9 volts on a disconnected circuit

[hbendillo]

Setup:

Strip Shopping Center with a 208-volt, three-phase circuit, Main panel metered by the power company, wiring trough fed by main panel and owner installed meters to provide services to tenants.

One tenant is a heath club with receptacles that serve treadmills. On at least one receptacle, the voltage to ground when the circuit breaker feeding the receptacle is turned off is 9 volts. Anybody care to venture a source of the problem?

 

[briand2]

Measured with a high impedance ammeter, induced voltages can be seen (certainly of the order of a few volts), depending upon load currents flowing in adjacent cables. Perhaps use a lower impedance ammeter (or just select the Low Impedance option, if it's got one) and see what happens?

Brian

 

[jbartos]

Suggestion: Longer power distribution runs or branch circuits may experience electromagnetically induced voltages. E.g. If a wire is run, as a laundry line, below high voltage transmission line, there could be enough induced power to heat an electric stove.

 

[busbar]

Basic concepts are discussed in

http://www.nema.org/engineering/papers/PhanBull88.html

 

[wbd]

I agree with briand2 - using a high impedance DMM will pick up small voltages. If you can get a analog meter, such as a Simpson 260, it will probably show zero volts.

 

[RalphChristie]

Is it a three phase, four wire system? (Three phases + neutral, like the 208V implies) There could also be a voltage to ground if the neutral is not broken.

RCC

 

[hbendillo]

I thought that there could be induced voltages but is 9-volts unusually high or normal? Yes, this is a 208-volt, 3-phase, 4-wire sytem.

 

[resqcapt19]

Nine volts does not seem high to me. I've seen induced voltages of over 75 volts on a 120 volt system when measured with a high impedance meter.
Don