Working in the home electrical installation, the usual light with a pair of two way switches.

Installation seems ok, things does what is expected. However, measuring with the voltmeter, I measure 80V between neutral and the line between switches "passive" (the one that, taken into account the position of the commutators, should not carry power), 230V in the active. Same if I measure from earth cable.

I do not understand why of this 80V voltage.

Worst, even if I isolate completely both sides of the cable, still 40V are measured.

Some explanation about these measures?

Additional information:

circuit is: power line (220V) <=> two way switch 1 <=> two wires <=> two way switch 2 <=> single wire <=> load (lamp) <=> neutral line

Wires between switches are about 5 m long, all wires running together inside a protective corrugate: power, neutral, earth and the two lines between switches.

a) Measuring with the multimeter, 40V-80V appears between one of the wires inter-switches and neutral or earth wire (where I expected near 0V), 230 v in the other one (this is ok).

b) Using as "measure" tool a simple incandescent bulb, no light where the multimeter says 80V.

c) Using as "measure" tool my hand (accidentally and, in fact, the reason I started to measure the state with the multimeter), an small discharge (how much sensitive is the hand?).

d) if I connect a wire between the line with the 80V and the earth wire, the voltage falls to 0V (at least ohm law still working) and security differential device doesn't triggers.

e) If I connect a 40W bulb lamp between the wire with 80V and earth, voltage falls to 0V, bulb made no light. Thanks.

  • \$\begingroup\$ Please draw a schematic. \$\endgroup\$
    – winny
    Jan 22 '17 at 11:47
  • \$\begingroup\$ @winny: schema added to the question. \$\endgroup\$ Jan 22 '17 at 13:21
  • \$\begingroup\$ That's not a schematic, but it is better than nothing. In electrical engineering, schemic is everything. \$\endgroup\$
    – winny
    Jan 22 '17 at 18:59

The isolated wire is bundled together with a live wire and capacitively couples to it thus, you will measure a voltage with a high impedance voltmeter.

  • 1
    \$\begingroup\$ Add a small load and remeasure. \$\endgroup\$
    – winny
    Jan 22 '17 at 11:55
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    \$\begingroup\$ If the voltage is not nearly the same as the live wire then the mechanism that produced it is highly likely to be capacitive. Using a lamp is a good test. \$\endgroup\$
    – Andy aka
    Jan 22 '17 at 12:08
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    \$\begingroup\$ @winny: done the test you suggest using a 40w bulb as load. Voltage fails to 0v. After this test, I think I will accept this answer as the most probable. \$\endgroup\$ Jan 22 '17 at 13:19
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    \$\begingroup\$ Yes, it seems resonable. Your meter's input impedance however, does not. What are you using? \$\endgroup\$
    – winny
    Jan 22 '17 at 19:00
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    \$\begingroup\$ It's a capacitive effect and can tickle a little bit for sure but it's harmless but don't make a habit of touching it LOL. \$\endgroup\$
    – Andy aka
    Jan 22 '17 at 21:49

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