Ground Fault

[Nate Scott]

Why do ground fault requirements for services and feeders over 1000 amps only apply when voltages are over 150 volts to ground.

 

[don_resqcapt19]

Because with voltages of less than 150 volts to ground the arcing ground fault is not likely to re-establish itself after the zero crossing. With higher voltages the arc is re-established through the ionized air and the impedance of the arc limits the current so the OCPD does not open on overcurrent.
Don

 

[brian john]

As Don noted above in higher voltage distribution systems an sustained arcing ground his an impedance that is below the thresholds of OCP's, the sustained arcing ground faluts can burn to the point where the ground fault current exceeds the OCP rating and opens the device or becomes a phase to phase fault generating sufficent current to operate the OCP device.

This has been covered in other threads

http://forums.mikeholt.com/showthrea...t=ground+fault

From that thread


Overcurrents and Undercurrents All About GFCIs”.
Author Earl W. Roberts,
1996, Mystic Publications

This book also covers GFPE, from chapter 12 (my abridged version)

While arcing faults can occur on 120/208 VAC systems the arc does not result in a restrike arc due to the voltage level. Studies have shown that to have a restriking arcing ground fault the voltage levels of 374 volts are necessary.

120X1.414 = 169.68, Peak Voltage
277X1.414 = 391.67. Peak Voltage

GFPE is required on 277/480 VAC distribution systems to protect against the devastating damage that occurs from these sustained restriking arcing ground faults, it is not required on 120/208 distribution systems as the restriking high impedance burn down arcing ground faults are not occurring.