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Short circuit contribution from downsteam equipment

Short circuit contribution from downsteam equipment

Short circuit contribution from downsteam equipment

(OP)
Hi,

I was questioning myself by looking at a short circuit report.

Let say we have a main breaker.  The SCC from the utility is 53 kA, and the contribution from the downstream motor is 5 kA.  What's the duty of the breaker?  53 kA or 58 kA (neglect the X/R)

The current that pass throught the breaker when a fault occur at the load side is the current from the utility only, not the motor contribution.  So i was asking if we have to consider it since the breaker doesn't have to interrupt it?

Thanks  

RE: Short circuit contribution from downsteam equipment

Correct, the largest current the main would be required to interrupt is the utility contribution. Any breakers other than the motor feeder will see the contribution of both. On the other hand, if you're worried about that small difference in fault current you've got too muchfault current for the breakers and need to use a higher rating.  

RE: Short circuit contribution from downsteam equipment

It is true that the breaker will only see 53kA maximum in this case.

The rub lies in where the UL listing (or equivalent) comes in for the switchboard as a unit. According to UL the switchboard assembly's SCCR rating is not greater than that of the smallest rated breaker installed in the switchboard. The switchboard still needs to be rated for 58kA and therefore you cannot have a 53 kA rated breaker in it.

Plus there is an issue of how bare bone design you feel comfortable with. While you can argue, not all battles are worth picking.

Rafiq Bulsara
http://www.srengineersct.com

RE: Short circuit contribution from downsteam equipment

(OP)
Thank David,

i understand what you mean.

I was asking this because the report was done with easypower, and the person who did the study put all the breaker in place in the software, and the duty for the main and the feeder was the same (utility + all motor contribution).  I was just asking why easypower doesn't remove the motor contribution by himself.

In large industrial complex, sometime motor contribution can be quite large and make a difference.

 

RE: Short circuit contribution from downsteam equipment

EasyPower has a feature called "SmartDuty" that computes the actual fault current through each device when an Equipment Duty report is run.  

It actually checks the fault current through the breaker in both directions to determine the max fault current that any device will see.  The basic short circuit calculation report does not reflect this, but the Equipment Duty report will.   

David Castor
www.cvoes.com

RE: Short circuit contribution from downsteam equipment

I would assume the motor is not an induction motor. Often there can be a confusion between breaker duty, and asymetrical fault current level.

Also it is better to have your breakers rated higher then your breaker duty, as this has an impact on the maintenance of the breaker contacts. Assuming these are not molded case breakers, which probally are not very accessable for maintenance.

RE: Short circuit contribution from downsteam equipment

(OP)
Thanks to all, that answer my question.

Motor are induction.  IEEE say to take them into consideration for low voltage circuit breaker duty at 1/2 cycle.

RE: Short circuit contribution from downsteam equipment

hat has been said so far is correct. This breaker will see 53 kA for a downstream fault.

The breaker next to this one will see 58 kA for its downstream fault case.

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