conduit practices in class 1, div 2 areas 1

[mike4168]

I am installing equipment in a hazardous (C1, D2) location. All the instrumentation is XP rated for this area. The enclosure is also located in this classified area. The enclosure is a NEMA 12 type with an argon purge. As I interpret my NEC manual, that should be suitable. I have used seals and meyers hubs to enter this enclosure. I have two "junction boxes" (NEMA 12, no purge)mounted closer to the action that are used simply as a manifold for routing cables to the individual instruments from a couple of conduits. Keep in mind that no terminations are made in these "junction boxes". If the terminal chambers on the instruments are XP rated (I believe that means they are rated to contain an explosion), do I need to use seals at the entrance to these terminal chambers? If so, then I have an alternate question. I have read a reply to someone's question concerning seal-tite and EMT conduit stating that EMT could not be used, and that seal-tite was limited to 6 feet for grounding purposes. If I use seals and supply a sufficient ground wire, then what is in between shouldn't matter, except for physical protection. Sorry I'm so long, but I do appreciate any advice.

 

[peebee]

I'm a little confused about the details of your installation. Is your "XP rated instrumentation" truely XP rated, or is it "intrinsically safe" power limited instrumentation? In either case, if it is "XP rated", why was it also installed in a purged enclosure?

If you're not clear on the intrinsically safe thing, this is a system that consists of a power supply that limits both voltage and current to levels below that required to ignite an atmosphere. In addition, all components fed by the power supply must have no energy-storing components (inductors or capacitors) capable of supplying sufficient energy to ignite the atmosphere. UL listed stuff. Intrinsically safe wiring can be installed in XP areas using general-purpose type wiring methods, although I believe seals are still required wherever you change Division.

For non intrinsically safe wiring:

Generally, seals are required in two locations: whenever you change classifications (Div 1/Div 2/Genl Purpose), and whenever you have an arcing device.

EMT is generally not acceptable.

Regular flex is permitted in Div 2. However, it is not permitted between the arcing device and the seal.

You are correct that the code would permit you to use flex longer than 6' with a grounding conductor. But I bet most AHJ's wouldn't let you. There's probably not much reason you would want to use such long flex, anyway, as 6' should be sufficient for working on your instrumentation.

 

[mike4168]

I guess I may not be as clear as I thought. The enclosure I speak of is the enclosure that the PLC and misc. relays, etc are installed in. The instrumentation, of course, is connected back to this control system. It is in the same room (lab)and within 6 ft. of the process. The process is contained within a vessel rated to contain the P max of the volatile component. The instruments I am using are just PT's, RTD's, and some I/P's with screw lids on the terminal chambers. I thought I understood that terminal connections are an explosion hazard, not just arcing contacts. So if I have a terminal strip, but no relays ,etc, in a junction box in this location would I need to seal and purge it? Please clear this up. Is it necessary to use breathers on all these devices where I have used seals?

 

[peebee]

It does not sound like you need to seal it. I'd need more detail to be more specific. Also, a quick read of NEC 501 is highly recommended.

 

[scott1971]

Mike:

I know I'm a Chem.E, but I've done a number of Class I, Div. 2 installations in my work. If I understand you correctly, your "field" components are all equipped with XP enclosures. So long as these components have no arcing contacts AND none of your wiring has quick-disconnects, you can use standard strain-relief type glands for your cables at these enclosures. Div 1 would require those Crouse-Hinds type sealed connectors. Check 501-5, Part B, Section 2, Exception No. 3.

You must be certain that there is no possibility of an ignition source in these enclosures. Even though you may have no making/breaking contacts, there might be fuses, lights, and other components that must be considered. You may have to go with the sealed fittings after all.

Another interesting requirement: If you have instrinsically safe circuits, make sure they are isolated from non-instrinsically safe circuits both inside and outside your enclosure. It is consided unsafe to run them parallel because the non-safe circuit could technically induce energy in the protected circuit eliminating its "instrisically safe-ness."

I recommed obtaining a copy of NFPA 496 (Standard for Purged and Pressurized Enclosures for Electrical Equipment.