Items which are simple (usually 2 wire1) with defined characteristics and can be shown to be inherently safe if a (barrier) controlled supply is applied to them are known as simple apparatus.
Such items can, but not exclusively, include:
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Although such items do not require to be certified it is often specified that an intrinsically safe certified box (assembly) is used.
In most cases this is a result of a misunderstanding of Intrinsic Safety as a system concept and the idea of an enclosure certified intrinsically safe became a marketing ploy pushed by enclosure vendors in the 1990's. The installation standard2 now calls for evidence of compliance, of the terminal box, to En 60079‑0 when multiple intrinsic safety circuits are inside.
Using a component certifed Ex e3 would ensure compliance to EN 60079‑0.
ASSEMBLY CERTIFICATION IS NOT REQUIRED, and therefore the enclosure does not have an external certification plate as it only needs component certification.
Although some vendors push an 'intrinsically safe assembly' with certification plate this can be misleading as the junction box certification plate may be completely different from the intrinsically safe circuits inside. The enclosure would require an identifying labels stating "Contains Intrinsic Safety Circuits" or similar to ensure it is clear this is part of an are intrinsically safe system.
In summary for Terminal/Junction boxes
All terminal boxes must have an intrinsically safe identifying label
For a single intrinsically safe loop: Enclosure must be physically suitable for environment
For an multiple intrinsically safe loops the Enclosure should have component certification to a relevant protection method e.g. Ex e
Usually 2 wire but 3 wire devices are allowed if they clearly meet the concept e.g. potentiometers which are can be visualised as to two linked resistors, however 3 wire semiconductors (transistors) would not be simple apparatus. ↩
En 60079-14 'Electrical installations design, selection and erection' ↩
En 60079-7 'Increased Safety' ↩