Breaker coordination with transformers

[steve66]

An arcflash textbook shows circuit breaker TCC curves. It also shows motor starting on the same chart. If the motor curves are always to the left of the breaker curve, the breaker will hold during normal operation of the motor.

But transformer inrush curves always seem to be to the right of the breaker curve. Doesn't that mean the breaker will trip??

For example, page 122 of Easypower's arc flash guide (www.easypower.com).

Steve

 

[jim dungar]

Re: Breaker coordination with transformers

I have not looked at your reference.
All of the TCCs I have seen show the transformer damage curve, which you want to the right of the OCPD, and the transformer inrush as a single point (usually an X) which you want to the left.

 

[ron]

Re: Breaker coordination with transformers

I agree with Jim.
When I draw a TCC for a xfmr, the curves to the right of the OCPD are damage curves (not operation curves), and hovering around the bottom would be the inrush point to the left of the OCPD curves.

 

[steve66]

Re: Breaker coordination with transformers

I just looked at the drawing again. Now I also see the inrush point and the full load current point. So those are damage curves. That makes sense. We want the breaker to trip before we hit that point.

Thanks:
Steve