Sewers, drainage and stormwater

What is ATEX Zone 0, and when does it apply to water assets?

• Three zones, graded by duration. Zone 0 continuous or long periods, Zone 1 likely in normal operation, Zone 2 unlikely and brief.
• The hour figures are convention. HSE is explicit that time limits have been proposed but none officially adopted.
• Zone 0 kit works in Zone 1 and 2. Not the other way round, which is why Zone 0 certification gets specified defensively.
• Certification covers the whole arrangement. A certified sensor wired through an uncertified device is not a certified installation.

The three zones

Hazardous area classification grades a location by how often, and for how long, an explosive gas atmosphere is present. HSE sets out the three gas zones as follows. Zone 0 is an area in which an explosive gas atmosphere is present continuously or for long periods. Zone 1 is an area in which such an atmosphere is likely to occur in normal operation. Zone 2 is an area in which it is not likely to occur in normal operation and, if it does, will exist only for a short time.

Numbers are frequently attached to those definitions, most commonly more than 1000 hours a year for Zone 0, between 10 and 1000 hours for Zone 1, and under 10 hours for Zone 2. It is worth knowing that HSE states plainly that various sources have tried to place time limits on the zones but none have been officially adopted. Those figures are the most commonly used values where somebody wants to quantify the definitions, not a legal test.

What follows from that is practical. A zone is a judgement recorded in a site area classification drawing, made by somebody competent, and it is that drawing which governs what equipment may be installed. It is not a figure you can derive yourself from an asset type, and a supplier cannot classify your site for you.

Where Zone 0 appears on water assets

Sewers, wet wells, rising main chambers, storage tanks and digesters all generate gas through anaerobic decomposition. Methane is the explosion risk. Hydrogen sulphide is both a toxic hazard and an aggressive corrosion problem in its own right, separate from any question of ignition.

In practice, many wet wells and chambers are classified as Zone 1 or Zone 2 rather than Zone 0, because ventilation and the pattern of gas generation do not sustain a continuous atmosphere. The classification depends on the specific asset, its ventilation and its duty, which is exactly why the site drawing rather than the asset name decides it.

Zone 0 equipment is nevertheless specified more often than a strict reading of the classification would require, and the reason is straightforward. Category 1 equipment is acceptable in all three zones, so specifying to Zone 0 removes any argument about precisely where a boundary sits inside a chamber, and it keeps the same device usable across a mixed estate. It costs more and it constrains the design, but it settles the question once.

What it means for a sensor

Equipment for Zone 0 must be Category 1, and the usual route to that is intrinsic safety, marked ia. An intrinsically safe circuit is designed so that it cannot store or release enough energy to ignite the surrounding atmosphere, and it has to remain safe under fault conditions rather than merely in normal operation.

That has real design consequences. The energy limit constrains available power, which is why intrinsically safe field devices tend to be battery powered with modest transmit budgets and conservative reporting intervals rather than continuous streaming. It also affects maintenance, because changing a battery inside the zone is not automatically permitted and depends on the certification and the site procedure.

The certification attaches to the device and its exact configuration, including cabling, barriers and anything connected to it. A certified sensor wired back through an uncertified logger or power supply is not a compliant installation, and this is the most common way an otherwise correct specification goes wrong. The same logic applies to any 4-20mA loop entering the zone, which will normally need an intrinsically safe barrier as part of the certified arrangement.

Because access is restricted and entry is costly, non-contact measurement is favoured in these locations for reasons beyond the hazard itself. A radar level sensor mounted in the chamber roof measures without touching the liquid, does not foul, and does not require somebody to enter in order to clean it.

What to ask for

Ask for the certificate rather than a claim, along with its number and the exact equipment marking, and check that the marking covers the gas group and temperature class relevant to your site rather than only the zone.

Establish what the certification covers. Sensor only, or sensor plus logger, cabling and barrier as an assembled system.

Check the device marking against your own area classification drawing, and if the drawing is old or the duty of the asset has changed, that is worth resolving before equipment is ordered.

Finally, settle who is performing the confined space entry for installation and under what permit, including gas testing and rescue arrangements. On most of these jobs the entry regime, not the sensor, is the substantial part of the cost.