Ex Explained #5; Pressurisation (Ex p)

You'll see it marked Ex p, with a further two letters, as Ex pxb, Ex pyb or Ex pzc, showing which level of protection is provided. It's the concept you're most likely to meet on large rotating machines, because at large frame sizes a flameproof enclosure becomes impractical.

The Principle: Purge, Then Pressurise

A pressurised enclosure works in two stages.

Purging. Before the equipment is energised, the enclosure is flushed through with a protective gas, typically clean air from a known safe source or an inert gas such as nitrogen, at a flow rate and duration sufficient to displace any flammable gas or vapour that may have entered while de-energised. Only once purging is complete, and confirmed, can the equipment be started.

Pressurisation. Once purged, a protective gas is continuously fed into the enclosure and maintained at a pressure slightly above that of the surrounding atmosphere. As long as that positive pressure is maintained, the surrounding hazardous atmosphere cannot migrate in through joints, cable glands or door seals.

Because the enclosure inside is kept free of an explosive atmosphere, standard industrial components, switchgear, terminals, control equipment, can be used inside it without themselves being individually Ex certified. This is one of the reasons pressurisation is an attractive option for large equipment: it doesn't require every internal component to be certified.

The Levels: pxb, pyb and pzc

Pressurisation protection is divided into three types, distinguished by what happens inside the enclosure and what happens if pressure is lost.

Ex pxb is suitable for Zone 1 or 21. If overpressure is lost, the equipment must be switched off, loss of pressure means loss of protection, as internal components do not need to be certified.

Ex pyb is also suitable for Zone 1 or 21, but the equipment inside must meet the requirements for Zone 2 or 22. If pressure is lost, the internal equipment still provides a baseline of protection.

Ex pzc is suitable for Zone 2 or 22. Given this is in an area where statistically there is unlikely to be an explosive atmosphere, loss of pressure typically requires only an alarm rather than an automatic shutdown.

The correct type depends on the equipment inside the enclosure and how the site's process can tolerate a shutdown if pressure is lost.

Why Pressurisation Suits Large Rotating Machines

Flameproof construction depends on precisely machined joints capable of containing explosion pressure and venting it safely, and the internal volumes of explosive mixtures will likely be very high. At the frame sizes used for large motors and generators, building a flameproof enclosure to withstand that pressure becomes heavy, costly and, in many cases, impractical and/or uneconomical.

Pressurisation offers an alternative: keep the explosive atmosphere out of the machine altogether, and the internal ignition risk becomes far less critical. This is why you'll find Ex p protecting large induction motors, synchronous motors and generators, particularly in power generation, oil and gas, and other heavy industrial applications where machine outputs exceed what flameproof designs can practically achieve.

Why This Matters for Repair and Maintenance

Because the protection depends on the enclosure remaining gas-tight and the pressurisation system functioning correctly, both the mechanical integrity of the machine and its associated control system matter.

Seals and joints must be maintained. Any repair that disturbs a seal, gasket or cable entry must restore it to its certified condition. A pressurised enclosure that leaks may not be able to maintain the required overpressure.

The pressurisation control system is part of the certified equipment. Purge timers, flow and pressure monitoring devices, and any automatic shutdown or alarm functions are as much a part of the protection concept as the enclosure itself, and must be maintained and tested accordingly.

Repairs follow BS EN IEC 60079-19. As with other protection concepts, work on Ex p equipment must be carried out by competent personnel, in line with the repair standard, with the equipment's certification documentation used to verify that any repair keeps it within its original specification.

Documentation and traceability apply as they do throughout this series. Any repair, overhaul or modification must be recorded, and the user informed in writing if the equipment can no longer be confirmed as meeting its original certification.

The Bottom Line

Pressurisation solves the explosive atmosphere problem by removing it from the equation: purge it out, keep it out, and standard equipment can safely operate inside. It's a practical answer for equipment too large for flameproof construction, but its protection depends entirely on the enclosure staying sealed and the pressurisation system doing its job continuously. Treat the purge and pressure control system with the same care as the enclosure itself, because the two are inseparable.

What's Next?

In the next post, we'll look at Ex t, protection for equipment operating in atmospheres where combustible dust, rather than gas, is the hazard.