Ground fault location on Ungrounded system

[kwired]

There is a minimal current that will flow in an "ungrounded system" if one line is accidentally grounded. That small current is capacitive and will depend on the total capacitance of the line.

To set the ground fault protection relays, we either do a "live test" or estimate the line charging current in these ungrounded systems. I tried to search the web and found this page for your info. LINK

Understand the capacitance, but would think it typically would be such a small current it wouldn't be that noticeable.

All that happens when one line faults to ground is to effectively change the system to a grounded system, so you have about same thing you would have with a corner grounded system - just the point that is grounded is not necessarily at/near the source. You also still are supposed to have equipment grounding conductors an a grounding electrode system - so you bring all those non current carrying metallic components and the faulted conductor all to the same potential, which is going to work against developing some of the capacitance that would occur if we didn't have proper grounding and bonding.

 

[topgone]

Understand the capacitance, but would think it typically would be such a small current it wouldn't be that noticeable.

All that happens when one line faults to ground is to effectively change the system to a grounded system, so you have about same thing you would have with a corner grounded system - just the point that is grounded is not necessarily at/near the source. You also still are supposed to have equipment grounding conductors an a grounding electrode system - so you bring all those non current carrying metallic components and the faulted conductor all to the same potential, which is going to work against developing some of the capacitance that would occur if we didn't have proper grounding and bonding.

Correct.

The other thing I would like to discuss is how the system elements would behave with a higher than normal voltage. With one line grounded, the other two lines will be at 1.732 x normal voltage. This will be a non-issue at LV but becomes critical at MV systems. 120V system --> using 250V-rated devices will be fine (1.732 x 120V = 207V). But a 13.8 kV system using 15kV-rated devices will be a different thing, especially if those equipment are of old age(13.8 x 1.732 =24 kV). Somewhere, at any bad location, some insulations may fail.