IEC 60079-26-2021 EXPLOSIVE ATMOSPHERES – Part 26: Equipment with Separation Elements or combined Levels of Protection.
3 Terms and definitions
For the purposes of this document, the terms and definitions given in IEC 60079-0 apply. ISO and IEC maintain terminological databases for use in standardization at the following addresses:
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4 Ex Equipment with two combined Types of Protection
4.1 General To achieve EPL Ga, electrical equipment may comply with the requirements of two independent Types of Protection each of EPL Gb. If one Type of Protection fails, the other Type of Protection shall continue to function. Then even during rare malfunctions, for example two independent faults, no ignition source can occur.
4.2 Basic requirements The independent Types of Protection shall not have a common mode of failure, except as specified below (see a) and b)). Combined Types of Protection of EPL Gb shall depend on different physical protection principles.
NOTE 1 An example for an unacceptable common mode of failure is an Ex “db” enclosure containing arcing components installed inside an Ex “eb” enclosure. Should the Ex “db” enclosure be compromised, it would also compromise the Ex “eb” enclosure.
NOTE 2 The combination of Ex “db” and Ex “q” both depend on the avoidance of flame propagation (same physical protection principle) and are not suitable in combination. In practice, some combinations are not suitable, for example the combination of liquid immersion “ob” and powder filling “q”. Both Types of Protection shall be assessed using the most onerous malfunction condition of the other Type of Protection. When combining intrinsic safety, i.e. Level of Protection “ib”, with another Type of Protection, the second Type of Protection shall be assessed after the application of faults to the intrinsically safe apparatus which result in the most onerous condition as specified in IEC 60079-1 1 .
Thermal dissipation shall be considered in case of a fault of one Type of Protection. When using two Types of Protection, which both rely on the same parameter (for example, the creepage distance when combining Ex “ib” with Ex “eb”), the most stringent requirement of both Types of Protection shall be applied. If two Types of Protection are combined which both rely on the enclosure, one of the following shall be met: a) if two enclosures are used (one totally enclosed within the other), each enclosure shall comply with the requirements of the respective Type of Protection; or b) if only one enclosure is used, the enclosure and the Cable Glands shall meet the impact test requirements of IEC 60079-0, using the Group I values. Examples of relevant combinations of two independent Types of Protection are as follows:
– inductive transmitters (for example proximity switches, electrical position sensors) with intrinsic safety “ib” enclosed by encapsulation “mb”;IEC 60079-26 pdf download.