Stray voltage

[making_1998]

Plumber has just completed upgrading heating system which includes 3 zone valves. When completing wiring for zone valves I encountered some strange readings. The switch on the zone valve is supplied with a pernament live(230v AC). When the valve is open the switch closes and switched live goes back to wiring center. My problem is that with the switch open I am getting 70volts AC coming back on the switched live. Disconnected pernament live and 70v disappeared. Verified correct operation of switch. Suspected fault in cable. Connnected with a new cable and same problem exists.
As part of trouble shooting, I connect one core of a short length of 3 core cable to a pernament live. I was then getting 70v AC on the 2 cores which were not connected to anything at either end ! Measurements were taken using a multimeter with CU earth terminal used for Com.
Any ideas on where these voltages are coming form ?

 

[aptsys]

Capacitive Coupling

 

[making_1998]

Thanks for the quick reply aptsys.
Okay so Capacitive coupling seems to be the problem. I am using a 4 core 1.5mm NYMJ cable of about 8 metres. 2 cores to power the valve on and two for the switch. Would it help if I replaced the switch wire with T+E ?
I am feeding the switched live to a PLC input. All is fine with one valve connected, but when the 3 valves are fed back to the PLC input, the combined stray voltage is >100v, which is enough to turn on the PLC input.(I want the input on when either valve is on).
Any ideas on how to fix this problem. (I don't have any spare inputs on the PLC)

 

[Spark123]

Put a resistor across the PLC input to sink the voltage, start with say a 100k ohm resistor and see if that does the trick.
Or you could run the cable back to a relay and use the dry contacts to feed the PLC.

 

[making_1998]

Will try the resistor option first. Any particular type of resistor ? Will the resistors in attached link suffice. http://www.maplin.co.uk/Module.aspx?ModuleNo=42453&criteria=resistor&doy=8m8

 

[Spark123]

If you work it out from when a 100k resistor has 240v across it then it will need to dissipate 0.576w hence I'd be looking at a 0.75 watt resistor minimum, the ones in your link are 0.25w so are no good.

D100K here: clicky would be a safer bet.

 

[making_1998]

100K 2W Resistor across the input solved the problem. Thanks for your help.