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Explosion proof, according to the National Electrical Code, applies to an apparatus enclosed in a case that is capable of withstanding a gas or vapor explosion. It means that, should there be an explosion, it will be contained within an enclosure.

The definition from FM Global says:

Intrinsically Safe Circuit:  A circuit in which any spark or thermal effect, produced either normally or in specified fault conditions, is incapable of causing ignition of a mixture of flammable or combustable material in air in the mixture's most easily ignited concentration. 

Therefore, Intrinsically safe means that an apparatus, such as a temperature transmitter is not capable of causing an explosion.  Explosion proof means that should an explosion occur, it will be contained within an enclosure.

MktgGenius wrote:

If you are not sure which is required, you can use intrinsically safe devices in an explosionproof assembly and cover all the bases. Typically, a barrier is also required; companies like Stahl, Pepperl & Fuchs, and MTL make those.

What exactly is a barrier? Why is it needed?

Color me "confused". How does a spark NOT cause "ignition of a mixture of flammable or combustable material in air in the mixture's most easily ignited concentration"? I thought that intrinsically safe meant that the energy to a circuit was limited, so that a spark could never form in the first place. I thought that the function of the barrier was to prevent accidental voltage spikes from propagating to the hazardous area.

We have both here(United States) with processes and equipment from all over the world SO we've had to learn both. Intrinsic safety basically means one can go into a plant, disconnect the IS wiring(blue colored here), make and break the wiring connection all day long and nothing will go KA-BOOM  because there is not enough energy. Here are some links for reading:

One GOOD one I use to explain it to new designers when they ask "Why do I have to....?". Note the wiring layout with dividers as the separation/barriers ARE important to prevent wire placement making the I.S. stuff NOT safe!

http://www.automation.com/pdf_articles/Applying_Intrinsic_Safety.pdf

From the WalMart of instrumentation:

http://www.omega.com/techref/intrinsic.html

From P&F(with advertisements!):

http://www.am.pepperl-fuchs.com/techresources/pdf/IS_in_a_nutshell-InTech-Dec2002.pdf

Since this facility is in the U.S., we have more ex-proof than I.S. installations. One overseas trip to Germany opened my eyes to the advantages of I.S. as the plant had NO CONDUIT!! All wire was underground(control room to process area) and then in wireways(Europe's equivalent of a cabletray?). Lots of bare wires from wireways(overhead) to field devices. Their operators must be more "careful" on the job than some I've seen here .

Also about I.S.: make sure your plant handheld radios(if you use them) are labeled by the manufacturer as I.S or approved for use in hazardous areas so they don't cause a boom when used.

Cell phones are a gray area BUT a close friend has relayed a tale of a vendor service engineer getting flash burns when hydrocarbon vapors around a compressor caught fire when the vendor answered his cell phone. I've turned mine off before entering a process area after my friend's story and it is now a plant safety rule here. I also leave my cell phone in my car when pumping gas(Class 1 Div 1 mostly, nice dose of benzene exceeding PEL).

Explosion proof has been covered already in this thread. Hope this helps.

Gilligan wrote:

Color me "confused". How does a spark NOT cause "ignition of a mixture of flammable or combustable material in air in the mixture's most easily ignited concentration"?

Some sparks or thermal effect are incapable of causing ignition of a mixture of flammable or combustible material in air in the mixture's most easily ignited concentration.  FM standard FM3610 contains appenidices B, and C that deal with quantifying a spark.  The spark ignition test is outlined in section XII of FM3610.

(http://www.fmglobal.com/approvals/resources/approvalstandards/3610.pdf)