National Electrical Code Top Ten Tips: Article 504 -- Intrinsically Safe Systems
- Article 504 covers the installation of intrinsically safe
apparatus in Class I, Class II, and Class III locations (defined
in Article 500.5) [504.1].
- An intrinsically safe circuit is one that won't ignite due
to spark or thermal effect of (flammable or combustible) material in
air under prescribed test conditions [504.2].
- If it's part of an intrinsically safe installation, you must
install it per the control drawing(s) [504.10(A)]. The installation particulars are often part of what makes the installation intrinsically safe, not just the component selection.
- You can use a general enclosure with intrinsically safe
apparatus [504.10(B)]. Often with the NEC, it is good to exceed the requirements to get the added benefits. That is certainly the case with the receptacle requirements for branch circuits (woefully inadequate for normal use) and with conductor sizing in general. But it is not the caswe when it comes to choosing an enclosure for intrinsically safe apparatus. You will get zero additional benefit by choosing an explosionproof one over a general one.
- You can use any of the wiring methods suitable for unclassified
locations, when installing intrinsically safe apparatus [504.20]. As with enclosures, going the hazardous location requirement route won't gain you any benefit. Rather than waste money that way, exceed the minimum conductor sizes to gain real benefit.
- You must separate intrinsically safe conductors from
nonintrinsically safe conductors [504.30(A)]. This requirement is
the core of Article 504 and its details make up about half of
Article 504. Think about why this requirement exists.
- You must connect intrinsically safe apparatus, enclosures, and
raceways (if metallic) to the equipment grounding conductor
[504.50]. That is actually a bonding conductor (metallic path, extremely low impedance). Do not confuse this with a "ground" connection such as driving a ground rod and connecting an enclosure or other item to that. Ground is a high impedance path, and it won't remove differences of potential for that reason.
- Bond all metallic objects of an intrinsically safe system, per
250.100 [504.60]. This puts all metallic objects at the same potential, eliminating the possibility of flashovers.
- Enclosures that contain only intrinsically safe apparatus don't
have to be sealed (except as required by 501.15(F)(3)) [504.70 exception]. Now, this is a case where you might want to exceed the NEC requirements. You might seal the enclosure just t okeep out corrosive vapors. Or to prevent a dust build-up and the ensuing extra maintenance involved. But sealing will not give you additional benefits in regard to preventing ignition.
- Intrinsically safe circuits must be identified at terminal and junction
locations, in a manner that prevents unintentional interference with
the circuits during testing and servicing [504.80]. This is where a high quality label maker saves you time and money.