I have following question. Why do we often use Exi pressure and temperature transmitter in II zone (methane) with appropriate Intrinsic Safety isolators whereas total cost of combination Exi transmitter with Intrinsic Safety isolators mounted in control cabinet is more expensive than Exd transmitter mounted in the field alone?
Thanks in advance for answers!
Good Question, why would you use Intrinsic Safe, I work in related industry but we seldom do any more. It's not just the hardware, but to do it right special cable, dedicated trays, segregated panels.
Occasionally you strike a situation where Intrinsic Safe is the only approval you can get for a particular instrument. I'm looking at a guided wave radar in a tank with Hydrogen gas. I dare say the barriers will be somewhere close by.
I don't see Exi in the Methods of Protection list Ex ia, EX ib, Ex ic. There are so many variations in hazardous location designation. Can you share the location of the project, complete hazardous zone designation?
The cost of Exi vs Exd is not just the barriers. Exd requires much more expensive flameproof metal enclosures, cable seals and the use of metal conduit, in North America.
"In North America metal conduit must be used for field wiring. In Europe and elsewhere suitably rated cable is connected directly to the equipment using certified flame proof cable glands."
Most transmitters are certified as Exi, Exd and general purpose Nema 4X. so there is no savings with different transmitter certifications.
Exi is usually less expensive to install and easier to work on.
I doubt the instrument manufacturers use a different enclosure metal from one rating to another. I think you must be referring to USA when you say Nth America. Here in Canada we can use cable for Class 1 Zone 1 (Rule 18-100), and even tray cable for Class 1 Zone 2 (Rule 18-156).
You really have to be careful when specifying instruments for different parts of the globe. Korea is a good example everything has to be KOSHA approved. Only a small percentage of instruments are. It's often difficult to nail down the requirements at the start of a new project. That's why I asked where the project is located.