The zone decides the equipment. Get the zone wrong in either direction and you either fail an inspection or spend money you never needed to.
| Zone | Explosive atmosphere present | Equipment category |
|---|---|---|
| 0 (gas) / 20 (dust) | Continuously, for long periods, or frequently | Category 1 |
| 1 (gas) / 21 (dust) | Likely occasionally in normal operation | Category 2 |
| 2 (gas) / 22 (dust) | Not likely in normal operation, brief if it happens | Category 3 |
Equipment suitable for a more demanding zone is acceptable in a less demanding one. The reverse never is.
Substances differ in how easily they ignite. Hydrogen and acetylene sit in the most demanding group, and equipment certified for a less demanding one is not suitable for them.
The maximum surface temperature the equipment can reach has to sit safely below the ignition temperature of your substance. This is the one most often overlooked.
Certification applies to correctly installed, correctly maintained equipment. A certified enclosure with a missing gland or a cracked window is not protected.
Handing an electrician a zoning drawing instead of a conversation is the difference between a job that goes smoothly and one that comes back with questions.
No, though they are two halves of the same picture. DSEAR is the UK duty on the employer to assess and control the risk. The equipment side sets out what protected equipment may be placed in each zone. In practice: DSEAR says where the zones are, the equipment rules say what may go in them.
The marking carries the equipment group and category, the protection concept, the gas or dust group, and a temperature class. All of it has to be suitable for the zone, the substance and the surface temperature that would ignite it.
You can, and people do, and it is an expensive way to solve a problem you may not have. Correct classification is usually the cheaper answer, because it often shows the zone is smaller than assumed.
Existing equipment in a zone has to be assessed for continued suitability. It does not automatically have to be replaced, but somebody has to have looked at it and written down why it is acceptable.
How the zones themselves are worked out.
The regulations behind all of this, in plain English.
A worked example of gas group and zoning in practice.
No charge for the conversation. Most quotes go out the same day.