Earth leakge relay 1

[markjp]

I was wondering if someone may have an opinion on the following issue. I have started working on a project for an underground mine and have noticed that the earth leakage relays on the 1000V motor circuits are wired to trip the contactor rather than the circuit breaker. I've have always seen earth leakage relays as a fault protection device and as such should be used to trip the circuit breaker via shunt trip, as a contactor isn't designed to interrupt large fault currents. Details of the installation are as follows: 1000V,3-phase fault level up to 25kA, NER limiting earth fault to 5A, earth leakage relays set to 500mA & 100ms, typical motor circuit with circuit breaker, electronic overload relay and contactor, 110kW and 7.5kW pumps. Theoretically the contactor should not have to operate under a fault condition where there is more than 5A earth fault current due to the NER. Also the 100ms time delay on the earth leakage relay should see the circuit breaker operate faster (say 50ms)to clear the fault under a major fault condition. So I suppose the installation looks okay but am looking for other opinions on the issue as I just haven't seen it done this way before. Thanks.

 

[RRaghunath]

markjp,
You said it. The installation seems OK.
Earth leakage relay will not respond to phase faults and also will not be affected by CT saturation during large magnitude phase fault currents. This is because it would have been connected to zero sequence CT, rather than in the residual circuit of phase CTs.
However, for MV system with no. of feeders connected to the busbar, 5A is too low a current to give comfort against over voltages during earth faults.
If you have been facing frequent cable / motor failures and recurrent phase-phase faults, you have some thing serious to worry about and may need to review the system earthing / fault detection principle in detail.

 

[rohra]

Many a times Motor feeders in LV & MV are contactor type having Earth leakage relay as protection relay.In motors which have both Phase fault & earth fault Relays (overcurrent & earthfault) earth fault is residually connected
thanks

 

[markjp]

Thanks for your help guys.

 

[ScottyUK]

A relay on a residual connection is ok if the ratio of E/F current to CT nominal rating is not too large. If the E/F relay is made too sensitive then CT matching errors cause problems with false trips occuring, especially during motor starting. Better to use a core balance CT if the ratio is high.


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If we learn from our mistakes I'm getting a great education!

 

[burnt2x]

markjp,

I usually set E/F at 20 to 40% of expected residual current and set trip delay to a definite time, which will not exceed 1 second as a rule of thumb. With your expected earth fault of 5A (NER limiting the fault current), a setting of 0.5A is 10% and may cause nuisance trips. 100ms trip time seems fast enough.

As posted above, check if you havn't got those trips for you to raise the setting, or lower it, if you have had insulation failures.

My two cents.