4-20ma cable question

[kiwisoundguy]

Hi Guys,

I know I have asked a similar question before about this, but I've another.

For a 3 wire sensor, (ground, +ve and signal) can the sheild in the cable also be the ground? +ve and signal lines are twisted. The cable in use is a 2 x 0.75 with sheild.

What do you think?

Cheers, Ben

 

[Tharon]

It should not.

In this case, "ground" is being used as the common point for the power to the sensor. Meaning that power going to the sensor would go back to the source over the "ground" wire. It is not just a physical earth connection.

This would mean that you'll have current flowing on your shield. And it would defeat the purpose of the shield.

 

[kiwisoundguy]

Thanks for that. I can see what you mean. I am unfortunatly restricted by the number of conductors in the cable. Could I swap the +ve and the 0v? For example the 0v and signal would be within the sheild and the power for the sensor carried on the shield. Better, worse?

 

[chrisk_81]

That's the same thing. Current flowing in the shield defeats its purpose. Your only choice is replace the cable.

 

[kiwisoundguy]

Hi Guys,

Thanks very much for the responses. It confirms what others have been saying also.

I've attached a small jpg with 3 wire optionsusing differnt cable. I woudl prefer to go with #3. Do you agree?

Can I tie the two 0v lines together - they come from the same PSU 0v rail.

Fig 1 and 2 have a foil shield which can not be terminated to
ground.

Would fig 2 be ok if I tied the two 0v lines together on the PSU side of the shield?

Fig 3 has a braided shield.

Sorry for the barrage of questions.

Many thanks in advance.

Cheers, Ben

 

[sthompson]

Even with a Foil Shield, there should still be an uninsulated Drain Wire in with it that can be terminated to ground.

Stu....

 

[kiwisoundguy]

So does that mean that all 3 examles wil be ok?

Tie the drain wire to ground and tie the two 0v lines together?

Cheers, Ben

 

[sthompson]

So does that mean that all 3 examles wil be ok?

Tie the drain wire to ground and tie the two 0v lines together?

Cheers, Ben

Yes, that should be fine if the +18VDC and +VE are the same Power Supply, or there -Commons are tied together, but my point was even the Cables with Foil Shielding should have a Drain Wire in them.

Stu.....

 

[kiwisoundguy]

Thanks stu, your a star!

So as a final pass by your learned selves, does the attached drawing look ok.

I've joined the 0v together before the sheild begins, tied the shield to ground and enclosed all the conductors within the shield.

Cheers, Ben