I just switched the wiring of a single-pole light switch from back wiring to side wiring and noticed that my voltage tester (non-contact pen-style one, Klein Tools NCVT-3) detects voltage in both the other black wire going into light fixsture and the ground wire when the switch is off. The tester beeped with one or two yellow lights on, so the voltage seems lower side.

I did continuity testing with my multimeter and the reading looked fine (overload when the switch was off and 0 and beeping when the switch was on).

From my completely novice point of view, the switch should cut off any current when it is off and voltage should be detected in the hot wire only but not in others. Is it not true? Does what I see indicate something's wrong or is it actually normal (like picking up voltage from the nearby hot wire)?

  • How many cables come into the switch box? Is that box shared with other switches? May 30, 2020 at 23:34
  • @ThreePhaseEel It is a 1-gang box, so 1 switch,. I see two romex wires - assuming one carrying power from the source and the other going to the fixture. Can it be voltage picked up by the nearby hot wire? What else can be tested? May 31, 2020 at 0:13
  • Non-contact voltage sensors and DVM's are high-input impedance devices, and can detect the tiny amount of current induced between a live and a floating insulated wire running in parallel. You'd need to draw a mA-or-so current, e.g. across a 100 or 200 kilohm resistor, to show that it is not insignificant leakage. May 31, 2020 at 3:27
  • "Did you check it with a wiggy yet?"
    – Mazura
    May 31, 2020 at 11:03
  • It does cut off current. You're sensing voltage. May 31, 2020 at 17:26

1 Answer 1


Yes, it is AC voltage that is transfered by the tiny capacitance between galvanically isolated wires. The test equipment (normally having an input resistance of a few MegaOhm) is very sensitive and can measure these voltages, because it does need only tiny currents. Physically, the voltage source has a huge resistance.

Another example would be the natural E field all over this planet, which is equivalent to ca. 100V per Meter height difference. There would be ca. 200V between head and feet of every human being. But this voltage source has an even higher resistance, and needs a very sensitive special test set.

  • That natural planetary field gave the USA fits in World War II. Seems the Bureau of Ordnance calibrated torpedoes for the magnetic field in Chesapeake Bay. It's not the same in the central Pacific. May 31, 2020 at 17:26
  • @Harper-ReinstateMonica Yes, there is also a magnetic earth field besides the lesser known electric field (earth = charged spherical capacitor). The history of the US WWII torpedoes really sounds like sabotage. The tests were only conducted with dummy war heads which did not match the war head's weight. The torpedoes missed many times since they ran too deep. Reports from the NAVY were not taken into account, the tests were continued with dummy torpedoes having incorrect weight over an unbelievable long time span.
    – xeeka
    Jun 2, 2020 at 9:01

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