)

 
It's probably because a ground fault will typically draw the fault current levels.

You could set some arbitrary lower level pickup point with enough time delay so the fuses can clear any fault that exceeds the contactor breaking capacity. You have a 50:5 CT so you can set it to say 25A or 50A. The likelihood of this setup being useful and actually tripping on a "low level" ground fault vs the ground fault blowing fuses is quite low.
 
Thank you. So, i'll file this one under No relay gf protection on a mv, solidly grounded, fused/contactor motor starter.

 
The fuse is sized such to also provide short circuit protection for the contactor. This will also provide you with high current ground fault protection. If you have a weak system or for high impedance fault you can use the ground fault protection for these low magnitude faults. Look at the TCC and coordinate the ground fault pickup somewhere below the fuse pickup with some delay. If it is a high current fault it will not matter because the fuse will below first and this can be your backup protection including single-phasing. If the ground fault current is low enough than this will be your primary protection.

"Throughout space there is energy. Is this energy static or kinetic! If static our hopes are in vain; if kinetic ù and this we know it is, for certain ù then it is a mere question of time when men will succeed in attaching their machinery to the very wheelwork of nature". û Nikola Tesla
 
Thanks VTer - That was my original intent, but the responses I received sent me down the road of removing all gf protection. I like the idea of having fast acting low magnitude fault protection. I guess I would just need to set pickup above the charging current value (1a/MVA) to avoid sympathetic tripping. Always learning...
 
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