Best way to implement redundant cabling on 4-20mA

I would assume a true redundant system would have a different input (different input card or rack). Landing to the same input seems more problematic than anything else.

I would agree, we basically have a device with a single 4-20mA output. Due to size restrictions we cannot put a second signal conditioner in to get a second true 4-20mA output.

I could potentially feed the same 4-20 input into separate PLC cards and use separate cabling, but it would always be a replication of the single 4-20mA input.
 
Cable failure is an order of magnitude less likely than sensor/input card/PLC failure/power failure, etc.

Barking up the wrong tree IMO. Just do a better job of installing and protecting the wire in the first place.
 
Cable failure is an order of magnitude less likely than sensor/input card/PLC failure/power failure, etc.

Barking up the wrong tree IMO. Just do a better job of installing and protecting the wire in the first place.

I think you haven't been exposed to certain industrial environments and possibilities to claim this. Also, protecting a single cable run isn't quite the same as protecting a bunch of cables. Which is a challenge with this application.

Ultimately, if the instrument is so critical, one more with a separate cable run connected to a separate input card would achieve far more than a second cable and protection.
 
Ultimately, if the instrument is so critical, one more with a separate cable run connected to a separate input card would achieve far more than a second cable and protection.

Yeah, that's what I'm saying.

Just curious though, what environment is this installation in?
 
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For example if you have two cables, and one gets damaged causing a short between say the 4-20mA and +VDC conductor. This will also be present on the second cable as they are essentially connected at the terminals at either end.

Therefore without putting fuses/diodes at the terminations at both ends, if one cable becomes damaged the second cable won't be of any use anyway?

Why do you not put fuses for any field equipment?
 
so a dead short would just pull the whole 24v system down to earth? or what ever the supply is? redundancy on cables or analog channels but no fuses? doesnt make sense to me.... but again im open to new ideas
 
Are you trying to make the supply redundant, the path of current redundant, the accuracy of the information being returned by the sensor redundant, or the ability to measure whatever the sensor is measuring redundant?

Each of those have different requirements.
 
Ultimately, if the instrument is so critical, one more with a separate cable run connected to a separate input card would achieve far more than a second cable and protection.

This I agree with, if you want true redundancy. 1 sensor with 2 cables (or any other combination of silly ideas) is just a fudge. Either do it properly or not bother.
 
so a dead short would just pull the whole 24v system down to earth? or what ever the supply is? redundancy on cables or analog channels but no fuses? doesnt make sense to me.... but again im open to new ideas

I didn't say that...

But if you put two current loops in parallel with fuse protection, your redundancy is only working when one of the loops opens, not when it short circuits as the short circuit on one loop would short circuit the other and blow all fuses on these lines.

So putting fuses, does defeat what was intended... the problem however is not the fuses being a poor practice, it's paralleling current loops that is.
 

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