Simple Apparatus

Electrical component or combination of components of simple construction with well-defined electrical parameters which is compatible with the intrinsic safety of the circuit in which it is used

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:

Switches & Contacts
Switch
Voltfree items e.g. push buttons, magnetic contacts, Mechanical Contacts
Resistors
Resistor examples
RTD's, Light dependant Resistors Potentiometers
Diodes
Diode Examples
Including Zener Diodes and LED's but not LED clusters
Thermocouples
Thermocouple
although they do generate very small voltages
Terminals
including terminal boxes fuses

Junction box

IS JB
Junction Box
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 intrinsicaly 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 no an external certification plate as the enclosure only needs component certification. Although some vendores push an i'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 identifiying 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



  1. 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. 

  2. En 60079-14 'Electrical installations design, selection and erection' 

  3. En 60079-7 'Increased Safety' 

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