Pressurisation uses a positive overpressure of clean air or inert gas inside the enclosure to prevent flammable atmosphere from entering. This allows non-Ex components to operate safely in hazardous areas. This unit covers the three Ex p types (px, py, pz), purge cycle requirements, safety interlocks, and inspection requirements.
Learning Objectives
Define the three Ex p types (px, py, pz) and state their zones of use and EPLs
Explain the purge cycle — minimum volume changes and purpose
State the minimum overpressure values for px/py and pz
Describe the mandatory interlock action on loss of pressure for Ex px
Identify the permitted and prohibited sources of protective gas
Ex p Protection Concept
Pressurisation prevents a flammable atmosphere from forming near equipment by maintaining a positive pressure of clean protective gas (air or inert gas) inside the enclosure. Safety depends on maintaining this overpressure continuously — if pressure is lost, the protection concept fails.
Before the enclosure can be energised, it must be purged to remove any gas that may have entered during the de-energised state. A minimum of 5 volume changes of protective gas must pass through the enclosure before energisation.
Three Ex p Types
px — Zone 1 (EPL Gb): automatic power disconnection mandatory on pressure loss. py — Zone 1 (EPL Gb): alarm mandatory on pressure loss; automatic disconnection optional. pz — Zone 2 (EPL Gc): alarm acceptable on pressure loss.
Protective Gas Requirements
The protective gas is normally clean dry air from a safe area source — motor-driven fan, compressor, or storage cylinders. When inert gas (nitrogen) is used, all accessible doors must carry asphyxiation warnings and be fitted with suitable locks.
Air from Zone 0 or Zone 1 is prohibited as a protective gas source
Air from Zone 2 is permitted with gas detectors at well below LEL
The protective gas must be non-toxic and free from moisture, oil, dust, fibres, and contaminants
Must not be oxygen-enriched
Purge Cycle and Energisation Sequence
The sequence for energising an Ex p system:
Ensure protective gas supply is connected and functioning
Purge with minimum 5 volume changes of protective gas (calculated by volume and flow rate, not just time)
For Ex px, automatic power disconnection on pressure loss is a mandatory certified safety function. Bypassing this interlock is a Category X deficiency. The panel must be de-energised immediately and the interlock restored before return to service.
Quick Check — 5 Questions
Test key concepts from this unit before moving on.
Q1Summary
For an Ex px system, what must happen automatically when the protective gas pressure falls below the minimum?
For Ex px (Zone 1, EPL Gb), automatic power disconnection is mandatory on pressure loss. This is a certified safety function — it cannot be replaced by an alarm or manual intervention. For Ex py, an alarm is mandatory and disconnection is optional. For Ex pz (Zone 2), an alarm alone is acceptable.Ref: IEC 60079-2 — Ex px Pressure Loss Action
Q2Summary
What is the minimum number of volume changes of protective gas required before an Ex p enclosure can be energised?
IEC 60079-2 specifies a minimum of 5 volume changes (5 × the internal volume of the enclosure and associated ducting) of protective gas before energisation. This removes any flammable gas that may have entered during the de-energised period. Manufacturers may specify more than 5 volume changes. The purge is calculated on volume and flow rate — not simply on time.Ref: IEC 60079-2 — Purge Requirements
Q3Summary
An Ex px panel is found with the pressure switch bypassed with a wire link. What deficiency category?
A bypassed Ex px pressure interlock is Category X. The automatic power disconnection is the only protection against energised non-Ex equipment being exposed to the external atmosphere on pressure loss. With the interlock defeated, the panel can remain energised even with zero protective pressure. De-energise immediately and restore the interlock before any return to service.Ref: IEC 60079-2 / IEC 60079-17
Q4Summary
Which source of protective gas is prohibited for Ex p systems?
Air from Zone 0 or Zone 1 is prohibited as a protective gas source. These areas may already contain flammable gas at or near explosive concentrations — using such air as the purge and protective medium would introduce the hazard directly into the protected enclosure. Air from Zone 2 may be used provided gas detectors are fitted and alarm at well below the LEL.Ref: IEC 60079-2 — Protective Gas Sources
Q5Summary
For how long should the minimum overpressure be maintained in an Ex py enclosure during normal operation?
Overpressure must be maintained continuously throughout operation for all Ex p types. The overpressure is what prevents external flammable gas from entering the enclosure. If overpressure is lost at any point during operation, the protection concept has failed. For Ex py, loss of pressure triggers a mandatory alarm; for Ex px, automatic disconnection.Ref: IEC 60079-2 — Overpressure Maintenance
EX Academy — independent CompEx-style preparation. Not official CompEx course materials.
Unit 6 · Ex01 & Ex02 · Full Manual
Pressurisation — Ex p
Pressurisation uses a positive overpressure of clean air or inert gas inside the enclosure to prevent flammable atmosphere from entering. This allows non-Ex components to operate safely in hazardous areas. This unit covers the three Ex p types (px, py, pz), purge cycle requirements, safety interlocks, and inspection requirements.
Learning Objectives
Define the three Ex p types (px, py, pz) and state their zones of use and EPLs
Explain the purge cycle — minimum volume changes and purpose
State the minimum overpressure values for px/py and pz
Describe the mandatory interlock action on loss of pressure for Ex px
Identify the permitted and prohibited sources of protective gas
6.2 Protection Concept — Ex p
Pressurisation maintains a positive pressure of clean protective gas inside an enclosure, preventing flammable atmosphere from entering and contacting non-Ex electrical components inside. This allows equipment that would otherwise not be suitable for hazardous areas to operate safely in Zone 1 and Zone 2 by keeping the hazardous atmosphere out.
The three types of pressurisation: px — Zone 1 and Zone 2 (EPL Gb); on pressure loss: automatic power disconnection mandatory. py — Zone 1 and Zone 2 (EPL Gb); on pressure loss: alarm mandatory, automatic disconnection optional. pz — Zone 2 only (EPL Gc); on pressure loss: alarm acceptable.
Typical applications: large machines, slip-ring or collector motors, MCC panels, analyser houses, control rooms adjacent to Zone 1 areas, oil drilling control consoles.
6.3 Protective (Safe) Gas
The protective gas is normally clean dry air, but may be an inert gas such as nitrogen for applications where air could react with the enclosure contents. Requirements:
Air from a safe area — via motor-driven fan, compressor, or storage cylinders
Air from Zone 2 is permitted with gas detectors alarming at well below LEL
Air from Zone 0 or Zone 1 is strictly prohibited
Must not be oxygen-enriched
Free from moisture, oil, dust, fibres, and chemical contaminants
Non-toxic — when nitrogen is used, doors must carry asphyxiation warnings
6.4 Minimum Overpressure
Minimum overpressure values specified in IEC 60079-2:
Type
Minimum Overpressure
px and py
50 Pa (0.5 mbar)
pz
25 Pa (0.25 mbar)
Overpressure is maintained by: continuous purging with protective gas; or leakage compensation (controlled flow that replaces gas lost through enclosure leakage).
6.5 Purging Requirements
Before energising any Ex p system, the enclosure must be purged to remove any flammable gas that may have entered during the de-energised period. Purge requirements:
Minimum 5 volume changes of protective gas through the enclosure and associated ducting
Purge gas must flow through all parts of the enclosure — dead spots must be avoided in enclosure design
Volume changes calculated from actual enclosure volume and actual purge gas flow rate — not assumed from time alone
Manufacturers may specify more than 5 volume changes for safety margin
Purge vent must discharge to a safe area — not back into the hazardous area
6.8–6.9 Safety Systems and Controls
Required monitoring equipment for Ex p systems:
Overpressure monitoring device (pressure switch or sensor at the most difficult point to pressurise)
Protective gas flow-rate monitoring device
Pressure gauge (visible from outside)
Pressure relief valve (if from compressed-air or cylinder supply — setting 75% of declared maximum safe overpressure)
Ex px Door/Cover Interlock
Where an Ex px enclosure can be accessed via doors or covers without tools or keys, an interlock must automatically de-energise the electrical supply when the door/cover is opened. Doors must display the warning: "Do not open when an explosive atmosphere may be present". Where enclosures contain hot surfaces or energy-storing components (capacitors), additional warnings must state the required delay after isolation before opening.
Unit 6 Knowledge Check — 10 Questions
CompEx-style questions covering the full unit content.
Q1Unit {num}
The purge vent of an Ex p system discharges into a Zone 1 area. Is this acceptable?
The purge vent must discharge to a safe (non-hazardous) area. If the vent discharges into a Zone 1 area, a reversal of pressure (e.g. during a process upset) could draw hazardous gas from the Zone 1 area into the protected enclosure. Category A deficiency.Ref: IEC 60079-2 — Purge Vent Location
Q2Unit {num}
An Ex px enclosure contains capacitors. What warning must be displayed on the door?
For Ex px enclosures with energy-storing components (capacitors), two warnings are required: (1) "Do not open when an explosive atmosphere may be present" — standard Ex px door warning; (2) a warning stating the required time delay after electrical isolation before the door may be opened safely — to allow capacitors to discharge.Ref: IEC 60079-2 — Door Markings
Q3Unit {num}
The protective gas for an Ex p analyser shelter is drawn from a Zone 2 area where the gas is monitored at 5% LEL. Is this acceptable?
Air from Zone 2 may be used as a protective gas source provided gas detectors are fitted and alarm at a level well below the LEL (5% LEL monitoring is typical). The critical prohibition is air from Zone 0 or Zone 1. If gas is detected at the Zone 2 air intake, the system should alarm — but this source is acceptable with appropriate monitoring.Ref: IEC 60079-2 — Protective Gas Sources
Q4Unit {num}
What is the minimum enclosure strength required for an Ex p pressurised enclosure?
The surrounding flushed/pressurised enclosure must withstand 1.5 times the overpressure experienced during normal operation. This provides a safety margin against pressure surges and flow control failures. This is much lower than the explosion pressure requirements for Ex d (which must withstand up to 13 bar for hydrogen).Ref: IEC 60079-2 — Enclosure Strength
Q5Unit {num}
During commissioning of a new Ex px MCC, inspection of the interlocking arrangements reveals the door interlock has not been connected to the power supply. What category?
An unconnected door interlock on an Ex px enclosure is Category X. The automatic disconnection function (which must operate when the door is opened) is a mandatory certified safety feature of Ex px protection. Without the interlock connected, the enclosure can be opened while energised with no automatic power disconnection — directly defeating the Ex px protection concept.Ref: IEC 60079-2 / IEC 60079-17
Q6Unit {num}
Why is the purge requirement expressed in volume changes rather than simply in minutes?
Volume changes ensure the actual gas purged through the enclosure is sufficient. A 10-minute purge at low flow rate may be entirely inadequate for a large panel, while the same 10 minutes at high flow rate could achieve 20 volume changes in a small enclosure. The standard specifies 5 volume changes based on the actual enclosure volume and the verified purge gas flow rate.Ref: IEC 60079-2 — Purge Calculation
Q7Unit {num}
An Ex p system uses nitrogen as the protective gas. What specific safety measures are required at the enclosure doors?
When nitrogen (or other inert gases) is used as protective gas and personnel can gain access to the enclosure, all doors must carry prominent asphyxiation warnings. Nitrogen displaces air and is odourless and colourless — it provides no warning before a person loses consciousness. Suitable locks should prevent accidental or unauthorised entry.Ref: IEC 60079-2 — Nitrogen as Protective Gas
Q8Unit {num}
An Ex py system has lost protective gas pressure. The control system has sounded an alarm but has NOT disconnected the power. Is this compliant?
For Ex py: alarm on pressure loss is mandatory; automatic power disconnection is optional (not mandatory). This situation — alarm sounded, power not automatically disconnected — is compliant with Ex py requirements. The operator must decide whether to manually disconnect. This is the key difference between px (automatic disconnection mandatory) and py (alarm mandatory, disconnection optional).Ref: IEC 60079-2 — Ex py Pressure Loss Requirements
Q9Unit {num}
Ex p protection is primarily used for which type of application?
Ex p pressurisation is the primary method for protecting large electrical panels (MCCs, DCS cabinets), analyser shelters, and control systems where the equipment inside cannot practically be made Ex certified. It is also used for large machines (slip-ring motors, large rotating equipment) and for instrumentation analyser houses with complex non-Ex analysers.Ref: IEC 60079-2 — Typical Applications
Q10Unit {num}
During Detailed inspection of an Ex p system, how is the pressure interlock function verified?
Functional testing of the pressure interlock is a Detailed inspection requirement. The interlock must be tested by simulating an actual pressure loss and confirming the correct protective response — power disconnection for px, alarm for py, alarm for pz. Wiring inspection alone is not sufficient — functional testing is required at commissioning and periodically thereafter.Ref: IEC 60079-17 — Ex p Inspection