Open circuit voltage

Status
Not open for further replies.
ptonsparky

ptonsparky

Tom
Location
NE (9.06 miles @5.9 Degrees from Winged Horses)
Occupation
EC - retired
We are changing out three services from what was three separate buildings to one service entrance and a feeder each to the other buildings. Two of the original services were single phase from the same transformer. The third was three phase 120/208 bank. All pole top transformers. The three phase hadn’t been used for ten years. The guys turned Off the 3 phase disconnect at the pole. Ran, the feeder and as they connected the EG noticed sparking. They took an amp reading of 5 amps. No doubt we paralleled the neutral back to the transformers, and assuming the primary neutral between the transformers. I will remove the three phase neutral bond tomorrow. That system is being Removed entirely next week. Transformers and all.

Clear as mud?

What would be the expected open circuit voltage when that bond is removed?
 
I am not completely clear on the setup, but I would say the EG was an alternate path for the POCO primary neuter (if its an MGN), or the EG was an alternate path for neuter current on the single phase service (were they being used?). IT can be alarming, but likely only a few volts of potential there. I was once replacing a bunch of feeders to subs that were temp fed with #6-2 NM and N-G bonded in the 120/240 panelboards. I dont remember exactly but I think I had terminated the new MC feeder cables into the cabinet of the live and in use panels and was working them back to the electric room. I would get small sparks when the cable touched grounded things like pipes and EMT. I never measured the voltage, but of course it would have been equal to the voltage drop of the neutral current on that #10 egc/neutral in the ramen. I doubt it could have been over 5 volts or so.
 
I would expect the open circuit voltage to be the utility NEV. This might be a few volts normally. If on a long radial tap, it could be more. I would not like to see more than about 5 volts, maybe 10 volts in extreme situations.

If you want to know what NEV to expect at your location, measure from the nearest utility pole ground wire to a remote ground (like a screwdriver stuck in moist earth at leat 10 feet from the pole and other electrical or metal stuff). It can vary during the day with utility system load.
 
Status
Not open for further replies.
Top