4-20ma sensor loop - which leads is preferable?

[uvroy]

Hi.

I have a cable consisting of 9 leads(wires) with a length of about 100-120 meter. 2 of the leads will be connected to a 4-20ma loop (pressure sensor). I have 3 spare leads, 2 of which is coax core + braid/shield. The last one is a 16 awg / 1,35mm2 lead.

What would be the best solution?
As i

1.
4-20ma supply -> coax core
4-20ma return -> coax braid/shield

2.
4-20ma supply -> 1,35mm2 / 16 awg lead
4-20ma return -> coax braid/shield

3.
4-20ma supply -> 1,35mm2 / 16 awg lead
4-20ma return -> coax core

 

[hacksaw]

Separate pair for each transmitter circuit. The outer shield is only for shielding, typically grounded at the sending end.

You want to avoid field connections for the coax shielding

 

[IRstuff]

Your description is confusing. Do you have one coax or two? IF the latter then 4-20 should be the two cores with shielding grounded on both, assuming the 4-20 is not in series with anything else. Otherwise, the return should be the least resistance connection; you otherwise run some risk of voltage compliance issues. Shields should not be used for current carrying purposes, regardless of whether other people violate this principle.

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[uvroy]

Sorry, I'll try to explain better. I will add 1 pressure sensor on this cable (2wire 4-20ma loop)

The cable consists of the following :
[ul]
[li]1.34mm2 STP[/li]
[li]0.5mm2 STP[/li]
[li]0.22mm2 TP[/li]
[li]1.34mm2 Single Core[/li]
[li]Mini Coax[/li]
[/ul]

Most of the conducters are in use, so what I have available is:
[ul]
[li]1.34mm2 Single Core[/li]
[li]Mini Coax[/li]
[li]Shield from the 0.5mm2 STP[/li]
[/ul]

 

[danw2]

Over the years, I've seen 4-20mA run over bell wire, romex, zip cord, automotive trailer cable, flat ribbon 4 conductor antenna rotor cable, cable TV coax, and some other cabling I couldn't identify. In most cases, the loop was functional. There have been a couple cases of VFD induced noise in untwisted, unshielded pair where STP fixed the problem.

Given what you've got, I'd probably hold my nose and use the coax, only because one conductor from one cable and one shield from another cable would be an addition to my list of "How Remarkably Robust 4-20mA Is Given the Vagaries of How it Gets Installed" and coax is already on my list.

Belden says its mini RG-59 has a 23AWG core. A random web calculator says 120m of 23 AWG is only 8.2 Ohms and the average 4-20mA loop can tolerate 15-20 Ohms of cable resistance.

Not clean, but likely to function.

 

[uvroy]

Sorry if I'm not understanding it correctly.
Are you suggestion to use coax core+coax shield for sensor?
[ul]
[li]4-20 ma supply = Coax core (pin 9 in attached picture)[/li]
[li]4-20 ma return = Coax shield (pin 10 in attached picture)[/li]
[/ul]

Or just the core of coax + some other conducter, so in example :
[ul]
[li]4-20 ma supply = Coax core (pin 9 in attached picture)[/li]
[li]4-20 ma return = 1,34mm2 lead (pin 8 in attached picture, less resistance than coax)[/li]
[/ul]
Here I have set the return on 1,34mm2 which by calculations should have less resistance than coax.

 

[danw2]

Whichever one of the shields is copper (not Aluminum).

 

[hacksaw]

In your original post, you did not specify which transmitters have a common source.