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There's a staircase light circuit in my house similar to the one shown below.

basic two way switch schema (source -> https://www.youtube.com/watch?v=opoEswRp_jg&t=144s)

I disconnected 'live' from the switch on the left and measured the voltage between its 'L1' terminal and neutral and then its 'L2' terminal and neutral. Both read 230 V.

I repeated the measurements after toggling the switch on the right and still obtained the 230 V readings.

Why do I get 230 V readings in both L1 and L2 positions? (I thought one would show 0V) Secondly, there is no continuity between the ‘L1’ and ‘L2’ terminals of the switch on the left and neutral. Is it so because of the high resistance of the bulb?

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  • \$\begingroup\$ It's quite common for meters to have a very high impedance and pick up stray voltages from long runs of cable. That or the switches aren't wired the way you think. Does the site you got that picture from not explain the operation of two-way switching? \$\endgroup\$
    – Finbarr
    Aug 2, 2023 at 18:39
  • \$\begingroup\$ It explains how the the switch works in the context but it does not cover answears for questions like this. Maybe it is because it is some sort of basic electronic basic knowledge I am missing. Just want to know the answear. By the way the switch circuit is definitely wired correctly as it wired and works according to the diagram, but I am curious about those technical details because as newbie with multimeter it got me seriously confused when tried to double check if everything works as it should \$\endgroup\$
    – Skyferix
    Aug 4, 2023 at 17:40

1 Answer 1

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What you see is capacitive coupling. Sometimes called phantom voltage.

Basically, a capacitor is just two conductors separated by insulation.

You have insulated copper wires running along each other so an AC voltage on the powered wire gets capacitively coupled to the diconnected floating wire.

The capacitive coupling is very weak and the meter you are measuring the voltage is an extremly light load so it does not affect the voltage much.

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  • \$\begingroup\$ Never would I guest that running wires side by side separated by insulation would create an equivalent of capacitor. Just to clarify this phenomenon occur only in the AC circuit as the current constantly switches direction? Also you mention that coupling is very weak. So the best way to test the circuit in this situation would be to add resistor (light bulb)? (As the reading multimeter reading is unreliably) \$\endgroup\$
    – Skyferix
    Aug 4, 2023 at 19:51
  • \$\begingroup\$ No it's not due to current changing direction, it's because voltage goes up and down like a sine wave with amplitude of 325V. Yes having a light bulb would present a huge load and prevent the wire from floating so the weak capacitive coupling can't have much effect on it. \$\endgroup\$
    – Justme
    Aug 4, 2023 at 20:23

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