To determine what type of protection is required on HVAC equipment we must first look at the application. National Electric Code (NEC®) which is a part of The National Fire Protection Association (NFPA) or also known as NFPA 70 and ANSI/NFPA 70 has definitions for all the types of protection that is used in hazardous or classified locations such as:
Explosion Proof
Electrical equipment housed in an enclosure that is capable of withstanding an explosion of a specified gas or vapor that may occur within it and of preventing the ignition of a speci- fied gas or vapor surrounding the enclosure by sparks, flashes, or explosion of the gas or vapor within and that operates at such an external temperature that a surrounding flammable atmosphere will not be ignited thereby.
Dust Ignition Proof
Electrical equipment enclosed in a manner that excludes dusts and does not permit arcs, sparks, or heat otherwise generated or liberated inside of the enclosure to cause ignition of exterior accumulations or atmospheric suspensions of a specified dust on or in the vicinity of the enclosure.
Dust Tight
Enclosures constructed so that the dust will not enter under specified test conditions.
Purged and/or Pressurized
Purged is the process of supplying an enclosure with a pro- tective gas at a sufficient flow and positive pressure to reduce the concentration of any flammable gas or vapor initially present to an acceptable level. Pressurized is the protected enclosure shall be constantly maintained at a positive pressure of at least 0.1 in. of water above the surrounding atmosphere during operation of the protected equipment.
Intrinsically Safe
This protection technique shall be permitted for equipment in Class I, Division 1 or 2; or Class II, Division 1 or 2; or Class III, Division 1 or 2 locations.
These definitions set the criteria that must be met by all components installed in hazardous (classified) locations.
Temperature Rating
The maximum surface temperature of the exposed surface of electrical apparatus must always be lower than the ingnition temperature of the gas or vapor mixture. Temperature classes are: T1, T2, T3, T4, T5, and T6.
Maximum Surface Temperatures for each are:
T1 = 842°F , 450°C
T2 = 572°F , 300°C
T3 = 392°F , 200°C
T4 = 275°F , 135°C
T5 = 212°F , 100°C
T6 = 185°F , 85°C
Equipment that does not exceed a maximum surface temperature of 212°F (104°F ambient temperature) is not required to be marked with a temperature (NEC).
