Possible CB coordination problem?

[MIEngineer]

Hello All,

I recently ran into a situation with a rooftop unit opening up its feeder circuit breakers. The RTU is fed by a 50A fused disconnect with current limiting fuses, upstream is a 70A "vintage" SQ-D circuit breaker. And all of this is fed by a 175A "newer" style SQ-D CB.

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175A )
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70A )
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50A \ fused disconnect
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RTU

Come to find out the main compressor of the RTU had a locked-rotor which was tripping its internal protection most of the time, the current was up to over 100A before it cut-out. We had mulitple situations before we discovered the problem where the main 175A CB was opening up, ALONG WITH the 70A CB and (1) 50A fuse of the (3) in the disconnect. Is this simply a coordination issue? I guess I would've expected any one of those to open up but not all three.

Thanks for any opinions or hypotheses on this matter.

 

[ron]

Depending on the amount of current that is being drawn, locked rotor current could be high and activate all three on the instantaneous or short time region.
All three may very well overlap.

 

[bphgravity]

Square D can provide you with "time-current curves" that will indicate where the overcurrent device operation will likely occure.

 

[brian john]

Bussman advertises that their dual element (David will chime in with the Class ECT.) fuses when sized at 125% or next standard size will offer proper starting and allow for better coordination, particularly in a situation like this where the sizing is SOOO close.

That's assuming the upstream CB are operating with in the manufactures published data and the CB do not have or are set at a low instantaneous trip setting.