MCCB Instantaneous Trip Setting

[minerman]

Does anybody know how to determine an MCCB instantaneous trip setting? I haven't done this in a couple years. I was thinking 10 or 15x the FLA?

Thanks.

 

[kingpb]

In general, it must be set to trip below the minimum fault current that it will see, assuming the maximum capability of the equipment is greater than the minimum fault current.

 

[Jraef]

Read Article 430, specifically table 430.52, and read the exceptions as well, and make sure you understand all of them. If you have more uestions after that, post them here again.

 

[raider1]

I think the poster is asking about a molded case circuit breaker.

If that is the case to determine the instantaneous trip threshold you need to get the circuit breakers trip curve data from the breaker manufacture.

Chris

 

[zog]

Does anybody know how to determine an MCCB instantaneous trip setting? I haven't done this in a couple years. I was thinking 10 or 15x the FLA?

Thanks.

Are you asking how to determine WHERE it should be set or how to determine WHAT it is set at?

 

[ATSman]

Does anybody know how to determine an MCCB instantaneous trip setting? I haven't done this in a couple years. I was thinking 10 or 15x the FLA?

Thanks.

To be compatible with the coordination study, it should be set at just above the inrush current of the largest connected load on startup. If the breaker is in an MCC then I would follow the old Westinghouse recommended procedure: 1. Raise the inst dial to max. 2. Start and stop the motor. 3. Lower the setting 1 notch each time you restart motor until the breaker trips, then raise the dial 1 notch. 4. Restart motor to prove no-trip. This will be the final setting. This setting should match the trip curves in the study and also takes into account cable impedance, as well.

 

[jim dungar]

To be compatible with the coordination study, it should be set at just above the inrush current of the largest connected load on startup. If the breaker is in an MCC then I would follow the old Westinghouse recommended procedure: 1. Raise the inst dial to max. 2. Start and stop the motor. 3. Lower the setting 1 notch each time you restart motor until the breaker trips, then raise the dial 1 notch. 4. Restart motor to prove no-trip. This will be the final setting. This setting should match the trip curves in the study and also takes into account cable impedance, as well.

When doing this type of "click and hope" adjustments, make sure you are not exceeding both the # of starts/hr and the time between starts of the driven equipment.