Can an ordinary electrical panel be made suitable for Zone 1 by pressurising it?
Abstract
Yes, if the panel and its contents become a pressurised enclosure Ex pxb, assessed and certified as a whole: purge system, overpressure monitoring, automatic switch-off, door interlocks and outlet barriers. A certified purge controller alone does not certify the panel.
Yes, this is exactly what pressurisation level pxb is for: it lets ordinary, non-Ex equipment such as PLCs, drives and relays work in Zone 1. But the result is a new piece of Ex equipment. The enclosure, its contents and its purge and monitoring system must be designed to IEC 60079-2 and assessed as a whole; fitting a certified purge controller to an ordinary cabinet does not make the cabinet or its contents certified.
Short answer
- Use Ex pxb (EPL Gb) for ordinary equipment in Zone 1. pyb also gives EPL Gb but only accepts contents that already have an EPL of their own (Gc or better); pzc is for Zone 2 only.
- pxb needs an automatic purge sequence, an overpressure of at least 50 Pa monitored by an automatic safety device, interlocked doors (where they open without a tool) and, in Zone 1, automatic switch-off when pressure is lost.
- The temperature class is set by the hottest surface, inside or outside, unless the certified design deals with hot internal parts.
- Anything that stays live when the purge system cuts power (incoming terminals, the purge controller, heaters) must be protected by another Ex concept or sit outside the hazardous area.
- In the EU the finished panel is ATEX category 2 electrical equipment: whoever builds it is its manufacturer and needs the notified body route for that category.
What has to be added to an ordinary panel
A standard cabinet becomes a pxb enclosure only when a complete set of measures is designed in and proven:
| Element | What it does |
|---|---|
| Protective gas supply | Clean air or inert gas from a non-hazardous area, enough to purge and to replace leakage |
| Purge controller (pxb) | Proves outlet flow and overpressure, runs the purge timer, releases power only at the end |
| Pressure sensor | Reads the enclosure directly, no valve in between; operates below 50 Pa |
| Power isolation | Contactor or breaker that the controller opens on loss of pressure, itself Ex protected or outside the zone |
| Outlet with spark and particle barrier | Needed when the outlet discharges into Zone 1; into Zone 2 when the contents normally produce incandescent particles |
| Door interlock | Doors that open without a tool cut the power first |
| Pressure limiting device | Keeps the enclosure below its rated maximum overpressure where the supply could exceed it |
| Enclosure | Strength, sealing (at least IP4X) and resistance to the maximum overpressure test |
The purge time and flow come from a test or, where allowed, from five enclosure volumes; see how long a pressurised enclosure must be purged. The safety devices themselves must be suitable for the hazardous area or mounted outside it.
Equipment that stays live
When the controller cuts power, some parts remain energised: the supply terminals and isolator upstream of the cut, the purge controller and sensors, and any anti-condensation heater or clock that is deliberately kept on. Gas can reach these as soon as the pressure has gone, so each must be protected in its own right, for example in an Ex d or Ex e compartment, with intrinsically safe sensor circuits, or with the isolator placed outside the hazardous area. Under IEC 60079-14, equipment that is suitable for the zone outside the enclosure does not need to be switched off when pressure fails.
Heat from drives and power supplies
Variable frequency drives (VFDs), power supplies and transformers make heat. In a pxb enclosure the temperature class must cover the hottest internal component, because gas can reach it before it cools if pressure is lost. A hotter component is acceptable only if the certified design allows it: either it qualifies as a small component under IEC 60079-0, or, in pxb only, the enclosure has a marked cool-down time before opening and the design keeps gas away from the hot part until it has cooled after a pressure failure (for example a gas-tight housing around it). If the contents need air flow to stay within the temperature class, that flow becomes part of the protection: the enclosure runs on continuous flow and the controller must act when flow falls below the minimum.
Who certifies it
EU (ATEX). A pressurised panel built for Zone 1 is category 2 electrical equipment under Directive 2014/34/EU. The panel builder is its manufacturer: the directive's definition of manufacturer includes anyone who makes a product for their own purposes. The builder must follow the conformity assessment for that category: EU-type examination by a notified body followed by production control or quality assurance, or unit verification of the single panel. The UK regulator's interpretation states that an in-house control panel is an assembly that must be conformity assessed; where every component is already assessed and installed to its instructions, the further assessment covers only the risks of combining them.
IECEx and national schemes. The complete pressurised enclosure is certified; a certificate for the purge controller or a "kit" is only one part. Check the X conditions on that kit certificate: they list what the panel builder must supply and verify.
Alternatives. If the effort is too large, consider placing the panel in a non-hazardous area, in a pressurised room or analyser house, or using equipment with its own protection (an Ex d enclosure, Ex e terminals, intrinsically safe I/O).
Common mistakes
- Buying a certified purge kit and calling the panel "ATEX". IEC 60079-14 states that a certified pressurisation control system does not pass its certificate on to an uncertified enclosure or its contents.
- Forgetting the live parts. Incoming terminals and the purge controller's own supply are still energised after a trip.
- Adding equipment later. A new drive or a partition changes heat load and purge paths; it is a modification of certified equipment.
- Venting the outlet into Zone 1 without a spark and particle barrier. IEC 60079-2 and IEC 60079-14 ask for one there; in Zone 2 it is needed when the contents produce ignition-capable sparks or particles in normal operation.
Related on excodex
- Pressurisation (Ex p): px, py and pz, purging and protective gas
- Pressurised rooms and analyser houses: making an area non-hazardous by ventilation
- What happens if the purge pressure is lost in an Ex p enclosure?
- ATEX equipment categories 1, 2 and 3 and how they relate to EPLs
- Standards spaces: IEC 60079-2, IEC 60079-14, ATEX 2014/34/EU
Standards and further reading
- IEC 60079-2, equipment protection by pressurised enclosure "p" (levels pxb, pyb, pzc, purge, safety devices).
- IEC 60079-14, design, selection and installation (pressurised equipment, ducting, spark and particle barriers, action on pressure loss).
- Directive 2014/34/EU, conformity assessment of equipment for potentially explosive atmospheres; Article 2 definitions: legislation.gov.uk
- IEC 60079-2:2014 text, incorporated by reference in US federal regulations: law.resource.org
- AS/NZS 60079.14:2009 (IEC 60079-14:2007 with national modifications), type of protection "p": archive.org
- HSE (UK), interpretation of the equipment regulations, assemblies and pressurised cabins: hse.gov.uk
- SafeWork NSW TestSafe, pressurisation Ex p: nsw.gov.au
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.