Hi, I have a 5 core cable 1.5mm CSA conductors pvc insulation x 10m long. Conductors, Live, Earth, and Neutral are connected to an AC 240V supply. The 4th and 5th conductors are for a digital dimming signal (currently not connected at either end). I'm measuring 240V between Live/Earth and Live/Neutral, and 6V between earth and neutral. I'm also measuring 110V between Live and each of the unconnected dimming 4th and 5th conductors. So there appears to be induced voltage into these conductors from the AC supply. Can anyone guide me as to how to calculate these voltage values? All the cables insulation test above 2 megaohms. Thanks, Rookie.

Think of it as a capacitive voltage divider.

Within the tolerances of the cable manufacture both legs will have the same capacitance so you'll see about 1/2 supply, the source impedance will be pretty high so your meter will load the voltage somewhat but you're seeing almost exactly 50% of your supply.

The length of the cable run won't change the voltage, but it will affect the source impedance, at 10m I wouldn't be too worried, but long runs can induce enough to actually give a noticeable shock.

Once you connect your dimming electronics the snubbers (which should be part of the design) at each end will pull down the voltage to negligible levels, do be aware in your electronic design that there could be considerable spikes coupled from the mains wiring onto your control wires.

Formulas are about but academic in this case, the measured voltage values will differ, dependent on whether analogue or digital instruments are used. There is a problem here I think, caused by using a common cable. What does the maker of the dimmer say?

Regards

Thanks for the info. The dimming system is normally provided by the installer so it could be one of many manufacturers. It is interesting to know that this capacitive voltage is present. No wonder 1-10V dimming systems insist on shielded twisted pair cables. Thanks, Rookie.