I am curious how a high resistance grounding system would prevent a motor's CB from tripping when a ground fault occurs. Recently we observed this type of incidence and also observed very unbalanced line voltages, (Phase A = 878vac, Phase B = 630vac - the grounded phase and Phase C = 477vac). The short occured at the motor junction box, Phase B had shorted to ground and the CB never tripped, I was told that because of the high resistance grounding system the CB would never trip, only when a line-to-line short occurs - even though line to ground faults account for 80% of all faults. We utilize grounding conductors of the same size as the main conductors for each motor. Also this question applies to the Canadian Electrical Code, but a general explanation of how this type of system prevents a CB from tripping would be helpful.
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High resistance grounding system
- Thread starter mjhyatt
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The high resistance in the neutral ground limits the ?-grd fault to a low value, less than normal load current. Normally, this would be below the pickup of the fault protection, but would be detected and alarmed so that the fault can be fixed before another ?-grd fault makes it a ?-? fault. See http://www.i-gard.com/technologyinfo.htm for more information.
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mjhyatt said:I am curious how a high resistance grounding system would prevent a motor's CB from tripping when a ground fault occurs. Recently we observed this type of incidence and also observed very unbalanced line voltages, (Phase A = 878vac, Phase B = 630vac - the grounded phase and Phase C = 477vac). The short occured at the motor junction box, Phase B had shorted to ground and the CB never tripped, I was told that because of the high resistance grounding system the CB would never trip, only when a line-to-line short occurs - even though line to ground faults account for 80% of all faults. We utilize grounding conductors of the same size as the main conductors for each motor. Also this question applies to the Canadian Electrical Code, but a general explanation of how this type of system prevents a CB from tripping would be helpful.
On a 480V high resistance grounded system, the ground resistor is normally chosen to limit the "first" ground current to maximum of 5A (typical HRG's are in the range of 2-10A). This low current is well below the tirpping point of even a 15A breaker.
Your voltage reading are meaningless as stated, without the other reference point. I assume your nominal sytem voltage is 600V.
What were the voltages:
A-B
B-C
C-A
A-G
B-G
C-G
G-N the voltage across the resistor (or the ground current) hopefully your system has this value displayed as it usually difficult to measure.
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