4-20ma Cabling - 3Wire

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.

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.