Difference Between Earth and Ground Fault on MV system

Wiki has a nice definition here

A/ your test voltage isn't high enough

B/ ground fault and earth fault are the same unless you're on a seismic fault plane.

Have you tried a 24V filament lamp connected between earth and ground?

Can you provide us your megger results .Maybe you have problem with ground fault protection. If I were you I should test it .Do you have recently some activity with connection of PT ori CT"s on your protection scheme ?

Good hunting . . .

Can you describe in more/clearer detail what is happening and what your setup is?

You have loose connection between ground and earth. Your senior collegues can help you on this, if you ask them. I got raise in salary after a such question to my Manager.

Not necessarily. We can't tell from his cryptic message what is tripping what. Many electric motors have rectifiers that can put huge current spikes on the line. Those trip ground fault circuits all the time.

There are a couple of issues that come to mind that will help you.

The definitions are handled well enough already.

Your motor protection relay will detect Ground / Earth fault by measuring the residual current ie., the current that does not return via the supply cables. There are two usual methods for measuring the residual current 1. A single ground fault torroid around all current carrying conductors. 2. A mathematical balance of the 3 protection CTs (I can't recall the correct name right now, someone will jump in with it I'm sure).

Instance 1. If the cables are not well centred in the torroid expect bogus trips due to uneven sums of the fluxes. The fault settings may set wrongly on the relay. A turn to turn short in one winding often shows up first in ground fault protection by upsetting the flux balance.

Instance 2. Ensure by current injection that all CTs are balanced and also that the cables are centred. Ensure the burden resistors are correct and connected.

Now to the Megger. "Megger results are OK", haven't I heard this statement a few times? OK isn't a result, always record the actual values. Now to save yourself a lot of trouble conduct a polarisation test on your motor. To achieve this draw up a table and record the initial reading and the reading every two minutes for 10 minutes. The end result should be a resistance at least twice as high as the initial reading. Polarisation will indicate if moisture is your problem and the overall condition of the insulation. Insulation should polarise in exactly the same manner as the dielectric in a capacitor. If you have a 10KV Megger use it but otherwise the 10 minute test will even it out somewhat.

From left field. Is radio transmission getting into your CT / Torroid wiring?

Feel free to log in, I and many others don't normally respond to "Guest".

Yesterday its tripping time increases i-e it trip within half hour. We megger the motor at 10,000 VDC. At this Voltage it shows abnormality

We decouple the motor and take out the rotor out of the stator. There was scratch on the rotor.Actually sometime rotor touches the stator and as it touches motor trips.

Upon inspection of the bearing, there was abnormality in the Drive side bearing

Thanks for all sharing your ideas regarding this problem

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