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  • Have a new circuit in the basement that includes a dimmer switch and 4 recessed lights, in addition to 6 outlets.
  • The switch and the lights are at the end of the run. The order is:: panel > AFCI outlet > 5 outlets > switch > 4 recessed lights
  • When a contactless tester (Kline Tools) is brought very near (about an inch) or touched any wire of the circuit, except the part between two last lights, it beeps/blinks red, which is correct.
  • Checked the wiring (after turning off breaker) and all seem to be ok.
  • All are new products from Home Depot. Tester is also ok and all outlets functioned ok, including AFCI outlet (trips and resets).
  • When the tester is at about 12-14 inches from the wire between the two lights at the end of the run, it starts to beep and blink.
    • Why is this happening?
    • I'm planning on rechecking the wiring/connections etc., but any tips would be greatly helpful.
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  • You should mark the fixtures or bulbs that wire goes to first, take out the light bulbs (or disconnect the fixtures but splice the wires through temporarily), then power up the circuit to check if it still happens when there is no load. Then add the loads back on to the circuit one by one, checking the wire in and the wire out as you add each load. Let us know the result.
    – K H
    Commented Apr 13, 2021 at 6:37
  • I take it the dimmer is turned on at this point? Is the dimmer a variac type (about 20 pounds) or a rheostat type (makes so much heat it needs a chimney)? If it's not either of those, it is doing something electronic and not outputting a sine wave, which means it's making Harmonic frequencies at 180 Hz, 300 Hz, 420 Hz, and other odd multiples of 60 Hz.
    – Harper - Reinstate Monica
    Commented Apr 13, 2021 at 16:56
  • Thanks and Leviton dimmer that comes as a 3-pack is what I'm using. Used other two in my main house to replace older dimmers and no issues at all.
    – Nick
    Commented Apr 13, 2021 at 17:58
  • @Harper-ReinstateMonica I wrote an answer with the assumption it's related to a dimmer, but then I deleted it because I noticed it's not happening on all of the loads attached to the dimmer which means there's a good chance it's something else. The harmonics should be present on all wires downstream of the dimmer.
    – K H
    Commented Apr 14, 2021 at 5:55

4 Answers 4

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Those testers detect electromagnetic fields.

It's downstream of a dimmer

There are three dimmer technologies. Variacs are a variable transformer which steps down voltage, giving a sine wave, but they are huge and heavy. Rheostats are a variable resistor that adds impedance, giving a sine wave, but they make stupefying amounts of heat. Both of these, being sine waves (as in (a) below), produce a 60 Hz electro-magnetic field. (50 Hz in the rest of the world). Which is what that tester is designed to detect. However both those dimming methods are totally impractical for the home.

enter image description here

So a third method is used: semiconductor dimming, which uses electronic switching to alter the sine wave (as in (b) and (c) above). However, nature likes sine waves, and nature will treat a mutilated sine wave as if it were a conglomerate of various sine waves summing up. The science of this is called "Fourier analysis".

Which sine waves sum up to (c)? Many of them, at odd multiples (3x, 5x, 7x, 9x etc.) of the base frequency. So 180 Hz, 300 Hz, 420 Hz, 540 Hz, 660 Hz. and so on. Janky shaped waves can create lots of these and they can be strong. Depending on the setting of the dimmer, these other frequencies will have different strengths, and depending on the room, will propagate certain ways.

The tester's sensitivity to these harmonics will depend on its own design.

I bet all these factors are lining up in a way favorable to detection.

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There may be phantom voltages coupled-in, and especially apparent when the circuit is off.

Testers can be quite sensitive, and your phantom voltage can be as high as half the line voltage, so 60V.

These are not necessarily an electrical problem, but could trip your tester.

To check for this, temporarily add a low-impedance passive load at the end of your run. An old fashioned 10W (or more) incandescent bulb is enough. The point is that it should draw a current even if the supplied voltage is coupled. Many modern devices, like LED drivers, draw next to nothing if the voltage under-load is below a threshold, thus keeping coupled charges on the line. If you don't have such a bulb handy, you can experiment with other loads. Whether they'll accomplish the trick depends on their electronic internals. Usually old fashioned appliances would work: incandescent bulb, fan, hair dryer, toaster, mixer, maybe radio, CFL bulb...

Safely wire up an outlet at the end of your run and plug in a load.

Phantom voltages that trigger a tester can arise from capacitive, magnetic, or electromagnetic coupling. The voltage or current is coupled from other wires or electrostatic charges in drywall/insulation when the circuit is off, since the charge has nowhere to go.

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In that kind of distance strong electromagnetic field may caused if the feed and neutral do not run close together. Bad practice of wiring from not enough qualified electricians. Feed and neutral to load may be in different conduits.

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  • Thanks. The lights are recessed LED, with plastic casing, that does not need a ground. I'm using 14-2 and all wiring are close together. Not sure how they can be further apart. I'll check the wiring.
    – Nick
    Commented Apr 13, 2021 at 15:57
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    @Nick the writer is talking about cases where the current returns on a different route. Current flows in loops, and to avoid spurious EMF emissions, all current coming out a hot must return on its partner neutral. If you put a clamp ammeter around a cable it should read zero, because the currents are equal and cancel each other out.
    – Harper - Reinstate Monica
    Commented Apr 13, 2021 at 17:03
  • For example the phase come to load in on conduit or cable but neutral connected from another cable. Yes, the wires are together but different cables.
    – user263983
    Commented Apr 13, 2021 at 17:11
  • Thanks; I'll check all the wiring and connections and trouble shoot soon, and will post the results.
    – Nick
    Commented Apr 13, 2021 at 18:10
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There are different classifications of testers with different sensitivity.

An example of this is a tester used for hvac circuits that will detect 18vac.

The 18v tester will detect 120v from almost a foot away so I think that may be what you have.

My standard non contact tester is 90-1000v but if I have it turned on when I open my transformer vault 34500v it goes off. Similar but different ranges based on the sensitivity of the tool.

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  • Thanks and made sure tester only detects 40V and above (green light). If less than 40V it used blue light.
    – Nick
    Commented Apr 13, 2021 at 15:59
  • Nick the detector only detects a “field” if non contact it can not tell how far away the conductor is so your detector may show a light red or blue and the source could be 277 yes. I have had many non contact testers even volt meters that use the electricians body as the ground reference. They can all be off and not only from real voltage but induced or phantom voltages. Get a real volt meter if you are concerned but there is nothing wrong with the detector. It could be wave shaping from dimmer controls we don’t know but without a real meter it could be many different reasons
    – Ed Beal
    Commented Apr 13, 2021 at 16:16

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