Three questions, asked in a fixed order, and the order is the whole method
Class says what the hazard is. Division says how often it is present. Group says how bad it is. Answer them in sequence and Article 500 stops being intimidating.
On this page
Class: what the hazardous material is
- Class I is flammable gases, vapours and liquids.
- Class II is combustible dust.
- Class III is ignitible fibres and flyings.
Three classes, in order of how the material behaves. A question naming a substance is telling you the class before it tells you anything else.
Division: how often it is there
Division 1 means the hazard is present during normal operation. Division 2 means it is present only under abnormal conditions, such as a container failing.
That single distinction drives the equipment requirements, because Division 1 assumes the atmosphere is explosive while the equipment is running.
A question describing a room where a process routinely releases vapour is Division 1. A question describing sealed containers that could leak is Division 2.
Group: which specific material
Within Class I and Class II, groups subdivide by the specific ignition characteristics of the material.
You do not memorize which substance is in which group. You look it up, and the article and its informational material tell you.
What is worth knowing is that the group affects what equipment is listed for the location, which is why it comes third rather than first.
Why bonding changes here
In any classified location, the continuity of metal parts has to be ensured by specific bonding methods regardless of the system voltage, and regardless of whether a wire-type equipment grounding conductor is present (NEC 250.100).
That is a genuine departure from the ordinary rules, where the voltage threshold and the presence of a grounding conductor both matter.
It is also reliable exam material because it is an exception to a rule people have just learned.
The zone system exists too
Alongside the class and division system there is a zone classification system, and a facility may be classified under either.
You are unlikely to be asked to convert between them. You may be asked to recognize that both exist.
What this page cites
- NEC 500.5 Classification of locations by class and division.
- NEC 500.6 Material groups.
- NEC 250.100 Bonding in classified locations, regardless of voltage.