Multiconductor Analog Cable - in China 1

[controlnovice]

I'm on a shutdown in China now and just discovered that they ran individual analog cable (twisted pair with shield) from the instruments to a junction box.

From this junction box they run multiconductor cable to the DCS cabinet. The multiconductor cable has 10 pair, but with an overall shield. The individual pairs do not have their own shield.

On the terminals inside the junction box they tie the multiconductor overall shield to one terminal for shield, and jumper all of the other terminals for the shields to this one. The individual instrument shields are tied on the opposite side of the terminals.

I don't think this is acceptable. But of course, we need to start up in a few days and we can't get the correct multiconductor cable (we had to wait 4 days just to get this cable from what they told me).

I think this will lead to much interference in the signals. We are running the multiconductor about 25-30 meters. What options are there? Wait for the correct cable and loose production? Purchase the cable now and cross our fingers that the existing will work until we get the new stuff in (then take another outage)?

What else?

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[itsmoked]

I had you right up until:

On the terminals inside the junction box they tie the multiconductor overall shield to one terminal for shield, and jumper all of the other terminals for the shields to this one. The individual instrument shields are tied on the opposite side of the terminals.

Why this second sentence. ?? Now I'm confused.

If they tied all the instrument shields to the one multicable shield while not optimum it will probably work okay.

If by that second sentence you mean they also tied instrument grounds to the this same multiconductor shield.. that will be a problem.

If the shields are well grounded at the instruments you may be able to not connect these shields at all and be okay.

What kind of manufacturing are we talking? Thermal stuff or spot welders everywhere? VFDs?

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[ScottyUK]

What are the signals involved? 4-20mA is pretty robust and might well be ok for service; 0-10V or an AC signal - e.g. an LVDT - will probably be troublesome at some stage.


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[Ashereng]

One option is to go ahead and start up. You don't really have much to lose (how much is one day of production)?

Once you have the correct cable, you can hot cut-over, one instrument (one pair) at a time. Put the affect loop on manual control, cut-over, and then put the loop back on automatic control (or whatever it was). One pair at a time.

"Do not worry about your problems with mathematics, I assure you mine are far greater."
Albert Einstein
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[controlnovice]

Thanks for all the replys. We're going to order the proper cable if we can (we'll find later today). But for now, we're going with what was installed.

The engineering contractor is adamant that this is no problem and that it's how it's done in China.

If the shield around a single twisted pair is to protect against interference from other signals, is there not a concern with interference from the adjacent 4-20mA singals that are in the same cable (if they don't have individual shields)?

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[ScottyUK]

Electrostatic shielding is only effective against capacitively coupled interfering signals. It has little or no effect on inductively coupled interference because that passes straight through the shield unless you make the shield out of mu-metal or something similar. It is difficult to capacitively couple anything significant into a low impedance loop such as a 4-20mA circuit unless you have a very powerful source. Low impedance circuits are susceptible to inductively coupled interference but this is controlled by the use of twisted pairs which effectively reverse the polarity of the induced signal with each twist so they sum to zero.


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Sometimes I only open my mouth to swap feet...