Can a control room inside a hazardous area be made safe by pressurisation?

Yes. A control room, switchroom or local equipment room standing in Zone 1, 2, 21 or 22 can contain ordinary, non-Ex equipment if it is protected as a pressurised room to IEC 60079-13. Clean air from a safe place holds the room above the outside pressure so that the surrounding explosive atmosphere cannot get in, after a purge has removed any gas already inside. The protection is only as good as its monitoring: pressure and flow sensors, door arrangements, alarms and a defined action, usually power isolation, when the overpressure is lost.

Short answer

• Where it works: a room located in Zone 1, Zone 2, Zone 21 or Zone 22, with no flammable source inside it. Zone 0 and Zone 20 are outside the scope.
• Levels of protection: pressurisation pb gives EPL Gb or Db (for Zone 1 or 21); pressurisation pc gives EPL Gc or Dc (for Zone 2 or 22). Both allow unprotected equipment inside, except the pressurisation safety devices themselves.
• Key numbers in IEC 60079-13:2017: at least 25 Pa overpressure with half of the outlets (not doors) open; a purge of normally 10 room volumes at no less than five air changes per hour before energising; 0.3 m/s outward through an open door if there is no airlock.
• Internal sources such as analyser sample lines need artificial ventilation "v" in addition; pressurisation alone keeps gas out, it does not dilute gas released inside.
• Equipment that must work when protection fails, such as the fan, its motor, the pressure switches and gas detectors, needs an EPL suitable for the unprotected area.

How a pressurised room keeps the hazardous area out

IEC 60079-13 covers rooms that people enter, from a single room to a complete building, including inlet and outlet ducts. The idea is the same as an Ex p equipment enclosure (IEC 60079-2), scaled up to a building with doors and people:

1. Clean air supply. For pb the intake must be in a non-hazardous area. For pc it may be in Zone 2 if the air is essentially clean in normal operation, but then the standard asks for gas detection at the intake, with automatic shutdown of the intake at 25 % of the limiting value, and another detector in the room.
2. Overpressure. The system must hold at least 25 Pa against adjacent spaces with 50 % of all outlets other than doors open, and a sensor in the room must detect loss of pressure and be able to raise an alarm. Loss of air flow is detected at the fan discharge.
3. Purge before switch-on. Gas inside must be brought down to not more than 25 % of the limiting value (the lowest LFL of the flammable components) before non-Ex equipment is energised. The minimum is 10 room volumes, ducts included, unless a smoke test or analysis proves a smaller volume; the flow must be at least five air changes per hour. For pb the purge sequence and timer must be automatic.
4. Doors. Either an airlock, with a continuous clean air flow of at least five airlock volumes per hour or gas detectors in it, or an outward air velocity of at least 0.3 m/s through the open door. Doors must be self-closing and marked to be kept closed.
5. Independent power. The pressurisation system is energised independently of the electrical supply to the room.

The control system for these safety devices must tolerate a single fault for pb and be reliable in normal operation for pc; IEC 61508 and IEC 61511 may be used to show this.

What happens when it fails

For pb, loss of pressurisation must automatically disconnect all equipment in the room that does not have EPL Ga or Gb (Da or Db for dust), unless operation continues under defined administrative controls, for example a short delay while a door is open and gas detectors confirm the room is below 25 % of the limiting value. For pc, power may stay on for a limited time, after which equipment without at least EPL Gc must be switched off unless administrative controls apply. Before re-energising, the room is purged again. The standard also expects measures against ignition from hot surfaces that remain hot after power is cut, and accepts that critical circuits such as fire and gas detection may stay energised with additional layers of protection.

Worked example: a 210 m³ control room in Zone 2

Illustrative values: a control room of 10 m × 6 m × 3.5 m (210 m³) stands in a Zone 2 area and is protected as pc.

• Purge: 10 room volumes are 2,100 m³. At the minimum five air changes per hour (1,050 m³/h) the purge takes two hours; at 15 air changes per hour, 40 minutes. The purge time must be marked at the isolating switch.
• Door: a single 0.9 m × 2.1 m door kept at 0.3 m/s outward needs about 0.57 m³/s, roughly 2,000 m³/h, which is usually much more than the leakage flow needed to hold 25 Pa. A small airlock usually costs less than a fan sized for an open door.
• Detection: if the air intake cannot be placed outside the hazardous area, a detector at the intake closes it at 25 % of the limiting value.

When pressurisation is not the right answer

• Zone 0 or Zone 20 surroundings, and rooms protected with inert gas, are outside the standard.
• Toxic gas. IEC 60079-13 does not deal with air quality for people. A room that is safe from explosion may still need toxic gas detection, intake shutdown and shelter-in-place arrangements under other rules.
• Location first. Placing the control room outside the hazardous area removes the need for purge, interlocks and alarm discipline.

In North America, NFPA 496 (current edition 2027) covers purged and pressurised control rooms, and UL 60079-13 adopts IEC 60079-13 with US differences. A third edition of IEC 60079-13 is in preparation; the draft clarifies that the scope covers analyser houses, will also replace IEC 60079-16, and keeps the 25 Pa minimum.

Common mistakes

• Fan, motor or pressure switch that is not Ex rated. These devices must keep working when the room has been invaded by gas.
• Doors wedged open or used as the main access without an airlock. The overpressure collapses.
• Manual bypass of the purge timer after a trip, to get the screens back quickly.

Related on excodex

• Pressurised rooms and analyser houses: making an area non-hazardous by ventilation
• Pressurisation (Ex p): px, py and pz, purging and protective gas
• What overpressure must a pressurised room or analyser house maintain?
• What happens when pressurisation fails in a pressurised room?
• Standards spaces: IEC 60079-13, IEC 60079-2

Standards and further reading

• IEC 60079-13:2017 (edition 2), equipment protection by pressurized room "p" and artificially ventilated room "v"; text of the 2018 bilingual printing incorporated by reference in US federal regulations: law.resource.org; IEC webstore: webstore.iec.ch
• Draft third edition, prEN IEC 60079-13 (enquiry stage, summary of changes): standards.iteh.ai
• IEC 60079-2, equipment protection by pressurized enclosure "p"
• UL 60079-13, second edition (2022): ANSI webstore
• NFPA 496 (2027 edition), purged and pressurized enclosures for electrical equipment, including control rooms: nfpa.org

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.