- 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.
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.
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.
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.
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.