Three additional protection concepts — oil immersion (Ex o), powder filling (Ex q), and encapsulation (Ex m) — provide complementary explosion protection for specific applications. This unit covers their principles, EPLs, zone suitability, standards, and why Ex m encapsulation is commonly found inside IS field instruments.
Learning Objectives
State the protection principle for Ex o, Ex q, and Ex m
Identify the IEC 60079 standard covering each method
State the EPL and zone suitability for each method
Explain why Ex m is used for IS field instrument electronics
Describe Ex s special protection and when it was applied
Oil Immersion — Ex o
Ex o — IEC 60079-6
Components capable of arcing or sparking are immersed in insulating mineral oil at a minimum depth of 25mm. Oil prevents atmospheric contact and limits temperature rise. Oil level check method mandatory. EPL Gb — Zone 1 and 2. IP66 minimum. Applications: HV circuit breakers, transformers, motor starters.
Powder Filling — Ex q
Ex q — IEC 60079-5
Igniting components are completely surrounded by quartz sand, glass beads, or similar powder — preventing arc propagation and atmospheric access. No voids permitted. Enclosure pressure test 0.5 bar for 1 minute. Minimum IP54. EPL Gb — Zone 1 and 2. Used for capacitors, ballast units in Ex edq lighting.
Encapsulation — Ex m
Ex m — IEC 60079-18
Sparking or heating components enclosed in solid compound (epoxy, thermosetting resin, thermoplastic, elastomeric). EPL: ma (Zone 0+), mb (Zone 1+), mc (Zone 2). Common for IS field instrument electronics — transmitter circuit boards are typically Ex m within an Ex ia housing.
Ex m in IS Instruments
Most Ex ia pressure, temperature, and level transmitters contain Ex m encapsulated circuit boards. The overall instrument is marked Ex ia; the internal electronics are Ex m. Ex ma allows Zone 0 — one of very few methods that does.
Ex s — Special Protection
Historical method for equipment that did not comply with any specific standard but could be shown safe for its use. No specific rules — "special" by definition. Much Ex s equipment used encapsulation. Now replaced by IEC 60079-33. Equipment with Ex s markings may still be encountered in service.
Quick Check — 5 Questions
Test key concepts from this unit before moving on.
Q1Summary
Minimum oil immersion depth for sparking components in Ex o?
All parts capable of producing arcs or sparks must be immersed at not less than 25mm depth. An oil level indicator (sight glass or equivalent) must be provided to check this without opening the apparatus.Ref: IEC 60079-6
Q2Summary
Ex m encapsulation is most commonly found protecting what type of equipment?
Ex m is primarily used for small electronic items — PCBs, solenoid coils, IS transmitter electronics. It provides protection where the electronics must be sealed against the surrounding atmosphere. Ex ma allows Zone 0 installation — notable because few methods achieve this.Ref: IEC 60079-18
Q3Summary
Which EPL levels exist for Ex m?
Ex m has three levels: ma (EPL Ga — Zone 0+1+2), mb (EPL Gb — Zone 1+2), mc (EPL Gc — Zone 2 only). Ex ma is notable as one of the very few methods permitted in Zone 0.Ref: IEC 60079-18
Q4Summary
An Ex o transformer has oil below the minimum sight glass level. What action?
Oil immersion IS the protection mechanism. Below minimum level, sparking components may not be immersed to the required 25mm. Withdraw from service and restore correct oil level with approved oil.Ref: IEC 60079-6
Q5Summary
Which IEC 60079 part covers powder filling (Ex q)?
IEC 60079-5 is powder filling (Ex q). IEC 60079-6 is oil immersion (Ex o). IEC 60079-18 is encapsulation (Ex m). IEC 60079-15 is Type N.Ref: IEC 60079-5
EX Academy — independent CompEx-style preparation. Not official CompEx course materials.
Unit 8 · Ex01–Ex04 · Full Manual
Other Methods of Ex Protection
Three additional protection concepts — oil immersion (Ex o), powder filling (Ex q), and encapsulation (Ex m) — provide complementary explosion protection for specific applications. This unit covers their principles, EPLs, zone suitability, standards, and why Ex m encapsulation is commonly found inside IS field instruments.
Learning Objectives
State the protection principle for Ex o, Ex q, and Ex m
Identify the IEC 60079 standard covering each method
State the EPL and zone suitability for each method
Explain why Ex m is used for IS field instrument electronics
Describe Ex s special protection and when it was applied
8.1 Oil Immersion — Ex o
Oil immersion protects by immersing electrical components in insulating mineral oil, preventing contact between arcing/sparking parts and the explosive atmosphere. The oil also quenches arcs and limits temperature rise.
Standard: IEC 60079-6. EPL Gb — Zone 1 and 2. Category 2. Minimum IP66. Typical applications: HV circuit breakers, power transformers, motor starters.
Construction requirements:
All parts capable of arcing or sparking: minimum 25mm immersion depth
Oil level check method mandatory (sight glass, dipstick, or equivalent)
Temperature limits for submerged components must be met
⚠ Some Oils Produce Acetylene and Hydrogen
Some mineral oils used in switchgear produce acetylene and hydrogen when internal arcing occurs. Because of this fire and explosion risk, Ex o application in Zone 1 and 2 has been generally restricted. BS 5345: Part 9 remains a useful UK reference as IEC 60079-14 and -17 do not provide specific guidance for oil-immersed equipment.
8.2 Powder Filling — Ex q
Powder filling surrounds potentially igniting components with quartz sand, glass beads, or similar fine powder — preventing arc propagation and excluding explosive atmosphere from sparking parts.
Standard: IEC 60079-5. EPL Gb — Zone 1 and 2. Suitable all Group II, T6.
Construction requirements:
No voids permitted in filling
Maximum distance live parts to earth/screen: depends on voltage (10mm at 250V to 50mm at 6600V)
Enclosure pressure test: 0.5 bar for 1 minute
Minimum IP54
Permanently sealed enclosures ≤500V: distances may be reduced by 5mm
Applications: capacitors, ballast units in Ex edq lighting, telecommunications equipment.
8.3 Encapsulation — Ex m
Encapsulation encloses components capable of ignition in a solid compound — preventing the explosive atmosphere from reaching them under normal and fault conditions.
Standard: IEC 60079-18. Three EPL levels: Ex ma (Ga, Zone 0+1+2), Ex mb (Gb, Zone 1+2), Ex mc (Gc, Zone 2 only).
Approved compound types: thermosetting, thermoplastic, epoxy resin, elastomeric — solid state, with or without fillers.
Safety design requirements:
Resistors, capacitors, diodes: operate at no more than 2/3 of rated voltage
Temperature rise of components and wiring: within T-class limits
No voids or air pockets (except intentional ones for relays)
Short-circuit fault conditions must be considered in design
8.4 Special Protection — Ex s
Ex s was used for equipment that could not comply with any specific construction standard but could be demonstrated to be safe. No specific rules — "special" by definition. Much Ex s apparatus used encapsulation. Now replaced by IEC 60079-33. Equipment with Ex s markings may still be encountered in service and must be assessed carefully during inspection.
Unit 8 Knowledge Check — 10 Questions
CompEx-style questions covering the full unit content.
Q1Unit {num}
What does "Ex ma" signify?
Ex ma is EPL Ga — the highest protection level, Zone 0+1+2 suitable. One of very few protection methods permitted in Zone 0 alongside Ex ia.Ref: IEC 60079-18
Q2Unit {num}
What IP rating is required for an Ex o enclosure?
IEC 60079-6 requires minimum IP66 for oil-immersed enclosures — reflecting the need to both contain the oil and exclude external water/contamination.Ref: IEC 60079-6
Q3Unit {num}
Ex q powder-filling enclosures must withstand what pressure test?
The enclosure must withstand 0.5 bar overpressure for 1 minute, confirming it can contain the powder filling without breach. Minimum IP54 is also required.Ref: IEC 60079-5
Q4Unit {num}
Ex m components must not operate above what fraction of rated electrical parameters?
IEC 60079-18 requires resistors, capacitors, and similar components within Ex m assemblies to operate at no more than 2/3 of their rated voltage, current, and power. This safety factor ensures long-term reliable operation within thermal and electrical limits.Ref: IEC 60079-18
Q5Unit {num}
A crack is found in the encapsulant of an Ex mb PCB module. What action?
The encapsulant is the entire protection mechanism. A crack allows atmospheric access to internal sparking components. Withdraw from service. Field repair of Ex m is not possible — replacement required.Ref: IEC 60079-17
Q6Unit {num}
Which is NOT a typical Ex q application?
HV circuit breakers in mineral oil are Ex o (oil immersion), not Ex q (powder filling). Powder filling is used for capacitors, ballast units, and telecommunications equipment.Ref: IEC 60079-5
Q7Unit {num}
An Ex o transformer oil level cannot be checked because the sight glass is corroded and illegible. What category?
Without a readable oil level indicator, the minimum 25mm immersion depth cannot be confirmed. Category A — the sight glass must be cleaned, repaired, or replaced. If the oil level is also below minimum, that is also Category A.Ref: IEC 60079-6 / IEC 60079-17
Q8Unit {num}
What compound material is NOT permitted for Ex m encapsulation?
Quartz sand is used for Ex q (powder filling), not Ex m (encapsulation). Ex m compounds must be solid: thermosetting, thermoplastic, epoxy, or elastomeric materials. The compound must permanently encase the components.Ref: IEC 60079-18
Q9Unit {num}
Ex m is described as "used mainly for smaller items of equipment". What specific Ex m sub-level is needed for Zone 0 applications?
Ex ma (EPL Ga) is required for Zone 0 applications. Ex mb is EPL Gb (Zone 1+2). Ex mc is EPL Gc (Zone 2 only). Only Ex ma achieves the Ga protection level needed for Zone 0.Ref: IEC 60079-18
Q10Unit {num}
During inspection, a powder-filled Ex q ballast is found to have a partial void when the enclosure is opened. What deficiency?
No voids permitted in Ex q powder filling. A void allows the explosive atmosphere to penetrate and potentially contact sparking components. Category A — replace the ballast unit with a certified replacement before return to service.Ref: IEC 60079-5