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mgbinspect

Current Curiosity

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Every now and then we get, at a receptacle on a wall switch, a faint light when the switch is in the OFF position. I've always assumed it was a leaky switch, but the thought occurs to me, it could be a ground or neutral problem. Does anyone know, for a fact, what causes that reading?

Also, about every three years I'll get a reading with all three lights glowing. That configuration isn't on any chart, and once I even called the manufacturer and they put me back to a guy on the technical support side who basically said, "Beats me." Does anyone know, for a fact, what that reading tells us?

Inquiring minds want to know...

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All I can tell you is what was going on on the ones I found.

The three light came from where the home owner had tie the neutral into a hot wire and the dim light came where the neutral was being switched.

Of course there could other reasons this happens.

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All I can tell you is what was going on on the ones I found.

The three light came from where the home owner had tie the neutral into a hot wire and the dim light came where the neutral was being switched.

Of course there could other reasons this happens.

That makes sense, Phillip, and since no one else is offering anything different, I take it as a home run.

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I can supply a theory for the faint light, but it is pure speculation.

If there are multiwire circuits present, 2 hots and one neutral, and there is current running through the unswitched circuit, it may be inducing a small current through the switched circuit. When you plug your tester in, you provide a path for that stray current.

If the neutral is switched, with the switch off, shouldn't the tester show an open neutral?

I've never had all 3 lights on at once, but would put it down to seriously "faulty wiring".

That would be what Phil describes, power to both the hot and the neutral with your tester providing each with a path to ground.

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I can supply a theory for the faint light, but it is pure speculation.

If there are multiwire circuits present, 2 hots and one neutral, and there is current running through the unswitched circuit, it may be inducing a small current through the switched circuit. When you plug your tester in, you provide a path for that stray current.

I've never had all 3 lights on at once, but would put it down to seriously "faulty wiring".

That would be what Phil describes, power to both the hot and the neutral with your tester providing each with a path to ground.

Thanks John. The switch in question was on a living room circuit on more than on e switch. I believe, over the years, I've seen all three lights about maybe three of four times. I've always assumed that it can't be good, and always recommended that an electrician figure out what's going on.

I was just curious, regarding both, exactly what causes such readings, and assumed that someone out there in TIJ land actually can offer a technical explaination for both - Doug Hanson, or Jim K perhaps...

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My understanding of 3-light testers is that each light represents voltage potential between a pair of the receptacle slots. Typically the middle light indicates potential between hot & neutral, the right indicates hot-to-ground, and the left neutral-to-ground.

The left light could glow from a receptacle that has a downstream load on the circuit (including a multiwire circuit). The voltage drop on the neutral becomes a potential to the grounding slot, which might be faintly visible, depending upon the sensitivity of the instrument. If all 3 lights were brightly lit, I would discard that theory and pull out the multimeter.

I believe the faint light you see on the switched half-hot receptacle is an illusion caused by the proximity of the hot slot to the unswitched hot conductor. Again this would vary depending upon the sensitivity of the instrument.

We should always remember the limitations of these devices. If the tester says that all is OK, there are 3 possible scenarios: (1) a false ground, (2) reversed polarity plus a false ground connection between the neutral and grounding terminals, meaning the ground is then hot, and (3) everything actually being OK. It reminds me of why Woody Hayes did not like the forward pass. He said "3 things can happen, and 2 of them are bad."

That said, if I were a home inspector I probably would own one of those testers, since they save a lot of time and are suitable for the first pass on most receptacles. I would also carry other instruments to double-check any unusual readings. In my own work I use a TASCO Inspector III, which I find to be very reliable, as well as the usual arsenal of multimeters and non-contact sensors.

Douglas Hansen

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Every now and then we get, at a receptacle on a wall switch, a faint light when the switch is in the OFF position. I've always assumed it was a leaky switch, but the thought occurs to me, it could be a ground or neutral problem. Does anyone know, for a fact, what causes that reading?

It could be a few things. But induced voltage should be high on the list. This is particularly likely if the circuit is ungrounded. Read about my theory here:

https://www.inspectorsjournal.com/forum ... IC_ID=9210

Also, about every three years I'll get a reading with all three lights glowing. That configuration isn't on any chart, and once I even called the manufacturer and they put me back to a guy on the technical support side who basically said, "Beats me." Does anyone know, for a fact, what that reading tells us?

That can, for a fact, be caused when the receptacle is wired with 240 volts. Other things might cause it as well, but terminating a 240-volt circuit on a 120-volt outlet will do it every time.

- Jim Katen, Oregon

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Also, about every three years I'll get a reading with all three lights glowing. That configuration isn't on any chart, and once I even called the manufacturer and they put me back to a guy on the technical support side who basically said, "Beats me." Does anyone know, for a fact, what that reading tells us?

That can, for a fact, be caused when the receptacle is wired with 240 volts. Other things might cause it as well, but terminating a 240-volt circuit on a 120-volt outlet will do it every time.

- Jim Katen, Oregon

With the angels singing in the background, I realize you've hit it out of the park, Jim. That makes perfect sense, and now that you've made that connection, I realize I should have been able to deduce that one - well duh.... [:-graduat

Thanks for the link. I'll check it out when I get back home this evening.

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