I have not worked on three phase for a while. A technician called and said he had a 3 phase feeder to an air conditioner. With the disconnect open he was getting A-B=208V, B-C=180V, A-C=180V, A-GND=120V, B-GND=120V, C-GND=93V. This sounds like an open phase to me. I just can't remember how to calculate the fact that you still have voltage on the C phase.
3 phase open
- Thread starter rich000
- Start date
- Status
jmsbrush
Senior Member
- Location
- Central Florida
It cold be a loose connection at the panel or there might be a junction box in the middle with a loose connection
jmsbrush
Senior Member
- Location
- Central Florida
Sorry about the mispell. "could"jmsbrush said:
Thanks all. I was thinking loose connection (some boxes are not easily accessible). This is a dedicated feed to the HVAC. I think I remember that you can get the low voltage reading on an open phase in 3 phase, I just don't remember. I would think it is the same as when you are checking fuses on a 3 phase circuit.
LarryFine
Master Electrician Electric Contractor Richmond VA
- Location
- Henrico County, VA
- Occupation
- Electrical Contractor
It could be that you're measuring voltage with a high-impedance voltmeter that is not loading the C-phase wire enough to eliminate induced "phantom" voltage.rich000 said:
crossman
Senior Member
- Location
- Southeast Texas
augie47 said:
This is my first bet. The "backfeeding" load could be located anywhere else, and not necessarily on the feeder involved.
Questions: Are there any other 3-phase loads in the building? Are those loads having issues? Have you measured the voltages on other feeders, or at the service? This can let you narrow down the location of the problem.
I went out and looked at the site. There was a junction box the technician could not see behind a vent and pipe. Accessed my standing on top of a freezer and squezzing between an HVAC vent and a drain pipe. He thought it was a straight run. Opened the box and the wires were trashed. Wirenuts melted, bare wires.
- Status