SWGR tripped when MCC MCB did not?

[Isaiah]

We have a SWGR feeder breaker to a 480V MCC with a MCB and 1600A horizontal bus
When all five of the induction motors fed from the MCC were energized —the SWGR feeder CB tripped
But the MCB on the MCC did not trip…does this indicate poor coordination?


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[ron]

If the upstream 1600A SWGR circuit breaker that feeds the MCC, is in series with a 1600A main in the MCC, then it is not possible to get coordination.
If one trips and not the other, the settings must be checked to see if they differ.

 

[Isaiah]

If the upstream 1600A SWGR circuit breaker that feeds the MCC, is in series with a 1600A main in the MCC, then it is not possible to get coordination.
If one trips and not the other, the settings must be checked to see if they differ.

Can the instantaneous trip on the SWGR (snd MCC) be set much higher to allow for motor inrush and leave the long time trip at 1600A, ie same as the horizontal bus?


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[ron]

Can the instantaneous trip on the SWGR (snd MCC) be set much higher to allow for motor inrush and leave the long time trip at 1600A, ie same as the horizontal bus?

Depends on the trip unit capabilities on the breakers. Some trip units are not adjustable. When they are adjustable, instantaneous can be adjusted separately from the long time pickup.

 

[D. Castor]

Increasing the instantaneous trip to maximum might work but it depends entirely on the available fault current. If there's enough fault current the instantaneous element may still trip.

 

[HarrySlother]

It's not really "mis"coordinated in my mind - whether the MCC main trips or the feeder breaker trips, the entire MCC is down. If you prefer the MCC main trip first, then you will need to do a coordination study.

*Do not increase the instantaneous settings (particularly on the feeder breaker) without an arc flash analysis done.

 

[TwoBlocked]

We have a SWGR feeder breaker to a 480V MCC with a MCB and 1600A horizontal bus
When all five of the induction motors fed from the MCC were energized —the SWGR feeder CB tripped
But the MCB on the MCC did not trip…does this indicate poor coordination?


Enviado desde mi iPhone utilizando Tapatalk

To me, the poor coordination is in the starting or running of the five induction motors.

 

[Jraef]

Depends on the trip unit capabilities on the breakers. Some trip units are not adjustable. When they are adjustable, instantaneous can be adjusted separately from the long time pickup.

True, but I cannot think of a 1600A breaker on the market that does not have adjustable mag trips at the very least and tripping on the startup of 5 large induction motors is going to be a magnetic trip event.

 

[Jraef]

Can the instantaneous trip on the SWGR (snd MCC) be set much higher to allow for motor inrush and leave the long time trip at 1600A, ie same as the horizontal bus?

That’s almost literally the * definition of a coordination study…

*Edit: “partial definition”…