What does 'Zone 2 NE' (negligible extent) mean?
Abstract
Zone 2 NE is a Zone 2 of negligible extent under IEC 60079-10-1: a secondary release so well diluted that, if it ignited, the consequences would be negligible. It is treated as non-hazardous for equipment selection, but must be justified and documented.
"Zone 2 NE" means a Zone 2 of negligible extent. IEC 60079-10-1 uses it for a secondary grade release that is diluted so quickly and so close to the source that the flammable cloud is tiny, and an ignition, if it happened, would have negligible consequences. On a drawing it means the location is treated as non-hazardous for equipment selection, but only because a documented assessment has shown that the conditions for negligible extent are met.
Short answer
- NE is defined in IEC 60079-10-1 as a zone so small that an ignition would have negligible consequences. Zone 0 NE, Zone 1 NE and Zone 2 NE exist; Zone 2 NE is the most common.
- In the standard's zone table, a secondary release with high dilution gives "non-hazardous (Zone 2 NE)" with good availability, and also with fair availability. With poor availability it gives Zone 2.
- The 2020 edition (edition 3) added a dedicated subclause on zones of negligible extent. As summarised in a 2023 HySafe paper, its natural gas example is a cloud of average concentration 50 % of the LFL smaller than 0.1 m³ or 1 % of the enclosed space, whichever is smaller.
- Ignition of the cloud must not cause harm through overpressure or heat. For releases from systems above 20 bar gauge the standard does not allow NE without a specific detailed risk assessment.
- Ordinary (non-Ex) equipment may be used in a Zone 2 NE location; safety equipment that must work during a release is still often specified for the hazardous area.
Where Zone 2 NE comes from
Area classification under IEC 60079-10-1 runs from source to zone: grade of release, release rate, degree of dilution, availability of ventilation, then zone type and extent. When the release is small and the air movement strong, the dilution chart gives high dilution: the gas falls below the lower flammable limit (LFL) within a very short distance and practically nothing remains once the release stops. In theory a Zone 2 still exists around the leak, but it is only a small volume next to the hole. Instead of drawing a zone that no electrician could respect, the standard lets the assessor record it as Zone 2 NE and treat the place as non-hazardous.
The definition is about consequences, which is unusual in area classification. Zones normally describe only how often an explosive atmosphere is present. The scope of IEC 60079-10-1 says it does not consider ignition consequences except in this one case.
The conditions, as far as they are public
The clause text of IEC 60079-10-1:2020 is not freely available. A 2023 paper from the MultHyFuel hydrogen refuelling project (Torrado and co-authors, HySafe) quotes and summarises it. In paraphrase:
| Criterion | What the paper reports |
|---|---|
| Cloud size, natural gas example | average concentration 50 % of the LFL, volume less than 0.1 m³ or 1 % of the enclosed space concerned, whichever is smaller |
| Other gases | reference volume may be scaled by comparing properties with methane; for hydrogen the authors derived 0.01 m³ or 0.1 % of the enclosed space |
| Consequences | ignition must not cause harmful overpressure, and a flash or jet fire must not cause harm or escalation by heat |
| Pressure | above 2000 kPa gauge (20 barg) NE shall not be applied unless a specific detailed risk assessment documents otherwise; between 10 and 20 barg such an assessment should be considered |
| Dilution | high dilution, and indoors a background concentration below the critical value (25 % of the LFL in their example) |
The paper also notes that the standard does not say what the specific detailed risk assessment must contain. National forewords, national guidance and industry codes can restrict the use of NE further, so check them before relying on it. Edition 4 of IEC 60079-10-1 was circulated as a draft in 2025 and keeps a subclause on zones of negligible extent; wording may change.
What it means for equipment and people
A Zone 2 NE area appears on the drawing so that the reasoning is visible, but for practical purposes it is not a hazardous area: equipment does not need an Ex marking, and the area does not need Ex signs. In the HySafe example, an enclosure classified Zone 2 NE could hold equipment not rated for hazardous areas, while the safety-related devices that act during a leak (the hydrogen sensor, pressure sensors and ventilation fan) were still specified for the hazardous area.
Zone 2 NE is also fragile. It depends on the release rate and on the ventilation. A larger hole size, a higher operating pressure, a blocked louvre or a fan with poorer availability can turn it back into Zone 2.
Example: A gas analyser cabinet with a small natural gas sample line sits in a mechanically ventilated, well mixed room. The secondary releases from its fittings give high dilution, the background concentration stays far below 25 % of the LFL, and the calculated cloud at 50 % of the LFL is well under 0.1 m³ and 1 % of the room. The assessor records Zone 2 NE around the fittings, documents the calculation and the fan monitoring, and allows ordinary equipment in the room.
Common mistakes
- Using NE as a shortcut. Without a calculation of the release, dilution and cloud size, "NE" on a drawing is just an unsupported claim.
- Copying the natural gas cloud size to hydrogen. Hydrogen burns much more violently; scaling the reference volume by gas properties, as the standard allows, gives a much smaller limit.
- Applying NE to high-pressure systems without the specific risk assessment the standard asks for.
- Forgetting confined spaces. Where people work in a small enclosure, even a small flash can injure them.
Related on excodex
- Ventilation in area classification: degree of dilution and availability
- Estimating the extent of a hazardous zone: release rates and dilution
- Can ventilation make a Zone 2 area non-hazardous?
- Laboratories, workshops and small quantities: when is a hazardous area needed?
- Standards space: IEC 60079-10-1
Standards and further reading
- IEC 60079-10-1:2020 (edition 3.0), definition of Zone NE and subclause on zones of negligible extent: preview
- Torrado and others (MultHyFuel), Zone negligible extent: example of specific detailed risk assessment for low pressure equipment in a hydrogen refuelling station, 2023: hysafe.info (PDF); abstract at h2knowledgecentre.com
- Choi and Byeon, Specific process conditions for non-hazardous classification of hydrogen handling facilities, Safety and Health at Work, 2021: koreascience.or.kr
- Draft edition 4 of IEC 60079-10-1 (CDV 2025), contents: preview
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.