4-20mA signal plus VFD motor cables in shared conduit 13

[bentov]

I know it's considered bad practice, question is practicality & likely worst case trouble arising . . .

We retrofit small (5-15hp 240V) ag drainwater pumps with VFD's (PWM, V/hz) & 4-20mA/24VDC 2 wire level transmitters (pressure membrane piezoelectric transducers), run PID for constant level control. The typical setup is a pump panel, short underground run (10-40ft) of PVC conduit to the sump location containing motor conductors plus existing LL conductors (to float switch or induction relay probes). We've done 5 so far, pulled shielded sensor cable through the existing conduit, seem to be working OK.

I see quite a spectrum of practice/suggestions on this, from 6" minimum separation/all steel conduits/shielded motor power conductors all the way to a suggestion that 4-20mA signals are bullet proof, don't care about higher frequency interference. Our experience tends towards the latter, but I definitely haven't put a scope to it, also don't camp out with these installations or log them for hiccups - I figure components are inexpensive, consequences of inadvertent operation are minimal, what the heck, let's see what happens.

Now though, we might ramp up, do lots of these and also some new complete installations. I'd hate to have to go back to 100 sites for warranty rework (especially if the transducers suffer damage somehow, most expensive part of the whole setup), but also don't want to add (significant) cost by insisting on separate conduits, different material, etc., if in fact it isn't important for this application.

I'd be grateful for any suggestions from those of you with experience in similar applications.

 

[ScottyUK]

Do Belden have a vested interest in selling expensive VFD cables

Shielded cable does a pretty good job of suppressing capacitively coupled interference. It has limited impact on magnetically coupled interference. At a point 'remote' from the cable the magnetic fields of the conductors interact and largely cancel each other, but 'near' the conductors (i.e. touching or close to the cable sheath) the local magnetic fields do not fully cancel in the immediate vicinity of the conductors. Having a signal cable in the area 'near' the cable is inviting problems. The best antidote to magnetically coupled interference is distance, while other options such as mu-metal screening are effective but expensive, or just impractical.


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