Switch turned off, getting weird multimeter reading

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JimO

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Hi, I am connecting a new heating element into a combination heater/exhaust fan/light/nightlight appliance in the bathroom ceiling. Each of the separate elements has an electrical cable that plugs into a corresponding socket in the base of the appliance. I was testing the current to make sure it hadn't caused the previous heater to overheat. I used a multimeter in the socket for the heater and fan, and got a little over 120. It was hovering around 121-123. I assume a little variance from exactly 120 is normal. Of course, I'm not an electrician.

The weird thing was when I tested the plug for the cable to the light and the one for the nightlight, the multimeter went way off! It hit 230 or even higher. And the wall switch was off! When I then turned the wall switch on, the meter went to 120, which is normal. Where is this 230 volt reading coming from? Do I have an unwelcome current going through my house? Is this from a grounding issue? Is this normal? Does it have anything to do with building the house on an ancient burial ground? Please share your wisdom. Thanks!
 

Jadnashua

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The input to your (typical) house is really around 240vac with a neutral line connected 1/2-way across the transformer, so that the voltage reads 120vac from one end to neutral. That's why your main breaker has two poles to it...across it, it's 240vac, from either side to neutral, it's 120vac. If you measure 240vac between two hot wires in a 120vac circuit, you're measuring from the hot of one side of the power transformer to the other side - IOW, you have two separate 120vac circuits in that box, and each is only 120vac when measured to the neutral (or ground). If you aren't careful, and were to tie those two lines that show 240vac between them, to the same point, you'd get a big zap. FWIW, the heater MIGHT be a 240vac heating element, but the fan and almost certainly the light bulb, are 120vac devices.

120/240 vac is just the nominal value...it can easily vary +/-10% on a bad day, but normally is less than that.
 

Lone Star Charles

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It's not unusual for the open circuit voltage to be really squirrelly, especially when you measure it with a volt meter that has a very high input resistance. In this case, transient induced voltages or even static charges can fool the volt meter into higher readings that would not be evident if a lower input resistance allowed those to bleed down.
 

Jadnashua

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Open circuit measurements can be very misleading. A typical DMM can have hundreds of Megaohms of input impedance...IOW, it 'looks' like an open circuit. Put the thing you are measuring into a complete, working circuit, and you may read very different values. When measuring an 'open' circuit, you're actually inserting what amounts to a HUGE resistor in series with the circuit - this is good when actually measuring a 'real' circuit, but can give major false indications when the meter itself becomes a significant part of the circuit. When measuring a 'real' circuit, you are putting the meter into it as a parallel connection verses a series connection. You need to have a feel for Ohm's Law to understand what's happening, but it is normal.
 

DonL

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It also helps to know if you are "testing the current" or testing the voltage.

Be careful playing with electricity.
 
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