3 Phase 240v B Phase Grounded

[DigitalOI]

I'm hooking up a CNC machine, along with a vacuum pump (100a breaker w/mechanical disconnect, 2 gauge). The service is 240v B Phase Grounded (3 wire, no neutral). I've wired other machines on these same panels with great success, but never something as finicky as a CNC can be with voltages, so I decided to check the line-line and line-ground voltages. 240 v across all line-line, b phase to ground is reading 0v which is normal, but A and C phase read 30 volts to ground.

Does this seem awkward or is the line-line voltage the important factor in equipment wired for 240v 3 phase?

Thanks for any help.

 

[Smart $]

It is [next to] impossible for your reported readings to be correct, even if the system lost an ungrounded leg. Recheck your meter and method.

Welcome to the forum... :thumbsup:

 

[meternerd]

I agree with Smart $. No way you should get that reading. Make sure your ground is solid and make sure you are testing to the grounded B terminal, not some remote ground that may have some impedance. You should read 240V from each ungrounded phase to ground. Are you sure it's B phase corner grounded? Have you looked at the wiring or going by what someone said?

 

[olc]

Can I ask where you are from DigitalOI? I recently ran into 240V-3phase-corner grounded for the first time ever in Buffalo.

See the two replies above. If you measured 240V from A to B and B is grounded then you measured (by default) 240V from A to ground.

 

[DigitalOI]

Thanks guys. I've read a lot on these forums in the past but never took the 30 seconds to actually register!

I'll re-check everything tomorrow. All of the panels are marked 240v B grounded, and were installed in the late 80's.

I'm from upper michigan.

 

[kwired]

I'm hooking up a CNC machine, along with a vacuum pump (100a breaker w/mechanical disconnect, 2 gauge). The service is 240v B Phase Grounded (3 wire, no neutral). I've wired other machines on these same panels with great success, but never something as finicky as a CNC can be with voltages, so I decided to check the line-line and line-ground voltages. 240 v across all line-line, b phase to ground is reading 0v which is normal, but A and C phase read 30 volts to ground.

Does this seem awkward or is the line-line voltage the important factor in equipment wired for 240v 3 phase?

Thanks for any help.

If solidly grounded A and C will be 240 volts to ground. You did mention this was late 80's install, maybe something has changed (naturally or intentionally), but your readings sort of indicate it is now an ungrounded system for some reason.

 

[GoldDigger]

If solidly grounded A and C will be 240 volts to ground. You did mention this was late 80's install, maybe something has changed (naturally or intentionally), but your readings sort of indicate it is now an ungrounded system for some reason.

Or a high resistance grounded system, also possibly unintentional.