Motor Contactor Fault Interrupting

[wiigelec]

This is becoming a hot topic for me so I would like to solicit the membership for opinion, experience, and information on the following:

Using air switched fused motor contactors (5kV) for interruption of phase-ground and phase-phase faults in a large (200+ hp) motor circuit using an instantaneous (50) relay device.

Let me offer three methods and get some feedback:

Method 1) Always employ the 50 relay device to use the motor contactor for interruption of P-G and P-P faults.

Method 2) Employ the 50 relay device on a case-to-case basis only after performing a short circuit analysis and determining the motor contactor is rated for the available fault current.

Method 3) Never use motor contactor to interrupt P-G and P-P faults, always coordinate with the fuses.

So which of the above methods, or any additional method, do you use when it comes to instantaneous P-G P-P fault interruption in a fused air contactor motor circuit?

Thanks

 

[jim dungar]

I use method 2.

Apply the 50 function for the contactor within its non-fused fault handling rating. Coordinate the 50 function with the fuses for large available fault currents

 

[wiigelec]

Thanks Jim. In the abscence of a short circuit study would you err on the side of method 3?

 

[wiigelec]

Additional information:

We have a 5kV motor bus which a grounded motor trips the bus feeder breaker. This has happened several times with different motors. The question is where to coordinate for selection of the motor protective device instead of the bus protective device. As stated previously these are fused air contactors with numerical multifunction motor protective relays.

Currently a short circuit study is unavailable but could be done somewhat fairly easily. I believe available fault current to be high as motor bus is fed straight from secondary of transformer. Eventually the air contactors will be retrofitted with vacuum contactors...

 

[templdl]

Contactors are not intended to interupt. The must be able to stay closed long enough during a fault to allow the OCPD to clear the fault.
Jim Dungar has a very good explaination in one of his replies:
http://forums.mikeholt.com/showthread.php?t=145914&p=1406212&highlight=#post1406212

 

[jim dungar]

Additional information:

We have a 5kV motor bus which a grounded motor trips the bus feeder breaker. This has happened several times with different motors. The question is where to coordinate for selection of the motor protective device instead of the bus protective device. As stated previously these are fused air contactors with numerical multifunction motor protective relays.

Currently a short circuit study is unavailable but could be done somewhat fairly easily. I believe available fault current to be high as motor bus is fed straight from secondary of transformer. Eventually the air contactors will be retrofitted with vacuum contactors...

Not having a calculated SCA is absolutely not an excuse for poor coordination.

Simply assume that there is an infinite amount of current available and then perform your coordination study and you will end up with conservative protection.

Your coordination should be:
Motor running thermal protection (i.e. 49RMS or standard overload protection)
Backup motor running protection (51)
Motor stall protection (50)
Motor Ground fault (50N)
Motor short circuit (fuses)

Don't forget
Additional protection functions ike RTDs, # of starts, low voltage.

The cost of a motor protection coordination study is absolutely minor compared to the cost of an improper tripping event.
This is not DIY, but it also isn't rocket science.

 

[wiigelec]

Thanks all after staring at the Ampgard coordination chart and with your help I finally understand what needs to be done...

http://www.eaton.com/ecm/idcplg?Idc...veAs=0&Rendition=Primary&dDocName=TD02003001E