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Westinghouse IQ-1000 II Ground Fault Protection - CBCT Required ?

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EddyWirbelstrom

Electrical
Feb 17, 2002
214
An existing Westinghouse IQ-1000 II motor protection relay does not have a Core-Balance CT, however its Ground Fault trip element is enabled and set.
The manual states that ground fault detection requires an ''optional ground fault (zero sequence) transformer be installed''.
If only 3 phase CTs are installed, does the relay determine the ground fault current from the residual current of the three phase CTs ? Or will the relay ignore the ground fault setting because there is no core-balance CT connected ?
 
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It's only 'optional' in the sense that you don't have to have earth fault protection. If you want earth fault protection then the 50:5 CT is not optional. The E/F CT is expected to have a 50:5 ratio and this can't be configured internally to a different value.

If your line CTs happen to match this ratio then you could probably use the earth fault input with the residual connection. I suppose you 'could' use it with a different line CT ratio, but the scaling would be off and possibly not sensitive enough if you have large ratio CTs.


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Thanks Scotty, I will connect the IQ1000 II earthfault terminals in the residual circuit of the three 1000/5 phase protection CTs and multiply the IQ1000 II earthfault setting range by 20.
To prevent nuisance tripping on starting I will use a pickup E/F current of 250A and definite time delay of 0.15 sec.
 
Actually there are two kinds of Ground Fault; Residual and Zero Sequence. If all you need is Residual, the relay will take car of that internally by summing the existing current signals, meaning you don't need an external Zero Sequence (core balanced) CT. Unless you have a resistance grounded system, Residual, aka Arcing Ground Fault, most people don't add the ZSCT on a motor.

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jraef,

The relay isn't really designed to accommodate a residual connection. If it were then the CT ratio would be configurable. It is intended only for a ZSCT. I wouldn't have thought it was a big deal to allow the earth fault CT ratio to be adjustable, but who knows what the designer had in mind - probably making the customer buy a matching CT!


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Ahh, I have to admit I didn't look at the specs of that particular relay, I just ASSumed that since it is so easy, any decent designer would have done it. I guess I underestimated the power of a marketing dept. over an engineering dept.

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— Kilgore Trout (via Kurt Vonnegut)

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