Created Date: March 06,2007

Created by: AZRoger

I think I remember someone giving a specific recommendation for shielded cable on this forum. I searched the archives and couldn't find it. So...

I have two current sensors in each of two circuit breaker power panels. The analog docs all say use shielded cable. I have one run that is about 50 feet and the other is about 15 feet. The solution I have in mind (I'm very open to suggestions) is Shielded Cat5 cable with grounding sockets at the CB panel and non-grounding sockets (the regular plastic ones) at the analog module end. I'd use Shielded Cat-5 Jumper cables (available pre-made in many convenient lengths) to connect the Power Panels to the PLC panel. Does this sound OK to those who have tried? Is regular Cat5 good enough? I suspect not. It's very easy to tell the difference between the 1s and 0s that normal flow on Cat5 without getting confused. Analog signals are something else.

Any advice would be welcomed.

Roger

Created Date: March 07,2007

Created by: Bent

Well, ideal would be a shielded twisted pair. With Cat 5 there's always the question of what to do with the extra wires. I would think that the next best thing with Cat 5 would be to use 4 of the wires for each line (assuming a 2 wire 4-20 circuit.)

However you do it, you should have each side if the circuit twisted with the opposite side going through the cable.

If you don't use some wires, it is probably best to ground the unused wires to the shield on the grounded side.

Nice thing about the low frequency analog is you don't have to worry as much about cable capacitance as with digital.

I 've not tried the Cat 5, but have done it with multiconductor, shielded twisted pairs as above with little problem.

For long runs with little good grounding between the instruments, I do favor isolated inputs or outputs to prevent issues. Then you can connect the shield to both sides (if it's got good isolation.) But then again, isolated analog tends to be expensive.

Regards,

BenT

Created Date: March 09,2007

Created by: AZRoger

Tom,

Thanks for the advice re: shielding. I bought a Belden cable that has two shielded pairs (one for each sensor). I could buy it in a 100 foot spool. To get exactly the one you mentioned I'd need to buy 1000 and have over 900 left over. The conductors are only 22ga which should be OK since my runs are short. I'll be converting the 4-20mA current to a voltage before the analog module (so I can put one transmitter signal into two ports). The resistor that grounds the signal at the module is a couple orders of magnitude larger than the wire resistance. To get it really precise, I can scale for loss when I do the engineering units conversion. However, the transmitter only guarantees 2% accuracy. So what's the point.

EDIT

I put 200 feet of wire in series with the sensor for a test. The results were exactly (16 bit exactly) the same as without the wire. The module just pumps out more voltage to deliver the required milliamps. Sweet.

END EDIT

Based on your comment about splicing, I'm going to try to do all the wiring with no splices. If I do need to splice, do I need to put the terminal strip in a metal box to maintain the integrity of the shield?

Thanks again for the advice.

Roger