Short Circuit

[timm333]

If a transformer feeds the MCC, then should the MCC be sized based on the "full" short circuit current from the transformer?

What if the transformer secondary has a current limiting fuse? Should the MCC still be sized based on the "full" short circuit current from the transformer, or is it possible to take the current-limiting effect of the upstream fuse and size the MCC for less?

 

[davidbeach]

Yes.
Yes.
No.

 

[PHovnanian]

timm333, take a look at the NEC*, paragraph 240.86 "Series Ratings" for situations such as the one you describe.

*If applicable in your jurisdiction.

 

[timm333]

Series Rating works for lighting and power panels, etc. But I am wondering can it be used for capacitors as well?

 

[davidbeach]

If you can find a listed series combination of a current limiting device and a capacitor, feel free to use it. The odds of it existing a somewhere between slim and none.

 

[rbulsara]

Series ratings are for overcurrent protective devices in series. Capacitor is not an overcurrent protective device.

Rafiq Bulsara

http://www.srengineersct.com

 

[timm333]

rbulsara, your responses are often annoying. It is not the issue that series rating can be used for capacitors or not. The issue is that current limiting characteristic of the upstream breaker can limit the current low enough so that a capacitor of smaller KA rating can be used at location of higher kA.

 

[rbulsara]

timm333:
That was clearly answered by davidbeach in his very first response. I was only responding to what was asked in your follow up question.


Rafiq Bulsara

http://www.srengineersct.com

 

[davidbeach]

Outside a listed series combination the only benefits of a current limiting device is to reduce the damage done. No ratings benefits to anything.

As part of a list series rating combination (listed as a result of successful testing, no calculated listings) a current limiting device may be used to increase the rating of another device in the same listing. It does not reduce the fault current to the lower rating, it increases the lower rating the the value in the listing.

 

[LionelHutz]

You've got the same answer from multiple people given in 2 different threads. Are you satisfied it's correct now? If not, what detail do you still need help with?

The thing in question appears to be some kind of control panel with switching devices and possibly a breaker or fuses. At any rate, it's likely not just a capacitor. You're not telling so we really don't know for sure. This "capacitor" could have devices where the series rating could be used. Also, the switching devices (contactors) might have a higher fault rating if you switch the panel to fuses instead of a main breaker. You'll have to investigate this stuff and find out.

Also, your capacitor kA rating terminology is still off. I've never seen a capacitor (just the capacitor can) with a kA rating. raghun described capacitor rupture in your other thread. If the panel containing the capacitors has a kA rating then it is due to the other devices in it.

 

[jghrist]

A capacitor does not have a kA rating, but it does have a rupture TCC characteristic. To prevent capacitor tank rupture, you coordinate the protective device TCC with the tank rupture TCC. See Capacitor Tank Rupture Curve Coordination at

http://www.cooperpower.com/library/literature/section.asp?productlineid=21.

A current limiting fuse TCC may coordinate better than a non-current limiting fuse, so it may protect the capacitor. It has nothing to do with series ratings which have to do with the interaction between current interrupting in two series OCPDs.