Induced Voltage In Domestic Wiring

[TPower]

I was doing some testing on my switchboard at home, and I’m trying to understand what is going on here..

When I put my volt meter across the line and load side of the MCB supplying the range circuit (range switched on), there is 0V when MCB is ‘on’ and 230V when ‘off’. This all makes sense, however when I switch the range off so it’s an open circuit, and test across the MCB (still in the ‘off’ position) there is about 200V.

For another test; I got a 50m roll of 2core TPS, put one end on the ground, with both cores isolated from each other and floating in air. With the other end of the cable I connected one core to the neutral bar, and the other core is just floating in air, with the insulation stripped back for testing. When I test between this core I’ve stripped back and the live terminal of the main switch, I’m getting roughly 200V, even though same as the first test with the range it’s an open circuit?

When I disconnect the core I connected to the neutral bar with the 50m roll of TPS, so it’s completely isolated, and repeat the same test as above, I’m getting about 20V. It’s a lot less, but I don’t understand why it’s not 0V.

This is all a bit of a mystery to me, I think it may have something to do with induced voltages? Is someone able to explain how these test results may occur?

Thanks!

[gregmcc]

Hi impedance meters don't load up the circuit under test and so induced voltages, add a bit of load like a 100w filament light bulb and the voltage will be zero

[TPower]

Thanks Greg

[AlecK]

With any multi-core cable; the cores are acting as a capacitor.
A 50 m run is enough to put real voltage on the conductor that's not connected to anything.

I once had a lighting circuit where the 2-way strap was about 50 m cable route length; which wasn't a problem until the original incandescent lamps were swapped to CFLs; which flashed at fairly regular intervals when turned "off".
There was enough voltage to charge capacitors on the CFLs until they fired, flashing the lamp.
And you could watch it rise using an analogue meter.

The same happens in relation to the earth conductor. This is one of the reasons for restricting the number of circuits that can be protected by a single RCD; as all that capacitance amounts to a standing "leakage" current for an a.c. circuit.

[JamieP]

Just a side note isn't stuff like this why 1.5.11.4 exists?

[AlecK]

yes it's related.

however capacitive coupling / induced voltage doesn't affect only unsused conductors.