What Is Ex o Oil Immersion Protection? Complete Guide to IEC 60079-6
Electrical equipment used in hazardous areas must be designed to prevent ignition of explosive gas atmospheres. Common protection concepts include Ex d flameproof protection, Ex e increased safety, Ex i intrinsic safety, and Ex m encapsulation. Another specialized method is Ex o, which protects equipment through liquid immersion.
IEC 60079-6 specifies the requirements for equipment and components using liquid immersion protection. The method relies on a protective liquid to prevent an explosive atmosphere from being ignited by the immersed electrical components.
The standard defines two protection levels: Ex ob, associated with EPL Gb or Mb, and Ex oc, associated with EPL Gc. Understanding these levels is important when selecting equipment, interpreting certification markings, or assessing existing installations.
What Is Ex o Liquid Immersion Protection?
Ex o is a type of explosion protection in which electrical equipment, or relevant parts of it, are immersed in a protective liquid so that an explosive atmosphere cannot be ignited by the protected components.
The protective liquid separates potential ignition sources from the surrounding gas atmosphere. Depending on the equipment design, it can also provide electrical insulation and influence heat transfer.
Unlike Ex d, which is based on containing an internal explosion, Ex o prevents ignition through liquid immersion. Its effectiveness depends on the equipment construction, the properties of the protective liquid, and maintaining the conditions required by the certificate.
The formal term used in IEC 60079-6 is liquid immersion, although the protection designation remains Ex o. Do not assume that any ordinary oil-filled electrical device qualifies as Ex o equipment. The design must meet the applicable construction, testing, marking, and certification requirements.
How Does Ex o Protection Work?
Ex o equipment is designed so that the required electrical components remain immersed in a protective liquid. The equipment enclosure and liquid-management arrangements must maintain the conditions needed to prevent ignition of the surrounding explosive atmosphere.
Important design considerations include:
- Immersion depth: relevant components must remain immersed as required by the certified design.
- Liquid level: the equipment must provide the required means of ensuring that enough protective liquid is present.
- Liquid properties: the protective liquid must meet the applicable requirements for the intended protection method.
- Temperature and electrical conditions: the equipment must remain within its certified operating limits.
- Enclosure integrity: the construction must maintain the protective arrangement under the conditions specified by the standard.
Depending on the protection level and equipment design, liquid-level indication, monitoring, or safety devices may be required. The precise requirements must be checked against IEC 60079-6 and the equipment's certification documents.
Ex ob vs Ex oc: What Is the Difference?
IEC 60079-6 defines two levels of protection for liquid immersion equipment. Their principal difference is the level of protection required under normal operation and specified abnormal conditions.
| Feature | Ex ob | Ex oc |
|---|---|---|
| EPL for gas atmospheres | Gb | Gc |
| Typical gas zone suitability | Zone 1 and Zone 2 | Zone 2 |
| Protection basis | Normal operation and expected malfunctions, as defined by the standard | Normal operation and regular expected occurrences, as defined by the standard |
| Selection principle | For applications requiring EPL Gb | For applications where EPL Gc is sufficient |
Ex ob is associated with EPL Gb, which is generally suitable for Zone 1 and Zone 2 gas hazardous areas. Ex oc is associated with EPL Gc and is generally suitable for Zone 2.
These protection levels must not be treated as interchangeable. Equipment selection must follow the hazardous area classification and the certified marking. Ex o by itself does not establish whether a particular device is suitable for a specific zone.
IEC 60079-6: Requirements for Ex o Equipment
IEC 60079-6 covers the design, construction, testing, and marking of equipment and components using liquid immersion protection in explosive gas atmospheres. It supplements the general requirements in IEC 60079-0.
Key areas addressed by the standard include:
- Construction of the liquid immersion arrangement
- Requirements for the protective liquid
- Maintaining the required quantity of protective liquid
- Protection against ignition from immersed electrical components
- Requirements for switching devices and associated equipment designs
- Testing and marking to demonstrate compliance with the applicable protection level
The standard also specifies voltage limits within its scope: for Ex ob, the rated voltage is limited to 11 kV RMS AC or DC; for Ex oc, it is limited to 15 kV RMS AC or DC. These limits are not a statement that every device operating at those voltages can be certified under Ex o. The detailed standard requirements and equipment design still apply.
Where Is Ex o Protection Used?
Liquid immersion protection is a specialized method used where the equipment design and electrical function are compatible with the use of a protective liquid.
Possible applications include:
- Oil-immersed electrical components designed and certified for hazardous areas
- Specialized switching devices using a protective liquid
- Electrical equipment incorporating immersed circuits or components
- Selected industrial equipment where the certified design uses liquid immersion to prevent ignition
Do not assume that a conventional power transformer or oil-filled switchgear is Ex o certified simply because it contains oil. The equipment must be specifically designed and certified for the applicable hazardous-area protection concept.
How to Read Ex o Equipment Markings
A simplified example of a gas-atmosphere marking is:
Ex ob IIC T4 Gb
The elements indicate the following:
- Ex: equipment intended for use in explosive atmospheres.
- ob: liquid immersion protection level ob.
- IIC: gas subgroup for which the equipment is certified.
- T4: temperature class, corresponding to a maximum surface temperature of 135°C under the applicable specified conditions.
- Gb: Equipment Protection Level associated with high protection for gas atmospheres.
This is an illustrative marking, not a substitute for an actual equipment certificate. The complete marking may include additional information, and suitability depends on the certificate, ambient limits, installation requirements, and the hazardous atmosphere at the site.
For more information, see How to Read IECEx Equipment Markings and Equipment Protection Level (EPL) Explained.
Ex o vs Ex m: What Is the Difference?
Ex o and Ex m both separate ignition-capable components from the surrounding explosive atmosphere, but they use different protective media and construction methods.
| Feature | Ex o | Ex m |
|---|---|---|
| Protection method | Liquid immersion | Encapsulation |
| Protective medium | Specified protective liquid | Encapsulating compound |
| Design considerations | Liquid level, liquid properties, and immersion arrangement | Encapsulation integrity, material properties, and thermal behaviour |
| Relevant standard | IEC 60079-6 | IEC 60079-18 |
The two methods are not interchangeable. The appropriate protection concept depends on the equipment construction, required EPL, certification, and application.
See also What Is Ex m Encapsulation Protection?.
Inspection and Maintenance Considerations
For equipment using liquid immersion protection, maintaining the certified protective arrangement is essential. The applicable maintenance instructions and inspection requirements should be followed throughout the equipment's service life.
- Check liquid level indicators and monitoring devices where fitted.
- Inspect for leakage, damage, or signs of deterioration.
- Use only the protective liquid specified by the manufacturer.
- Check for contamination or changes in liquid condition where required by the maintenance instructions.
- Confirm that safety devices and automatic shutdown arrangements operate as specified.
- Ensure maintenance, repair, and replacement do not invalidate the equipment certification.
Inspection intervals and acceptance criteria must come from the manufacturer's documentation, the applicable standards, and the site's maintenance procedures. Generic visual checks alone cannot establish continued compliance.
Frequently Asked Questions
What does Ex o stand for?
Ex o is the designation for protection by liquid immersion. A protective liquid is used to prevent an explosive atmosphere from being ignited by the protected electrical components.
Which standard covers Ex o protection?
IEC 60079-6 covers equipment protection by liquid immersion “o”. IEC 60079-0 provides the general requirements for equipment used in explosive atmospheres.
What is the difference between Ex ob and Ex oc?
Ex ob is associated with EPL Gb or Mb, while Ex oc is associated with EPL Gc. For gas atmospheres, Ex ob is generally suitable for Zone 1 and Zone 2, while Ex oc is generally intended for Zone 2.
Can Ex o equipment be used in Zone 0?
Ex ob and Ex oc do not provide EPL Ga for gas atmospheres. Equipment intended for Zone 0 must meet the required EPL Ga protection requirements and have suitable certification.
Is every oil-filled transformer Ex o certified?
No. Containing oil does not automatically make a transformer or other electrical device Ex o equipment. It must be specifically designed and certified for the relevant liquid immersion protection requirements.
What is the difference between Ex o and Ex m?
Ex o uses a protective liquid to isolate ignition-capable components, while Ex m uses an encapsulating compound. They are different protection concepts covered by different parts of the IEC 60079 series.
Conclusion
Ex o liquid immersion protection is a specialized explosion protection method covered by IEC 60079-6. It uses a protective liquid to prevent the surrounding explosive atmosphere from being ignited by protected electrical components.
The standard defines Ex ob, associated with EPL Gb or Mb, and Ex oc, associated with EPL Gc. These levels have different application requirements and should be selected according to the hazardous area classification and equipment certification.
For engineering, procurement, inspection, and maintenance, the key is to verify the complete marking, certificate, protective liquid requirements, and installation conditions rather than assuming that any oil-filled device qualifies as Ex o equipment.
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