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Bedroom Voltage Question


JEuriech
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Bedroom Voltage Question

Last week I inspected a 10 year old vacant bank owned house. The buyer's father showed up at the beginning of the inspection to determine how hard it would be to install fans in two of the bedrooms.

Just before he left about ten minutes later, he told me that the switched outlets in the bedrooms had 40 volts on them while the switch was in the off position. I asked him to show me what he was talking about.

The wall light switch that controls the top half of an outlet on the opposite wall, showed 120 volts with the light switch on. With the light switch off, his digital meter had 40 volts. The father then took off.

Later in the inspection , I took my own analog meter and checked the outlet. This is what I found:

From hot to neutral with the light switch on , I had 120 volts.

From hot to ground with the light switch on , I had 120 volts.

From ground to neutral with the light switch on , I had 0 volts.

From hot to neutral with the light switch off , I had 18 volts.

From hot to ground with the light switch off , I had 18 volts.

From ground to neutral with the light switch off , I had 0 volts.

The bottom half of the outlets worked normal.

The switched outlets in both bedrooms worked the same way. Why would there be voltage on the outlets when they were in the off position? My cheap three light tester didn't show any type of problem. Only by using a volt meter does something show up.

Thanks

Jeff Euriech

Peoria Arizona

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I've seen some old switches which had all sorts of crud, dust, spiders, whatnot in them. This stuff can sometimes conduct a bit of electricity.

Also soda pop, coffee, etc. can conduct a bit of electricity. A kid may have poured something into the wall switch.

That would be the first thing I would replace.

If that did not fix the problem, pull all the switched outlet covers and look for crud between the top/bottom hot screws. Then after that I would start looking for a nail or screw through a wire.

BTW the term computer bug was originally from a bug crawling on the computer wiring and conducting electricity! This caused random problems with the computer...

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. . . The switched outlets in both bedrooms worked the same way. Why would there be voltage on the outlets when they were in the off position? My cheap three light tester didn't show any type of problem. Only by using a volt meter does something show up.

It's actually a rather common problem. Sometimes you'll find it because the three light tester lights up very dimly.

There might be a few things going on:

Were the switches the indicating kind, with little neon lights inside? If so, you'd just be seeing the potential across the neon. That'd be perfectly normal and expected.

Were they decora switches? I seem to find this problem more often with decora switches. I don't know why. When you remove the switch and troubleshoot with the bare wires, the problem goes away.

And lastly, you might find that you're reading induced voltage. This one is a huge pain in the rear to troubleshoot. It's similar to troubleshooting the travellers on three-way switches. If you've ever had the misfortune to try that using a multimeter you'll find all sorts of strange low voltage readings that seem to be induced by the three-wire cables. I've found that abandoning the multimeter in favor of a wiggy is the best way to deal with these. The wiggy won't read the induced voltages.

If it were a bank owned home and I were alone, I'd start by taking the switch out of the equation.

- Jim Katen, Oregon

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The light switch was an ordinary flip light switch. Nothing special about it.

Is induced voltage considered a safety hazard? At what point should we be calling it out? Above 10 volts?

The house was vacant and there was nothing turned on. Is it possible to get induced voltage say from another cable if is not currently in use?

Thanks for the info

Jeff Euriech

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The light switch was an ordinary flip light switch. Nothing special about it.

That pretty much leaves induced voltage as the likely culprit. I also noticed, on reading your original post again, that the Dad, using a digital multi meter, found 40 volts and you, using an analog one, found 18. That supports the induced voltage idea as well because the digital meter draws pretty much nothing from the circuit while your analog model only draws next to nothing. A wiggy would draw quite a bit more than either of them and would probably show 0 volts.

Is induced voltage considered a safety hazard? At what point should we be calling it out? Above 10 volts?

If it is induced voltage, it's not a safety hazard. It isn't even real voltage, just sort of a ghost image of voltage. However, if you're getting induced voltage, it probably means that the switch loop is ungrounded. That might be a hazard, though not much of one.

The house was vacant and there was nothing turned on. Is it possible to get induced voltage say from another cable if is not currently in use?

Yes. Think of it like this. There's a two-wire switch loop that runs from the receptacle box to the switch box. When the switch is in the off position, one wire is energized and the other wire isn't. The second wire will pick up a voltage from the first wire's electric field. Since the second wire isn't connected to anything at either end, it just sits there with this sort of reflection of voltage on it. If you were to include a grounding wire in the switch loop, the field would be drawn down and there would be little, if any, voltage on the second wire. In this off position, if you were to touch the wire or connect it to a turned-on lamp, the voltage would also be drawn down and disappear.

If this theory is correct, we ought to be finding traces of low voltage on every ungrounded switch loop out there when testing with a digital multimeter.

- Jim Katen, Oregon

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