What happens to the zone classification if the ventilation fan fails?
Abstract
A correct classification already allows for fan failure through the availability of ventilation. A zone that relies on a fan needs good availability (standby fan, or an interlock that stops the release) or a more severe zone; loss of flow should alarm and trigger set actions.
If the classification was done correctly, the drawing does not change when the fan stops, because fan failure was already allowed for. IEC 60079-10-1 rates the availability of ventilation as good, fair or poor, and a less reliable fan leads to a more severe zone or an added surrounding zone. What must not happen is a classification that assumes the fan runs and has no plan for when it does not: then the actual atmosphere can exceed what the installed equipment was chosen for.
Short answer
- Availability is part of the classification. Good means ventilation is present virtually continuously; fair means it is expected in normal operation with only rare, short interruptions; poor means neither, but without long interruptions.
- A single fan that is running and well placed is usually rated fair. Good availability normally needs an automatic standby fan, or flow monitoring that stops the release (for example closes the gas supply) when the flow is lost.
- Lower availability changes the zone: for a primary release with medium dilution, good gives Zone 1, fair or poor gives Zone 1 surrounded by Zone 2; for a secondary release with high dilution, poor turns non-hazardous (Zone 2 NE) into Zone 2.
- Ventilation that does not even reach "poor" is ignored in the assessment, which means low dilution.
- Rooms that depend on fans need flow monitoring, alarms and defined actions on failure. IEC 60079-13 sets these out for artificially ventilated rooms.
How the standard builds fan failure into the zone
IEC 60079-10-1 asks two separate questions about ventilation: how well it dilutes a release when it works (degree of dilution), and how reliably it is there (availability). The zone table combines both with the grade of release. The rows that change with availability:
| Grade | Dilution | Good | Fair | Poor |
|---|---|---|---|---|
| Continuous | high | non-hazardous (Zone 0 NE) | Zone 2 | Zone 1 |
| Primary | high | non-hazardous (Zone 1 NE) | Zone 2 | Zone 2 |
| Secondary | high | non-hazardous (Zone 2 NE) | non-hazardous (Zone 2 NE) | Zone 2 |
| Continuous | medium | Zone 0 | Zone 0 + Zone 2 | Zone 0 + Zone 1 |
| Primary | medium | Zone 1 | Zone 1 + Zone 2 | Zone 1 + Zone 2 |
| Secondary | medium | Zone 2 | Zone 2 | Zone 2 |
The "+" zone covers the periods when ventilation is reduced or lost. With low dilution, availability no longer matters: the result is already the most severe (Zone 1 or Zone 0 for primary and secondary releases, Zone 0 for continuous).
Ineris guidance (2022) adds that ventilation which does not reach even poor availability should not be counted as contributing to the ventilation of the location, so the assessment falls back to low dilution.
Raising availability: the usual measures
| Measure | Effect |
|---|---|
| Flow switch or pressure switch with alarm | failure is noticed at once; if the alarm triggers a defined stop of the release, Ineris accepts it as a route to good availability |
| Duty and standby fans with automatic changeover | ventilation continues after a single fan failure; can support good availability |
| Interlock that isolates the release on loss of flow | no release without ventilation; can support good availability |
| Interlock that isolates power to non-Ex equipment | equipment cannot ignite gas that builds up after failure |
| Fans and power supply covered by maintenance and testing | without proof testing, no availability claim is credible |
HSE guidance on compressor houses and gas turbine units makes the same point from the other side: where mechanical ventilation is used to reduce hazardous areas, the safety case should show the ventilation performs as designed and should consider the consequences of a loss of power.
For rooms protected by artificial ventilation (protection "v"), IEC 60079-13 makes the designer define the actions taken when ventilation fails. As summarised in the catalogue entry for the draft next edition, they depend on the protection level needed inside the room and on the equipment installed, and include alarms, isolation of power, isolation of the flammable source, or backup ventilation where permitted. The 2017 edition is current; the new edition passed its European enquiry in 2026.
What to do when a fan actually fails
The response belongs in the operating procedures, and the classification report should say what it is. Typically: the alarm alerts operators, the standby fan starts or the interlock acts, the release source is isolated if the design requires it, non-essential electrical equipment that is not suitable for the more severe zone is switched off by the interlock rather than by hand, and nobody enters the room until the atmosphere is confirmed safe. If the room is gas-detected, the detector reading decides re-entry.
Example: A gas analyser room has a sample point that is vented into the room for a few seconds several times a day in normal operation, a primary grade release, and one well-placed extract fan without flow monitoring. With the fan running, the assessment finds high dilution. A single running fan gives fair availability, so the zone table gives Zone 2 around the sample point: electrical items inside that zone need EPL Gc or better. The owner adds a flow switch, an automatic standby fan and an interlock that closes the sample supply if both fans fail. Availability is now good, the release can be treated as a zone of negligible extent (Zone 1 NE, non-hazardous), and ordinary equipment can be used, provided the fans, switch and valve are tested and the reasoning is documented. Compare a room with only secondary sources such as flanges: with high dilution, the single running fan (fair) already gives Zone 2 NE, and only poor availability would make it Zone 2.
Common mistakes
- Rating availability by fan quality. A reliable fan with no standby or monitoring still gives fair availability at best.
- Interlocks that are never tested. An interlock is credited in the classification only if it is proof tested and maintained.
- Fans and detectors not suitable for the failed state. Safety equipment must be suitable for the zone that exists when ventilation is lost.
- Bypassing alarms during maintenance without temporary precautions such as stopping the release.
Related on excodex
- Ventilation in area classification: degree of dilution and availability
- Pressurised rooms and analyser houses: making an area non-hazardous by ventilation
- What is the difference between natural and artificial ventilation in area classification?
- What happens when pressurisation fails in a pressurised room?
- Standards spaces: IEC 60079-10-1, IEC 60079-13
Standards and further reading
- IEC 60079-10-1:2020 (edition 3.0), classification of areas, explosive gas atmospheres, with criteria for availability of ventilation: preview
- IEC 60079-13:2017, equipment protection by pressurized room "p" and artificially ventilated room "v"; scope of the draft next edition: standards.iteh.ai
- Choi and Byeon, Safety and Health at Work, 2021, reproducing the zone table of IEC 60079-10-1 edition 2 (2015), which the 2020 edition keeps as Table D.1: koreascience.or.kr
- Ineris, Principes de ventilation en zone ATEX (in French), 2022: ineris.fr
- HSE, Hazardous area classification and control of ignition sources: hse.gov.uk
This article is general technical information prepared by the excodex editorial team. It does not replace the applicable standards, the equipment certificates and instructions, national regulations or the assessment of a competent person.