Sound Reinforcement - Forums for Live Sound Professionals - Your Displayed Name Must Be Your Real Full Name To Post In The Live Sound Forums > AC Power and Grounding
2 volts too much?
Bill McIntosh:
We recently had a touring Christian Band You May Have Heard Of play at our church. The rider called for 2 x 100 amp connections 120 volt 3 phase. Our electrical contractor tied their tails into panels.
The band's tech lead was reluctant to connect the tails to the PA distro - he measured 2 volts between neutral and ground. They connected after a discussion with our electrician and show went with no issues.
Is 2 volts a concern that we need to resolve?
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Scott Helmke:
Rule of thumb in the live sound world is that 2 volts will mean a noisy/buzzy show.
John Roberts {JR}:
not a human hazard but evidence of a problem.
JR
Mike Sokol:
--- Quote from: Bill McIntosh on November 13, 2017, 12:07:06 PM ---Is 2 volts a concern that we need to resolve?
--- End quote ---
In any subpanel with a load on it, there will nearly ALWAYS be 1 or 2 volts between the ground and neutral. That's because the ground and neutral should NOT be bonded at the subpanel, only at the incoming service panel. So if you have 4 volts drop on that panel due to a heavy load, then typically 2 of those volts are on the line conductor, and the other 2 volts are the drop on the neutral conductor. If you measure between the neutral and the ground on that subpanel you'll come up with 2 volts. I get worried when I measure EXACTLY 0 volts between neutral and ground on a subpanel because that suggests a secondary neutral-ground bond on the subpanel. That 0 volts is because you've now contaminated the ground with the voltage fluctuations on the neutral due to load variations. And THAT'S what causes ground loop hums.
Stephen Swaffer:
A bit a swerve-but relevant I think? Can you even get a UL listing for equipment that has neutral connected to a chassis? That would create a secondary ground/neutral bond? I know chassis tied to neutral's used to be standard fare. If there is no connection the voltage difference is irrelevant.
I agree with Mike-a 20 amp imbalance with 1 ohm of resistance in a neutral = 2.0 volts differential. That is pushing good design practice for voltage drop in a circuit.
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