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I have connected armoured 19 core 1.5 sqmm cable to a 110v ac supply. When one pair is connected to supply even if I keep the other end open, voltage is induced in all wires and the supplied 110v is dropping to 66v ac with respect to ground. We have solved this problem by using twisted multi-pair cables. Why is it happening in multicore?

Because the twisting gives you field cancellation
and the multicore does not?

Regards

cww

The coupling you see in the 19 core cable is due to the capacitance between the conductors as the nearby wire goes from positive to negative then back again. In the twisted cable each wire runs next to one of the AC wires then the other thus canceling out the effect. Going one step further a grounded screen (shield) is placed between each pair.

Although you see voltage induced in your 19 core if you put your multimeter on AC current you will see very little. In most electrical circuits the induced voltage has little effect because it's shorted out by the load. If you want to see induction at its worst try opening a circuit where there is a coil e.g. solenoid the induced voltage can spike to many times higher that the supply voltage as the coils magnetic field collapses. You will often find Metal Oxide Resistors fitted to a coil to minimize that.

Next time you see those old fashioned bare copper telephone lines look closely, you will find they twist as they go down the road either by moving from one insulator position to another or with jumpers every few poles.

Regards,
Roy