Ground fault
Ground fault
(OP)
What would be the difference (setting, working principal, etc) between a ground fault relay for a motor circuit, and the ground fault relay of a non-motor circuit. Thanks.
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RE: Ground fault
RE: Ground fault
Now I need your advice for the following:
For a 415 V, 45 KW motor supplied from solidly earthed 415V system, what should be a typical Earth Fault setting. Our Consultants have recommended 10 Amps with 0.1 sec time delay. The Earth fault protection unit gets the input from the CBCT.
In my view the sensitivity can be in milli Amperes (mA), say 500 mA with a time delay say 1 sec. Does it make sense?
RE: Ground fault
My question is that why do manufacturers say that the "motor ground fault relay" cannot be used for the ground fault protection of non-motor circuits? My understanding is that if the sensitivity of a motor ground fault relay is lowered to mA range (for example 30 mA), then it should be Ok to be used for the ground fault protection of non-motor circuits.
RE: Ground fault
RE: Ground fault
Motor starting inrush current can saturate a GF CT or at least cause errors and false trips. A feeder circuit may not have the large inrush currents seen on motor circuit. Alsao, a feeder circuit probably has downstream overcurrent protection devices that should coordinate with the feeder GF protection. A feeder GF relay typically will be set with a longer time delay than a motor circuit GF relay. The motor circuit GF doesn't have to wait for any downstream device to trip.
RE: Ground fault
RE: Ground fault
Now I recall why the motor feeders should have relatively higher time delay with high sensitivity (like 500mA). If the fault is with higher earth fault current, which is exceeding the contactor SC Current breaking capacity, the E/F element should allow the fuse or MCCB to clear the fault. If the earth leakage current is sustained at a low level, then it is a confirmed earth leakage case. Such a small leakage current cannot be detected by other protective devices in the motor starter. Also it is safer for the contactor to open the for the earth leakage current.
It may be noted that in a solidly earthed 415V system, the earth fault current could be as high as (exceed) the phase fault current.
The question of why do we need such a sensitive setting for the motors? Yes the sensitive setting would easily detect the weak winding and trips the motor before the motor fault makes the faulty motor beyond repair. In other words, it minimizes the extent of damage to the motor.
dpc,
Please tell now whether my explanation makes sense.
RE: Ground fault
RE: Ground fault
The fourth conductor may, in some cases, be the metallic raceway from the source to the motor. More often it is a fourth wire.
This is called the equipment grounding conductor.
In the event of an internal earth fault in the motor, some of the current will return to the source via the equipment grounding conductor.
If the ground fault is an arcing ground fault, it is important to interrupt the current before the stator laminations are damaged.
It won''t take much arc damage to the stator laminations before the stator is scrap metal.
Bill
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"Why not the best?"
Jimmy Carter
RE: Ground fault
If a system is resistance grounded, then for 3-phase 5-wire feeders (L1+L2+L3+N+Gnd) we will need the Second type ground fault relay.
Question: If a system is resistance grounded, then what type ground fault relay (First type or Second type) should be used for 3-phase 4-wire (L1+L2+L3+Gnd) feeders?
RE: Ground fault
Regarding resistance grounding - at 415 V, this must be high-resistance grounding, so there is really no purpose to the ground fault relay of either type. The whole point of high-resistance grounding is that tripping is not required for a single line-to-ground fault. You might want ground fault protection on the mains and feeders to help in the event of a double line to ground fault, but I've never seen motor ground fault relays applied on a high-resistance grounded low voltage system unless they were already there and the system was converted from solid grounding to resistance grounding.
RE: Ground fault
RE: Ground fault
25 A is not high resistance grounding at 415 V. It will almost always be less than 5 A. I think you have it backwards. Ground fault CURRENT detection is generally used for low-resistance grounded systems. That is one of their advantages - selective coordination is possible. With high resistance grounding, we generally do not trip. At least in my corner of the planet.
RE: Ground fault
RE: Ground fault
RE: Ground fault
There may also be code requirements that outline under what conditions you can leave the system energized with a single ground fault on a high resistance grounded system, i.e. must annunciate the presence of a ground fault on the system with a visual/audible alarm, system voltage 5 kV or less, 10A or less resistor rating for Canadian code (10-1102). If this is a mining application, there may also be additional requirements.
A 10 millisecond coordinating time interval between your feeder and mains would not be long enough in any case, for a tripping scheme on a high resistance grounded system.