Motor Ground Setting
Motor Ground Setting
(OP)
Is there any recommendation for setting the ground pick up level for an induction motor. Our current setting is just based on co-ordination with upstream protection. However, outside of that, is there any standard setting to protect different motors from ground faults?






RE: Motor Ground Setting
RE: Motor Ground Setting
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RE: Motor Ground Setting
RE: Motor Ground Setting
For earth fault protection in the residual connection:
Relay must be set above any false residual current that can result from the unequal motor starting currents. (and CT saturation during this time) This is difficult to predetermine - a rule of thump is 10% of motor FLA. A time delay can also be used until the starting offset has decayed, but this delays tripping for actual faults.
For earth fault protection with core-balance protection scheme:
This setting can be very low (high sensitivity) and settings of 1%-2% FLA (and sometimes even lower) can be obtained. You can use a definite time delay, normally 1-5 seconds are sufficient, but it can be increased if nuicance trips are experienced.
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Download Chapter 19 (AC motor protection)
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RE: Motor Ground Setting
RE: Motor Ground Setting
RE: Motor Ground Setting
Probably you are correct, but just in my opinion:
The reasons I'm stating an 1-5 second time delay on a core balance protection scheme are:
The earth-fault current is reduced so I assume you can have it longer on your system.
Normally we use a core balance scheme together with a residual scheme. The core balance scheme looks for the smaller earth faults that can be longer on the system while the residual scheme looks for bigger faults and normally act faster on big faults. If you have just the core balance scheme as earth protection I would also start with a much shorter time.
Depending on the start-up time of the motor we experienced nuicance trips on too short time delays with a core balance scheme.
86ranger:
On which protection scheme do you experienced the trips?
What is your CT-ratio?
What are your settings? (Pick-up, time delay and curve selection)
What are your motor start-up time and your motor starting method?
Many times the setting of a core balance sceme is a trial-and-error method.
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RE: Motor Ground Setting
The GF setting is 20A, no time delay (50G).
Thanks.
RE: Motor Ground Setting
The cause of your tripping is the instantaneous time setting. This is too short. Reduce your pick-up setting and increase your time setting.
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RE: Motor Ground Setting
RE: Motor Ground Setting
Cable shield ground wires not running back through the zero sequence CT caused false trips on other units. Cable terminations were on the line side of the CT so the CT saw the cable shield currents. Running the ground wires back through cancelled out shield currents.
If the cables are not centered in the CT, there can be nuisance trips on motor inrush due to asymmetrical saturation of the CT.
We have gotten by with 5 A pickups and 50 mSec time delays on similar installations to yours.