tgreif,
I too am having problems with your descrition of "capacitive coupling effect" preventing a contactor coil from dropping out, as you described it. I think the other posters here were describing the effects of voltage drop preventing the circuit from being able to energize or hold the circuit in, but I can't see how you could ever have enough capacitance in 2 control wires to hold a contactor in when the power has been removed by opening a PB contact. There might be a brief, and I mean millisecond, delay in interrupting the circuit, but surely it would have to be one hell of a capacitor to maintain enough energy on the contactor coil to keep it energized, and even then it would not be continuous!
What is a common occurrance however is an INDUCTIVE coupling effect on control circuits when proper attention was not givien to the wiring system design. If you run the control wires in the same conduit as the 480V motor power wires, and the distance is long enough, and the contactor coil seal-in power is low enough, and the control conductor resistance is low enough, you may have the 480V induce enough voltage on the control wires to maintain the contactor coil closed even if you open the source voltage with the PB contact. I have seen this happen a lot. Inductive coupling is essentially a transformer effect from the higher to the lower AC voltages. The right solution is to keep them in separate conduits, the magnetic fields can't penetrate very far. Switching to a DC control relay circuit as you have done will also do the trick because you can't induce DC voltage.
To that end, there is no hard and fast rule as to how long the distance is before it becomes a problem. As I noted above there are many factors that go into it. For instance if you have a large contactor coil with a high sealed current draw, it would be virtually impossible to induce enough energy on the control wires to keep it in. If however you run the 3 wire start/stop circuit through a small 120V pilot relay controlling a large motor circuit, it is very easy to have that problem.
Safe bet? Don't combine the wires.
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