4-20mA wiring

We have some really long runs of current loop signals (700 ft. or more) that were all run with TPS cables. Needed to add some new sensors and ran out of TPS spares in the trays, so tried out with TP unshielded spares, and saw no issues, even with redundant high accuracy measurements side by side TP and TPS. Not the way I would have set out to do it, but we found it worked.

For short runs in a rack, have seen no issues with single conductors either.
 
despite what others will say, in my experience these sorts of short runs are not significantly affected by not using screened cable, provided you don't have high current / high noise generating equipment in close proximity.

The plc input card isn't installed in a faraday cage after all.

Field wiring, for sure - screened cable all the way to the device.
+2
 
Shielded twisted pair for field wiring is like chicken soup - it can't hurt. It is a cheap way to improve EMI/RFI protection. If you are lucky and don't run through any strong fields from motors or lighting ballasts you can get away without it. However, for field wiring it is good practice and adds little to cost.
 
My interpretation of original topic is that the wiring in question is internal in a control cabinet., from conditioners to input.

In that case I'd say no shielding/twisting necessary. Never had problems inside a controls cabinet if there's no VFD, magnetizing transformers or something like that in there. Only know one customer that practices twisted wires internally in a control cabinet. All others, no and never had any problems.

If it's fieldwiring we're talking, even if short, I'd definitely use shielded/twisted cabling.
 
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Originally posted by dogleg43:

For NOW there is only low voltage wiring in the duct and we all know that no one will EVER run anything else in it.

...and if I set a 6000lb steel ingot in the trunk of my Saturn the frame is going to break in two.

Design to the application, build to the design. There is really very little any of us can do to stop the end user who chooses to do something outside of the design envelope. There is no end to the depth of the rabbit hole we end up going down if we start designing and building to account for something someone MIGHT do that is outside of the design.

As a general rule we also use shielded cable only for runs outside enclosures. I have yet to run into issues with this method.

Keith
 
Lots of good responses, the ones that match up with my training and experience.

1. Short runs inside the panel, single conductor is ok, but keep it to a minimum length.

2. Twisted pair is effective, easy and low cost. The example with a shared common in a multi-core non twisted cable may work just fine in some cases and not in others. I'm thinking if some high frequency noise comes in on one of those signals, all the signals in that cable will get it.

3. Shielding is also effective, easy and low cost. But you have to be careful with installation. Ground one end only, period. If you don't ground it, it's less effective than non-shielded twisted pair. If it gets grounded more than once it can be seriously bad. Current flow through the shield couples very well. I have seen cases where a jacket got scraped by a conduit burr and the signal was reading more than 10% upscale until we lifted the intentionally grounded end. The cable was less than 30 ft. and was in rigid conduit. That the conduit connected to equipment on a different floor probably accounted for the difference in ground potential.
 

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