Can you run 4-20mA analog & 125VDC contact sensing circuits as shielded prs in same multipaired

Question

Can you run 4-20mA analog & 125VDC contact sensing circuits as shielded prs in same multipaired

bdn2004 (Electrical)

(OP)

13 Apr 13 11:36

I know the conventional design is to run digital signals in one cable, and analog signals in another cable. I question why this is so if the individual pairs are shielded?

In our particular case we have some signals we want to pick up from about 500' away and there is a cable with individually shielded twisted pairs already in place. These signals are either just contact sensing - no power or outputs, or 4-20mA. I suppose the concern is induction onto the 4-20mA from on/off....?

itsmoked (Electrical) · Answer 1 · 2013-04-14 03:14:05

Yes digital and analog in that same cable over that distance will likely prove to be problematic. Don't do it. The shielding is only for capacitive, electric effects, it is useless against magnetic effects.

If you don't care if the analog values hop around during switching of limit switches or whatever, then you could probably 'get away with it'.

Keith Cress
kcress - http://www.flaminsystems.com

dpc (Electrical) · Answer 2 · 2013-04-15 13:43:13

Agree with Keith. The switching transients associated with 125 V dc control circuits can be substantial. Twisting helps some, but it makes sense to avoid self-inflicted problems when there are so many other problems out there to deal with.

Best approach is to put the 4-20 mA level stuff in a separate metallic conduit.

automatic2 (Industrial) · Answer 3 · 2013-04-18 21:39:52

500' feet is a long parallel run. Your looking for trouble. However, your translation to output can be effectively filtered with many controllers. Mind code as cables within a conduit must not be from seperate sources.

automatic2 (Industrial) · Answer 4 · 2013-04-18 21:42:02

I reread and see it's DC running alongside. You should have no problem as the most basic filtering will likely suffice.

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Can you run 4-20mA analog & 125VDC contact sensing circuits as shielded prs in same multipaired