Hey everyone,

I have a question about the method of detecting a ground fault on an ungrounded DC battery circuit. According to the equipment I've been researching (Megger BGFT), the leads of the device are placed in parallel to the DC source and ground. An AC voltage from the device is then ramped until an AC current is measured back at the device. By using the provided "receiver", which is essentially a clamp-on amp probe which you place around branch circuits downstream from the DC source, the fault can be traced to whatever branch the ground happens to be on. My question: How is the AC voltage able to induce a current when placed in parallel with the DC source? I've done some searching and can't seem to find answers that favor AC and DC sources in parallel without consequence. My understanding seems to have the AC voltage in series with the DC voltage to function correctly. The ungrounded DC circuit is 125 Vdc and the Ground Fault Tracer voltage output ranges from 0V to 50Vrms. The measurable ground resistance is 1 to 399 kilohms @ 50Vrms.

Thanks!

 

An AC "signal" is imposed on the conductor's of the DC system. Your "test leads" will parallel the installations permanent conductor(s), but your AC signal circuit will use part of your DC system's conductor(s). Meggers instruments will detect and help to locate a ground fault by essentially detecting the instability of that AC wave, which is slowly introduced, and letting you see or hear the most damped point, by the lowest amplitude of that signal. I believe that you must test the + and - conductors separately.

At least that's been my experience.

 

They do make UL approved GFIC's for DC systems.
https://www.westernautomation.com/wp-content/uploads/2016/03/GFCIs_For_AC_DC_Systems.pdf