bruinfan wrote:
I have attached a pdf of a coordination problem that I have. I have one major boundary condition which is inrush current. My load is 150 HP motor. I know that I can't set my circuit breaker settings low because it would trip upon motor start-up. As one can see in the attachment that I have coordination problems at the instantaneous and overload region of the TCC.
The instantaneous portion is the most important to me. It appears that if a large fault occured that it has potential that all three OCPD's would trip open. Is there a solution to this? Can this be coordinated in any way?
Thanks in advance for any help!
Your TCC looks pretty straight forward. One motor circuit protector, one upstream molded case breaker and one current limiting fuse further upstream.
Yes it does appear that if the fault current is high enough, you could trip all three devices. That is quite common with molded case breakers in the instantaneous region and there is not a whole lot you can do about it. However, how long is the motor feeder?
Typically the curves are cut off at the maximum short circuit current as you have shown. I am assuming this is the current at the MCC or starter. If there is long run of conductor to the motor, then the fault current may not be as high for a fault at the motor – the condition most likely to take out all three devices. However a large fault at the MCC may still be a problem.
Setting the motor breaker lower won’t really help since the mis-coordination is a function of the upstream thermal magnetic breaker’s instantaneous setting.
An electronic trip breaker with Long, Short, time features would help but is economically impractical. Switching to all fuses might help but then, it also might be impractical – and you have single phase problems.
Looks like it is as good as it gets with what you have.