Ex p — pressurization protection against ignition (IEC 60079-2)

The general ATEX and explosion-hazard guide covers the most common protection concepts at a high level: flameproof enclosure (Ex d), increased safety (Ex e), and intrinsic safety (Ex i, see the Ex i barrier guide). This article covers a fundamentally different approach: Ex p, protection against ignition by pressurization (purge), per IEC 60079-2.

The principle: keeping the atmosphere out instead of containing or limiting it

Ex d safely contains a potential internal ignition inside a flameproof enclosure. Ex i limits the energy in the circuit itself below the ignition threshold. Ex p does neither: the enclosure is filled with a protective gas (usually instrument air or an inert gas) at a pressure higher than the surrounding atmosphere, so the explosive gas or vapour atmosphere outside the enclosure physically cannot enter it. The electrical equipment inside therefore does not itself need to be explosion-proof, because it never comes into contact with the explosive atmosphere — as long as the overpressure is maintained.

Purging before energizing: the mandatory purge sequence

Before the equipment inside the enclosure is energized for the first time (or after the enclosure has been opened), the protective gas must purge the entire internal volume a specified number of times, to fully expel any explosive atmosphere that may have entered while the enclosure was de-energized. The purge duration and the required number of volume changes are equipment/enclosure-specific and set by the manufacturer; energizing the equipment is normally electrically interlocked until the purge sequence has completed and the minimum overpressure has been reached.

Continuous pressure monitoring during operation

Once in service, a differential pressure sensor continuously monitors whether the internal overpressure stays above the specified minimum relative to the surrounding atmosphere. If the pressure drops below that threshold — due to an enclosure leak, a protective-gas supply fault, or an opened door — the system must respond; the exact nature of that response (alarm, automatic de-energization, or both) depends on the protection level selected.

Ex px, Ex py, Ex pz — three protection levels

Ex px — intended for Zone 1: the enclosure may contain equipment that could itself normally be an ignition source (e.g. switching contacts). On pressure loss below the minimum, immediate automatic de-energization is required, since no secondary protection measure remains active.
Ex py — also intended for Zone 1, but the equipment inside is already protected against causing ignition by an additional protection concept (e.g. Ex e). On pressure loss, an alarm is normally sufficient instead of immediate de-energization, since the second protection layer remains active.
Ex pz — intended for Zone 2, where the probability of an explosive atmosphere being present is already low. On pressure loss, an alarm is normally the only requirement.

Note: the exact fault behaviour (alarm versus automatic de-energization, and the permitted response time) is defined in the certificate and manufacturer documentation of the specific Ex p enclosure — this article covers the underlying principle, not a ready-made decision table for every product.

Practical relevance

An Ex p installation requires periodic inspection per IEC 60079-17 — just like the other protection concepts covered in the periodic ATEX inspection guide — but with its own p-specific checkpoints: the operation and calibration of the differential pressure switch, the condition and capacity of the protective-gas supply, the operation of the purge interlock, and a functional test of the alarm/de-energization response under simulated pressure loss.

Common mistakes

Shortening or skipping the purge sequence under production pressure, so that any explosive atmosphere that entered during the de-energized period is not fully expelled before energizing.
Not periodically calibrating the differential pressure switch, so that pressure loss is detected too late or not at all.
Treating px, py and pz as interchangeable — the permitted fault behaviour and the suitable zone differ materially between levels.
Adding equipment to an existing Ex p enclosure without reassessing the original certification — an uncertified component can undermine the assumption on which the entire pressurization protection rests.