Two wire instruments powered using two pair cable

Hello anyone else seen this?

All twisted pair cables.
Multipair home run cable from Junction box to marshalling cabinet 10 pair x .8mm2 cable

Junction box to instrument 2 x 2 pair 1.13mm2 with both pairs connected in JB and instrument.

Pair 1 used as positive and pair two used as negative.

First time seeing such an installation and seems like overkill. Unless trying to save money on using small home run cable diameter and compensating for volt drop by using 1.13mm2 from JB to instrument.

Any thoughts?
 
Agree, it seems like overkill.

The home-run-to-JB is the longer run, isn't it? I2R voltage drop for 20mA is pretty low for a typical JB-to-instrument run, so I doubt was the consideration. But who knows?
 
Correct, home run to JB is the longer run. 200meters approx. Jb to instrument less than 30 meters so volt drop on the home run is the dominant factor and not really an issue. Can't figure the benefit of using slightly larger cable from jb to instrument and terminating both pairs at instrument and Jb by using double crimp use pair 1 as positive and pair 2 as negative. Seems very unorthadox as the black and blue cores in pair 1 from the two pair cable now has the colour blue at the positive terminal of the instrument. First time seeing this and has me baffled. Can't be for redundancy as home run cable is multipair and each pair is for one 4-20mA hart signal. Jb to instrument cables on skids are contained in metal cable tray and are all SWA cable so little risk of mechanical damage and all cables are meggered before install. Anyway who am I to argue, someone smarter than me has decided this is the standard to be followed and have a loop drawing showing same issued by a world wide known EPC company to follow.
 
The probability of using 2 pairs instead of single pair may be simply using what was readily available.
I've seen it installed at various sites, although generally the 2nd pair is tied back serving no useful purpose.
 
It sounds like this installation resulted in the positive and negative wires not being twisted together, making them more susceptible to common-mode noise.
 
The probability of using 2 pairs instead of single pair may be simply using what was readily available.
"Using what's readily available." Boy, does that cover a lot. I think I've seen 4-20mA wired with everything in Ace Hardware's electrical department (except lacquer coated motor/coil winding wire): Zip cord (typically used for US lamps), Romex, bell wire (solid, non-twisted pair), speaker wire. It's remarkable how resilient a current loop is.
 
The probability of using 2 pairs instead of single pair may be simply using what was readily available.
I've seen it installed at various sites, although generally the 2nd pair is tied back serving no useful purpose.
Agree, would have expected spare cores brought to earth as its Ex equipment in a hazardous area
 
"Using what's readily available." Boy, does that cover a lot. I think I've seen 4-20mA wired with everything in Ace Hardware's electrical department (except lacquer coated motor/coil winding wire): Zip cord (typically used for US lamps), Romex, bell wire (solid, non-twisted pair), speaker wire. It's remarkable how resilient a current loop is.
This installation is in the middle east so plenty of cable suppliers from different countries in the area. Its not a last minute decision to do the install like this, shown this way on the company standard typical drawings and project went through FEED design so plenty of time to order the cables. Even the Foundation Fieldbus installations use two pair as trunk cable. Again both pairs connected to the device coupler in the JB instead of using one pair and terminate spare pair to earth. Trunk cables again two pair and terminated same as Analog 4 to 20mA signals. Alls two pairs are quad formation so are twisted pairs.
 
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