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Ground fault
4

Ground fault

Ground fault

(OP)
I have 208/120V panel fed from a delta / wye transformer. Upstream of the transformer is a 480V switchgear with electronic trip breaker. The GF on this breaker is a residual sensor and is set at 140A. The transformer is about 150' from the switchgear, and the panel is 20' from the transformer. Last night an eltcrican was installing a new breaker in the panel and unfortunately he neglected to install the terminal shield. The dead-front cover contacted one of the terminals, and the result was that the terminal grounded itself and blew the metal away on the dead-front. The GF in the upstream breaker did not trip.

Did the breaker not trip becasue:
1) The residual current was too small due to it being on the primary of transformer?
2) The fault blew away the panel cover and cleared itself before the breaker tripped?
3) Other reson?

RE: Ground fault

The 480V primary didn't see a ground fault, it was isolated by the transformer.

Did the 208V main breaker not open?

Best to you,

Goober Dave

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RE: Ground fault

(OP)
No the 208V main in the panel did not open either.

RE: Ground fault

Must be a big panel.

To elaborate, the sum of currents on the 480V delta side of the transformer remained at zero through the fault. All fault current returned to the 208V side of the transformer's ground (it's a separately derived system, so must have its own ground per NEC).

Best to you,

Goober Dave

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RE: Ground fault

(OP)
It is a 600A panel.

So the 480V breaker did not trip, because it did not see any fault current due to the delta/wye transformer. The blast from the fault blew the cover away from the terminals and removed the fault. That's pretty much what I thought.

Thanks for the help.

RE: Ground fault

3
The primary saw the current from the fault as a line to line load, not a ground fault.
If the 120:208 panel had ground fault protection it should have tripped.

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Ground fault

I keep reading above that the primary doesn't see fault current in this situation. It is true a delta/wye will filter the zero sequence, but conservation of energy dictates the primary must provide all the energy. Fault current on the primary side will consist of positive and negative sequence and look like a phase to phase fault. Transformer and cable impedance may reduce the current to look more like load than fault. Star for Waross.

RE: Ground fault

Is there a reason why I never see a negative sequence time overcurrent protection on a delta-grounded wye transformer? I usually see as backup to the differential a phase overcurrent that is set as close as possible to .57 pu.

RE: Ground fault

(OP)
I was able to get a copy of ETAP and model this circuit.
The line to ground current on the 208V side was 5.6kA.
The three phase currents at the 480V switchgear were:
A phase - 1.46kA -69.5 degrees
B phase - 0A 110.5 degree
C phase - 1.46kA 110.5 degrees

All three phase currents sum to 0, so the residual would not trip. And the phase currents were not high enough to trip the 480V breaker. At this current, the TCC showed it would take around 45 seconds to trip.

Thanks for all the help.

RE: Ground fault

Anthony,
Just not looking in the right places I guess. I use it in this application routinely.

RE: Ground fault

How on a delta-wye transformer do you get it to coordinate when for a ground fault negative sequence current flows into the fault and for a phase to phase fault negative sequence current flows out of the fault? I don't quite understand how you coordinate when the direction depends on the kind of fault.

RE: Ground fault

I disagree with your premise, current direction is from the source to the fault in both cases. Time and instantaneous overcurrent elements don't care much about direction anyway. Coordination is easy when you use fault simulation software to assist.

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