A commercial kitchen is classified as a Class I, Division 1 hazardous location due to the presence of flammable vapors from cooking processes. An explosionproof enclosure is used for electrical equipment. According to NEC Article 500, what is the primary design characteristic of an explosionproof enclosure that makes it suitable for this location?
- AIt uses intrinsically safe circuits that operate at low energy levels.
- BIt limits the surface temperature to below the ignition temperature of the hazardous substance.
- CIt is designed to contain an internal explosion and prevent ignition of the external atmosphere.
- DIt prevents the entry of hazardous gases into the enclosure.
Show answer & explanationAnswer & explanation
Correct answer: C. It is designed to contain an internal explosion and prevent ignition of the external atmosphere.
NEC Article 500 defines 'explosionproof enclosure' as 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 specified gas or vapor surrounding the enclosure by sparks, flashes, or explosion of the gas or vapor within, and that operates at an external temperature that will not ignite the surrounding flammable atmosphere. The primary characteristic is containing the internal explosion and preventing external ignition.
Why the other options are wrong
- A. This describes intrinsic safety as a protection method, not an explosionproof enclosure.
- B. While temperature limitation is a characteristic, it's a secondary effect; the primary is containment and prevention of external ignition.
- D. This describes a purged or pressurized enclosure, not an explosionproof enclosure.
Explosionproof Enclosure Principle
An enclosure designed to contain any internal explosion of a hazardous atmosphere and prevent the ignition of the external atmosphere through its joints or by excessive surface temperature.
- Withstands internal explosions.
- Prevents spread of explosion to external atmosphere.
- Limits external surface temperature.
- Required in Class I, Division 1 locations.
Memory trick: Explosionproof: Keep the boom inside, so the outside won't ignite.