Analog, 24v dc, and 115ac in the same conduit

Tom,
I never meant to say that separate conduits for different voltages are not useful - they certainly will help to reduce the effects of induced voltages and EMI/RFI. My point is rather that the termination cabinets should not be ignored (as they often are) just becasue the separate conduit method is used. In some situations the benefit of the separate conduits can be nullified by mixing unshielded cables inside the temination cabinets. The deluxe method is to use separate conduits, shielded cables, and then keep different voltage equipment separated inside the control panels as much as possible. If you are separating your devices and wireways, then this is probably the reason that you have never had a problem with induced voltages.

As you said, EMI/RFI noise is peculiar. It is certainly dependent on the frequencies involved, the arrangement of the wires, size of the cabinet, and so on. At 60 Hz the problems are different than at 400 HZ, for example. I have seen reports that whether or not you should ground both ends of a shielded cable strictly depends on the frequency level.
 
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