10 VAC between line and ground when circuit breaker is off?

[PaulKraemer]

Hi,

I am preparing to replace a control panel for an industrial machine. The new control panel is a direct replacement for the old control panel that has been in use for nearly 30 years - it just has new components that are still supported, whereas the old control panel has many unsupported components that would be difficult to replace if they failed.

Supply power for these control panels is 208 VAC 3 phase 60 Hz. In preparation for the upgrade, I turned off the circuit breaker that supplies the old control panel. To make sure I turned off the right breaker, I measured the AC voltage between each line and ground at the inbound side of the old control panel disconnect.

I was surprised that I measured 10 VAC between each line and ground. This machine is in a large building with a lot of equipment. In the hope of boosting my electrical knowledge, I am wondering if anyone here can explain what might be the cause of this, and if there are any adverse effects that might result from it?

Thanks in advance,
Paul

 

[stevenal]

Gotta love those modern high impedance digital multimeters. Every little bit of capacitive or inductive coupling from other circuits is captured. 10 V is a small fraction of the normal 120 V to ground, so you have the right breaker. No adverse effects.

 

[waross]

Most probably you have a long feed to a panel or sub panel.
Add to that unbalanced single phase loads causing neutral current.
The result is a voltage drop on the neutral conductor.

 

[IRstuff]

And/or just capacitive coupling. You could check for that by adding a 1k resistor in parallel to the voltmeter and see what voltage is generated.

 

[stevenal]

https://dam-assets.fluke.com/s3fs-public/2105317_A_w.pdf?VersionId=ay9a7MLeqmnY7PfwYfAwHUEmUfd5yh_a

 

[PHovnanian]

This is why many electricians swear by a Wiggy tester.

 

[TugboatEng]

If you're still concerned, take a resistor that will limit current to some harmless number. 200 to 2000 ohms would do well in your case. Short the lines to ground with the resistor and take the voltage measurement while shorted with the resistor.