Ground Fault Setting

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ikhwanmy

Member
Location
Malaysia
Need advice to setting the Ground Fault for the new motor install: How do I set below setting. If it is base on FLA what is the percentage?
1. GF Inhibit Time
2. GF Trip Delay
3. GF Trip Level
4. GF Warn Level
 

Bugman1400

Senior Member
Location
Charlotte, NC
In general, if the motor is connected 3PH, you can set the ground pickup 10% to 30% of the phase pickup. However, this only applies if you are using the residual of the phase CTs and not one of them 50:5 window CTs. The time delay will depend on the motor curve. You will need to ensure that the delay will coordinate with the upstream device but, generally, 0.2-0.3 seconds is used. As you can see, there are a lot of "generalities" here and more info is needed to give a more informed response.
 

zog

Senior Member
Location
Charlotte, NC
:)
You can guess or the engineer can guess. I would expect no GF current, so I set it pretty low (not zero, due to coupling) and adjust if it trips.

Engineers never guess :)

Seriously though, what the OP needs is to have a coordination study done to determine the settings.
 

ron

Senior Member
Seriously though, what the OP needs is to have a coordination study done to determine the settings.

Even with a coordination study, they could lay the trip characteristics of the new GF trip next to an upstream device to gain selectivity, but it is still a guess on the setting relative to the motor characteristics. This is GF and not phase, so it is more of a guess.
 

ikhwanmy

Member
Location
Malaysia
Even with a coordination study, they could lay the trip characteristics of the new GF trip next to an upstream device to gain selectivity, but it is still a guess on the setting relative to the motor characteristics. This is GF and not phase, so it is more of a guess.


:happyno:. Thanks Guys. I shall set with the default setting and let see if the ground fault triggered.
 

topgone

Senior Member
Even with a coordination study, they could lay the trip characteristics of the new GF trip next to an upstream device to gain selectivity, but it is still a guess on the setting relative to the motor characteristics. This is GF and not phase, so it is more of a guess.

If it is humanly possible, you can do away with the usual guesswork->do a live test and measure the fault current to ground: ground the cables at the farthest end and then supply the line/equipment with a low-power AC source.:)
 

Ingenieur

Senior Member
Location
Earth
vfd or fvnr makes a difference
hp, voltage would help
what does it trip? Shunt, uv, etc
start the motor measure with a clamp on
should see almost nothing under any normal circumstance
time til running and use that as the inhibit
the trip delay, low 6 cycles or less, no need for any really other than as a filter to avoid nuisance
I would set it at 5-10 times what you measure

but if a small lv motor I would not mess with any of that
use a zero seq relay
inhibit 5 sec
trip delay 6 cycles
trip 10% of fla or 10 A max, the lower the better
warning 1/2 of trip
 
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