MDS 520 Circuit Breakers

[Kingstonrupp]

I have a situation where we have two 1.2MW Generators feeding a 5600A bus in parallel. The generator breakers are MDS 520 and have been specified as 2000AT/2000AF. I had a senior engineer go through the single line and he was concerned that having a 2000AT trip unit installed in the 2000AF breaker was a violation of the 80% rule in the code. I have since done some research on the breaker and found that they are actually rated for 100% continuous operation.

My question is, if a breaker manufacturer is fabricating CSA approved breakers that have trip units and frames sizes of the same value (i.e. 2000AT/2000AF) can we not assume that we are not in any violation of the code. My interpretation of the code CEC 8-104 is that your load and conductors should only be 80% rated if your breaker is not marked 100% or is marked 80%. I did not interpret this rule to indicate that you cannot install a 2000AT unit in a 2000AF breaker when they are designed to operate together by the manufacturer. The senior engineer wanted to change the breaker to 2000AT/2500AF if it was only 80% rated. Luckily it was 100% rated so no argument.

But even if the unit was 80% rated, I still would have argued that you can use a 2000AT unit in a 2000AF breaker, just that your load should be sized for no more than 1600Amps. Am I wrong on this?

Thanks in advance.

 

[unclebob]

These trip units are calibrated to permit 100% amps rating. That said, the pick up value of the long time is set at 110%. No trip should occur below 2200 +/- 10%.