MCC Rating

[chadwiseman]

Our MCC's have a symmetrical fault current rating. I was wondering if that was 1/2 cycle or 3-5 cycle fault current rating? Also, for what applications would one use the 1/2 cycle vs the 3.5 cycle fault levels (eg. which one for relay coordination)?

Thanks.

 

[RalphChristie]

Symmetrical and assymetrical current ratings will be that of the first 1/2 cycle, because this is when the fault-current is at its maximum. This peak current is used by equipment manufacturers as the design criteria, maximum mechanical forces on breakers, buses, etc. will occur during this time.

For relay coordination it depends. Some equipment such as fuses and molded case breakers can interrupt fault currents during the first cycle, while other equipment such as high-voltage breakers 1.5 to 8 cycles can take to open. This is typical for instantaneous settings. For time delayed settings the fault current will decay and you'll probably need the current after 0.3-0.5 seconds or after whatever time-delay you want.

See also:

http://www.ab.com/mcc/6_Specs_Design/3_Ratings_Tables/2_Fault_Currents_in_MCCs/06_03_02_01.html

 

[chadwiseman]

I understand that the fault current is greatest during the first 1/2 cycle. Is this the value of fault current I would use in selecting a properly rated MCC?

Also, if coordinating protection in a MCC and say a high-voltage breaker on the other side of the transformer, which fault level would I use in the TCC (1/2 cycle or contact parting .. 3-5 cycles).

Thanks.

 

[RalphChristie]

I understand that the fault current is greatest during the first 1/2 cycle. Is this the value of fault current I would use in selecting a properly rated MCC?
Yes, your MCC must be able to handle this max. fault-current. (if there is a fault)

Also, if coordinating protection in a MCC and say a high-voltage breaker on the other side of the transformer, which fault level would I use in the TCC (1/2 cycle or contact parting .. 3-5 cycles).
What protective elements do you use? (O/C Relays, instantaneous relays, fuses, etc.?) Dou you have to coordinate with downstream devices?