indused voltage

It's also important that the cable shields actually be connected to something - your system ground or earth ground. You'll get the best shielding if you connect the cable shields to ground at both ends, however, this may cause ground loops to occur due to differences in potential between the two grounds. It's possible that the induced voltage you're seeing may be due to these ground loops.

I would recommend:

1) Check the shields on your cables. If you don't have shielded cables, replace the cables with shielded cables.

2) If the shields are not connected to ground, connect them at one end to ground and see what your results are.

3) If the cables are connected to ground on both ends, disconnect them on one end. Generally the end closest to the control circuit is the best place to connect the shield on the control cable. Generally the end closest to the power source is the best place to connect the shield on the power cable. But you can try either way and see which works the best.

More information is required...

What are the cables types used for the high voltage and control wiring? What is the voltage level of the "high voltage" system?

I think you are saying:

I have two cables in the same cable tray: one is high voltage and the other is a control cable.

With the control cable breaker isolated, I still read voltage in the control cable.

I agree with techno (and you) that you might have induced voltage.

Of course, if you have full, ordinary control voltage, then you may have a modified machine, in which the control breaker has been rewired. This can happen when someone (usually a nut case) decides that the PLC (for example) must not be shut off (because it has a hard time booting up), and so powers it from the line side of the control breaker, instead of the switched side. This can lead to a dangerous condition for maintenance workers who are not careful enough to check voltage before beginning work.

But if the voltage is low, it is very likely being induced. If you wanted to verify that, you could put a light bulb (of the appropriate voltage) across the control voltage cable. If the bulb lights, there is probably something other than induction at work.

Please don't laugh at this but it could be also virtual voltage, I must agree with Ken on this one. Try putting a load on it, lamp or a resistor and recheck voltage. Also depending on frequencies and voltages it could be a result of ferro resonance. I recommend putting a pull down resister on the control cables at there termination point. If the voltage is coming and going, try putting .047mf caps on the ends of the shielding, it almost sounds like a standing wave reflection from the tray itself.OK now you can laugh.

Control systems in commercial laundries have problems with induced voltages even when only switching 240 v AC.

I have found empirically that a signal wire which has been switched 'open' at one end of the wiring loom can be considered to be effectively connected to 240v ac via a 2.5meg ohm resistance.

Despite my best endeavours I can't persuade the guys who wire up the panels to make the wiring switch down to some fixed voltage, even via a load... hence a wire which should have no signal voltage will often measure between 90 and 170 V on a DVM.

PS.

To ensure the floating voltages don't trigger our circuits (opto isolated inputs) we put 150K 1W resistors across the signal....this keep it down.