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?
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
Did the 208V main breaker not open?
Best to you,
Goober Dave
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RE: Ground fault
RE: Ground fault
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
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
If the 120:208 panel had ground fault protection it should have tripped.
Bill
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"Why not the best?"
Jimmy Carter
RE: Ground fault
RE: Ground fault
RE: Ground fault
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
Just not looking in the right places I guess. I use it in this application routinely.
RE: Ground fault
RE: Ground fault