4-20ma cable again...

[kiwisoundguy]

Hello all,

I have a 3 wire sensor (Ground, 4-20ma and +12v) which I normally run down 3 core (24awg) plus foil & drain wire to a display. The cable (50m long) runs along quite a few 3 phase power cables (6 x 415v 3ph, each pulling about 10amps each but do switch on and off quite regularly) This works out great with no noise issues, as expected.

However, I have a need to use this same 3 wire sensor but the only cable available to me is two core (24awg) plus foil & drain wire. This cables is 100m long and again runs along side the same 3ph mains cables. I have thought of two possibilities of utilising the cable but I would like some of your opinions...

Option 1:
Shield: Ground
Core 1: 4-20
Core 2: +12v

Option 2:
Shield: +12v
Core 1: 4-20
Core 2: Ground

I would test either set up but the cable is fixed in place and the end user needs it to installed rather quickly. Any thoughts as to the ideal solution with the cable available to me would be greatly appreciated.

Thanks alot!!

 

[cjd1965]

Any reason you cant use the correct cable? As a fudge I would use option 1

 

[kiwisoundguy]

I would love to use the correct cable but unfortunately the cable type is being dictated to me by others.

I thought option 1 might work best but my thoughts with option 2 were that the 4-20 signal and ground would be as close as possible to each other, i.e trying to simulate a twisted pair.

 

[marcinski]

Don't use any of the two solutions.
Get 3 conductor cable attach it to 2 conductor cable and use the 2 conductor cable as your fish line.
Your solutions can work on test bench but not in real life application. You should never use shield or drain as your signal or current carrying conductor.

 

[kiwisoundguy]

Thanks marcinski, I know that I shouldn't use either of the solutions that I have put forward but I have to find a solution with the cable that I have been given. There is no option to run another cable in as the cable run is inaccessible. Believe if I could, I would.

Short of powering the sensor locally, do you have another solution with the available cable?

 

[cjd1965]

Been thinking about this...What about some other idea..... like 4-20---rs232---4-20 converters ?

Just thinking outside the original question

http://www.bb-elec.com/Products/Serial-Connectivity/Serial-Converters/Current-Loop-Converters.aspx

 

[Bit_Bucket_07]

Use option 1 if you're going to cheat. Powering the instrument locally is the better option.

 

[Jeev]

Use the 2 core and just put 2 runs of cable in. Send your analogues down one and your power down the other.

 

[NevisGroup]

My two cents worth. (you are entitled to a full refund)
Is the sensor Loop powered or is it truly 3 wire. I have run into many of each. Check the docs on the sensor but it may work fine loop powered.
In that case option 1 is most correct. If not then (although counter intuitive) option 2 makes more sense. You want The current carrying conductors to have the greatest common mode rejection i.e. twisted pair.

Otherwise I would go with cjd1965's plan and think outside the box. Perhaps 4-20 from the sensor to proper 2 wire 4-20 isolated loop back to the other end. I think bb-elect has isolators.

 

[shooter]

use option 1, but disconnect the ground on the sensor side, so the cable can not have any ground loop.
no reason why this should not work.

 

[Bit_Bucket_07]

use option 1, but disconnect the ground on the sensor side, so the cable can not have any ground loop.
no reason why this should not work.

Seems to me that the instrument would have no power supply without a ground connection.

 

[rdrast]

Change the instrument to one that is loop powered.

 

[cjd1965]

Sounds like the OP is having to cover up someone elses mess, with no budget

 

[Helliana]

Would it be possible to use the shielded pair for the 4-20mA and the ground since that is the signal carrier, and use a spare wire for the +12 since it is just powering the sensor and noise isn't as big of an issue?

 

[Fredlaroche]

Here's what I would do:

Option 1 but with this subtlety:

Connect the ground pin of the sensor with a ground strap between your sensor and the closest ground you can find around the sensor. Sprira-band the ground strap with the cable and make it look clean.

Use shield as intended.