I've noticed that using my mains tester the light in the screwdriver lights up regardless of the position of the light switch near the door. It seems to only not light up when both switches are in some "off" position (although you can't see which is on/off).

I thought that the lamp is after the two switches so even if one of them is in "on" position and the other one in "off" the wire should be dead and only be hot if both are in the "on" position.

So... if this isn't the case at my appartment does this mean that the lamp is actually between these two switches so that when the first switch is in "on" position the wire at the lamp is hot?

(Actually yes, there's no such thing as on/off position. In usual multiway switching they just need to be both in the same position for the circuit to be closed.)

  • 1
    There is no 'off' with multi-way switching. There is only "On A" and "On B". The switch flops one for the other. When all switches agree, the light lights. This happens 50% of the time by definition. One hot or the other is hot all the time, as is the supply if it runs backwards along the switch. The two that switch are called messengers. It's also possible to wire it so the lamp is in the middle, but that is weird and a bit dangerous. – Harper - Reinstate Monica Jul 17 '17 at 15:30
  • You say "hot" - but are you actually using a multimeter to make that determination? It would be helpful to know the exact voltages you are seeing. – SDsolar Jul 18 '17 at 3:53
  • No. I just use what's called a "phase tester" here. It's a screwdriver with a lamp that lights up to check which wire is the neutral one and which is not and wether the electricity is off or not. – mroman Jul 18 '17 at 6:54

I'm guessing your are using a proximity type tester. Some are very sensitive to any electrical current even static charge. So if you have a wire next to a hot wire yet is disconnected from the circuit, mutual induction will cause some trace voltage with very small (milliamp) current and yes it will register on your sensor. Thes type of sensors will tell you current is present but they don't do much more than that.

If you are concerned, try using an ammeter it has settings that will give you voltage and current strength.

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