×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Multiconductor Analog Cable - in China

Multiconductor Analog Cable - in China

Multiconductor Analog Cable - in China

(OP)
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?

______________________________________________________________________________
This is normally the space where people post something insightful.

RE: Multiconductor Analog Cable - in China

I had you right up until:

Quote:

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

RE: Multiconductor Analog Cable - in China

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.
 

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

RE: Multiconductor Analog Cable - in China

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
Have you read FAQ731-376: Eng-Tips.com Forum Policies to make the best use of Eng-Tips Forums?

RE: Multiconductor Analog Cable - in China

(OP)
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)?

______________________________________________________________________________
This is normally the space where people post something insightful.

RE: Multiconductor Analog Cable - in China

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.
 

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

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources