4-20ma Cabling - 3Wire

Sorry, but I am a bit confused by this. If you have a wire for the signal (i.e. carrying a current of between 4mA and 20mA) then where is the current going? Surely there needs to be a return current, so which is the return wire?

The benefits of twisted pair are lost on a 3-wire loop device, as there are no two wires where the the currents are the same, but flow in opposite directions.


3-core screened would be better.


Basically, the current in the common wire should sum to the total current in the remaining power wire and the transmitter (signal) wire, so these three should be as close as possible and within the same cable.


So, whilst 3-wire transmitters might be cheaper, they have their down-sides.

Graham,

Thanks for the explanation - I am now somewhat less confused, and your explanation makes perfect sense and seems to answer Les's question.

Thanks, excellent response Graham, I suppose a follow up question comes where a digital signal wire is run in the same cable.

• If that signal is run in a four wire cable with the three wire 4-20 device then could that in any way effect the 4-20 signal?

• And if using a two wire 4-20ma in a twisted pair then presumably the digital signal could be run in a single wire of the second pair without concern?

I assume you are talking about sourcing or sinking switched digital signals otherwise known as "volt-free contact" (VFC) type signals, rather than comms (which would require a separate twisted pair cable normally_?


It's not recommended to run that type of switched digital signal in the same cable as 4-20 mA loops as the VFC affects the 4-20 mA signal, especially 3-wire.


In the main, volt-free contact signals are usually run in the equivalent of the "3-wire" type arrangement (common + switched).


If you have to run them in the same cable, they should be separated by a screen - 2-wire approach recommended and each twisted pair in its own screen. The only exception to this is if you're using actual relay contacts/coils rather than semiconductor source/sink, as these can really foul up 4-20 mA signals, (and comms).

Of course, the previous post is "best practice" - and certainly if you've safety-related controls, and/or long runs it definitely applies.


Where there is only a short cable to a single piece of electronic equipment, it may be possible to run a common +, the 4-20 mA signal and 0 V, and a few semiconductor-switched VFC - IF this meets manufacturer's recommendations for equipment on BOTH ends.


SO, for example, where the manufacturer of the transmitter device is the same as the manufacturer of the controller, and their instructions say it's OK, the mixed signal approach would be OK.


But if you read, for example, installation instructions from a PLC manufacturer like Allen Bradley they will give you the advice I have regarding segregation of different I/O types, e.g.:


 - Keep VFC signals together in one cable (Segregated from other signal types)

 - Keep 4-20 mA and 0-20 mA (current loop analogue) together in one cable (segregated from other signal types)

 - keep 0-10 V (voltage signalled analogue) together in one cable (segregated from other signal types).


This is, of course, if you want their EMC certification to be valid, and also achieve the maximum cable distances or loop resistances stated in their manuals.