Induced current or shorted wire

[Chris4u13]

I'm trying to figure out why two wires from my thermostat cable are inducing a current. Both units and both cables. When the unit is started I'll get about 6 to 10 volts AC between those two wires and it seems to increase when the blower turns on. Wires are connected to N/O setpoints and are not connected to any volage source during testing. Has anyone ran into a issue like this? I’m trying to eliminate that current because I’m using it for dc signaling.


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[HVAC_Marc]

If you disconnect the wires from the board (I know it's N.O.) do you still have the induced current? If yes, then the problem lies in where the wire is running to and from. If not, then the problem lies in the board.

Signaling cables are usually shielded. you may have to run different wires, too.

[Chris4u13]

Voltage can be read whether it’s connected or not. There is a static voltage and as 24v components get turned(relays,contactors), on voltage increases. By passing yellow cooling wire and turning on contactor from r shows no voltage increase. On those setpoints.current induced on green and white wires.


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[HVAC_Marc]

If it's truly disconnected from everything and you have just wires end to end, AND there's induced power, you need to start looking where the wire runs:

- next to ballasts
- along other high voltage wiring
- near transformers

and the like

[Chris4u13]

I’ve notice this same issue on another system under similar Conditions. It seems the extra non used thermostat wires within the same sleeve of the live ones induce voltage, I’ve read about 2 to 4 volts ac. Is this your similar experience?


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[HVAC_Marc]

There was a problem similar to that with Lennox thermostats. The wires often picked up "signals" from other wiring. The solution was to use either shielded cables or a very specific way of grounding the unused wires.