Wiring instruments 1

[garfio]

This a a totally simple question for you but I hope I can get your assistance.

I an classified area of a plant (Class I Div. 1), I have instruments with a intrinsically safe installation communicating with the DCS using Profibus PA protocol. The barriers are inside junction boxes.

I need to add a couple of instruments that are intrinsically safe 4-20mA. Is it possible to use the existing junction boxes and barriers or I need to install new for these instruments?

 

[danw2]

Insturments are installed with every instrument on its own I/S barrier.

 

[garfio]

Sorry but I dindn't understand the answer. I guess my question wasn't clear. I was wondering if I can wire the 4-20 mA instruments using the same junction box that I already have for Profibus instruments. The junction box has a barrier block with spare places that I could use, but wasn't sure if there is a restriction about mixing different signal in the same box.

Could somebody help me out?

 

[itsmoked]

Certainly you may use the same barrier box. BUT! You must add new barriers for each of your unique new signals.

Furthermore you would want separate circuits/barriers so a problem with one signal doesn't bring down others.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[JLSeagull]

itsmoked is correct. The intrinsically safe wiring cannot be run with any wiring that is not intrincically safe. We tend to use explosion-proof fittings, seals etc. instead of IS wiring. However you could use the same transmitters that I use but with the IS certification.

 

[itsmoked]

Yes! It is absolutely paramount that you do not EVER let IS wires share conduit, boxes, runs, anything with non-IS wires. As a mechanical problem could hook up an incendiary source to the IS wiring. (Picture idiot with a power drill.)

I was once asked to come in and upgrade a control system at a huge ETO sterilization facility. The sterilent, ETO, is rabidly flammable, only thing worse is Acetylene. I got there and removed the old control system and installed the new one. The barriers were in a wall mounted box all by themselves, tidily laid out. All the resulting IS wire's exited into an adjacent conduit box<don't recall the name of that type box..> that was about 6"x6" with a cover. Seemed excessive for those IS wires. I pulled the cover.. It was packed with 12AWG wires which I followed back to a 480V panel. They had to change a lot of stuff to fix that.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[danw2]

Keith,

Did you, by any chance, work at the Ontario, CA, where the ETO went boom ! ! ? ?

Chemical Safety Board has a video on the incident at

http://www.chemsafety.gov/index.cfm?folder=video_archive&page=index

Ethylene Oxide Explosion at Sterigenics International, Ontario, California

Dan

 

[itsmoked]

That particular site was not the one I was referring to. I worked at several in Ontario but that one was a bit newer. I designed all the controllers used at Sterigenics. They made a classical set of mistakes there with predictable results. They blew the entire roof off the huge facility and destroyed all production ability. Several people were injured, luckily no one fatally. (Lesson 1: don't put large glass windows overlooking something that can explode.)

I can tell you precisely what went wrong but only if you actually care.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com