Standalone NGR monitor vs 50/51G relay functions

[tumbleweed1]

Looking for some indication as to what is a better system to use- having an NGR with a standalone NGR monitor similar to the Littlefuse SE-330 (

http://www.littelfuse.com/products/...resistance-grounding/se-330/se_330_20_11.aspx

) or utilizing relay functions 50G/51G within any feeder breaker.

I'm talking 4160V level.

Or, am I thinking about this all wrong and you should have the NGR monitor similar to the SE-330 which provides detection and outputs to the relay when there is a ground fault detected, for the relay to trip accordingly? Seems to me that the relay would have the logic curves that the NGR monitor would provide anyway.

Any input?

Thanks again.

 

[iceworm]

I'm pretty familiar with the Littlefuse relay - it is the Startco relay. However, all were on 480V HRG, 5A or 10A. These are designed to continue running with one phase grounded. The Startco relay provides an alarm and a pulsing circuit for troubleshooting.

I've only seen one 4160V with an NGR, 50A. It was set to trip in 1sec. A Startco relay would not add any benefit.

ice

Harmless flakes working together can unleash an avalanche of destruction

 

[tumbleweed1]

That is right, Startco was bought by Littlefuse.

So you say it is alarming only, no trip function? I did look through the manual and figured the outputs could be used as a trip signal to a feeder relay. Either way, I think the feeder relay should be sufficient to cover this NGR.

 

[iceworm]

tumble - I'll try to be clearer:
The Startco has a trip function and an alarm function. The trip function is not used on 480V HRG - just alarm. It also has a pulse function; used on 480V HRG for troubleshooting GFs. The reason for 480V HRG is to keep the power running even if there is a GF. So one needs alarm and troubleshooting capability.

The only 4160V NGR system I have seen did not have any use for a Startco. The CB GF functions were set to trip in 1sec. So no way to use the pulse function and the trip function was redundant.

ice



Harmless flakes working together can unleash an avalanche of destruction

 

[3I0]

Typically another benefit of using a resistor monitoring relay like a Startco SE-330 is that if it's used in conjunction with a sensing resistor (part # ER-5KV) it monitors the continuity of the neutral-ground connection through the resistor. I believe the sensing resistor is also used to derive the neutral-ground voltage, so you also get a 59N element as well for backup protection (CT shorting link left in, etc).

In some places (i.e. mines in Canada - CSA M421) the resistor fault monitoring/tripping function is a code requirement as well. For medium voltage systems it's common for grounding resistors to only be rated at full current for 10 seconds so they can get damaged/burned out from excess heat.

In the case where I have downstream 50/51G elements on the feeder relays I typically set the NGR relay to a longer definite time interval than the feeder relays and hit a lockout relay if it trips, as that should only happen for a ground fault in the buswork/transformer secondary/secondary feeder cable/downstream breaker failure.

 

[tumbleweed1]

Interesting read regarding stator protection schemes:

http://www.ccj-online.com/2q-2013/g...ss-grounding-issues-in-rotor-stator-windings/