AI and DI in the same Junction Box

[leomiranda]

I'm Electric/Instrumentation Engineer from Brazil and I work with projects for Oil rigs (Petrobras). Sometimes I see projects with analog and digital signals in the same junction box, so in the same multicable.
Can the bought signals (LIT and ZSH for example) "live" in the same multicable without problems, if all pairs are shield cover?
I have in my hands one skid that was made as I described above. The costumer does not fell sure about the following situation. Can I leave as described or I HAVE to change this?
Tks

Leonardo

 

[nbucska]

It depends how much errorrate your digital circuit can
tolerate. With errorcorrection theoretically it is possible
to disconnect the line for an hour and restore the missing
info using the error correction alone -- the trade-off is between the delay and redundancy vs. error tolerance.

The interaction can be reduced by using twisted lines
for both analog and digital. No interaction i.e. 0 error
probability is impossible so the design has to take
the effect of an error into consideration and mitigate them.



----------------------------
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[dpc]

You did not mention the analog signal levels or the type of digital signal...

But in general, for low level digital signals and 4-20 mA analog, if wiring is individually shielded twisted pairs, you should not have too much problem.

 

[nbucska]

... for a reasonable distance.
You can reduce the error probability:

For the interaction A--->D :
Reducing the A level and speed to minimum, use hysteresis
and debouncing technics in the D.

For D--->A:
Increasing the level of A.
Reducing the bandwidth of A and slowing down the edges of D.

Contradictory? well, this is what design is about.
Every engineering job is impossible at the beginning,
compromisses may make it possible.



----------------------------
Please read FAQ240-1032
My WEB: <

http://geocities.com/nbucska/>

 

[controlnovice]

If the AI and DI are all 24Vdc, then they can be in the same junction box without concern.

If they are mixed voltage, they should have separate entrances and exits, and they should be separated as much as possible in the junction box, but it can be done. In the states, ALL of the cable in the junction box has to have the same insulation rating.

So, if you are using 120Vac for the DI, this is usually 300V insulation. Many times the 24Vdc AI signals are 150V. If these are in the same J-Box, all of the cables have to be rated for 300V. (at least that's how I remember it)

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

 

[leomiranda]

All signals are 24VDC. Thanks!

 

[JLSeagull]

24 Vdc digital inputs to a DCS or PLC are not a problem when mixed with 4-20 mAdc. Don't run 120 V digital inputs or outputs with 4-20 mA. 24 Vdc digital outputs pull some current. Solenoid valves are not a problem.

I am one of the left overs who prefer 600 V insulation for all this stuff.