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
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
RE: Short circuit contribution from downsteam equipment
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
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
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
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
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
The breaker next to this one will see 58 kA for its downstream fault case.