Zone 0 — Continuously Present Explosive Atmosphere
Zone 0 is the highest risk hazardous area classification for gas atmospheres. An explosive gas-air mixture is continuously present, or present for long periods. Zone 0 locations are rare in most industrial facilities but must be correctly identified and equipped.
Typical Zone 0 Locations
- Inside storage tanks containing flammable liquids — the vapour space above the liquid
- Inside open containers of flammable liquids
- Inside enclosed ducts carrying flammable vapour
- Inside process vessels where flammable gas or vapour is present continuously
Equipment Requirements for Zone 0
Zone 0 requires EPL Ga equipment — the highest level of protection. This means equipment that remains safe even with two independent faults occurring simultaneously. In practice, Zone 0 electrical equipment is very limited:
- Ex ia intrinsic safety (EPL Ga) — the most common choice for Zone 0 sensors and transmitters
- Ex ma encapsulation (EPL Ga)
- Specially certified Ex d equipment for Zone 0
Standard Ex d (EPL Gb) equipment is NOT suitable for Zone 0. Installing EPL Gb equipment in Zone 0 is a serious non-compliance. In practice, almost all Zone 0 instrumentation uses Ex ia intrinsic safety. If you encounter electrical equipment in a Zone 0 location, verify its EPL on the certification marking before assuming it is compliant.
Zone 0 in Practice
Most electrical workers rarely encounter Zone 0. The majority of process plant Zone 0 locations are instrument-only — a thermocouple, level transmitter, or pressure sensor installed directly into a vessel. Installation is typically carried out by instrument technicians holding CompEx Ex05.
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