The Black wire should be negative DC return in 4-20mA loops 1

[mbellow]

thread196-100316
John wrote that "It is common for 24 VDC 4-20 mA loops to use black and white pairs, typically with 24 VDC source on the black wire. Black and red is also used.

I would like to say that in over 27 years it has only been over the last few years that people have tried to promote using 'Black' as positive DC voltage and not as the negative DC return as it has traditionally been.

In asking around and reading comments, I believe that this trend is due to using electrical engineers for the designing and electricians to wire systems up and not instrumentation engineers and technicians.

What color is the negative (return)wire going to the DC battery in your car and used in American produced electronics?

Comments are requested.

 

[JLSeagull]

You correctly suggest that red is normally the positive connection on an automobile battery cable and that black is normally the negative connection. However, no clear color code is established for 4-20 mAdc wiring insulation by codes or standards.

Check ISA 5.4 for loop diagrams. The minimum content description note 4 advises to identify interconnections with numbers or colors. None of the drawing examples include colors.

ISA 12.06.01 lists light blue for conductor insulation, raceways, trays, JB's, etc.

PIP PCSCP001 mentions AC power to be color coded per ISA RP60.8; and thermocouple extensions to match the termocouple.

ISA MC96.1 1982 for thermocouples addressed the color code topic as follows:
“Much discussion was involved in the use of the color red to designated polarity, since red is used popularly in electrical circuits to indicate positive. No nationally-accepted code known to the committee covered this point. Research into manufacturers' records showed that, in thermocouple circuits, the red negative had been in use for more than forty years.”

For duplex thermocouple wire, red is negative for type E, J, K and T. For single conductor extension wires the negative is red with color tracers. Also, Black is positive for types J, R and S thermocouple extension wire.

Clearly little concensus exists regarding the conductor insulation color codes for 4-20 mAdc signal cables.

 

[xJAyx]

I assume this stems from the north american AC wiring convention of having white as the neutral and black as the hot wire.
For my projects I always insist that in DC circuits the black wire is the negative if it is supplying power or is single pair. In a multipair cable that is carrying milliamp signals, I find that it is actually better to have the black be the positive so that the individual signals are colour coded. If a lot of multipair cables are used with a black common wire between the pairs, it would almost make sense to standardize the whole thing and keep the black as positive. The proper solution would be to just select a multipair cable where the common colour in the pairs is red or white.

Thermocouple wire is not as consistent as you think, for example, green/white is nearly as common as yellow/red for type K thermocouples. Typically the rule of thumb with thermocouple wire is that the jacket colour matches the positive, however I have seen some german wire that doesn't even follow this convention, but it is their standard wire, not some random, poorly produced off the shelf type stuff. This just shows that colour codes are far from standard once you start breaking down country lines.