Intrinsic Safety - Ex i

Ex e Triangle

Ex i EN 60079-11 Explosive atmospheres. Equipment protection by intrinsic safety "i"
Unlike all other protection methods intrinsic safety is a system concept, not just an individual entity but a combination of interconnected entities.

Intrinsically safe circuit
A circuit in which any spark or any thermal effect produced in the conditions specified in IEC 60079‑11, including normal operation and specified fault conditions, are not capable of causing ignition of a given explosive gas atmosphere.
The principle of this protection method is to keep the power level below that which will spark or heating effect causing ignition and consequently can be the highest level of protection1 and consequently, the with one2 exception, the only method of protection allowed in Category 1G.

Simple I.S. circuit Assuming a device is not self-contained i.e. portable then there will be at least two items (plus cable) to form a circuit knowns as a loop.
Each item in the loop can be one of 4 types:

  1. Associated Apparatus
    Electrical apparatus which contains both intrinsically safe circuits and non-intrinsically safe circuits and is constructed so that the non‑ intrinsically safe circuits cannot adversely affect the intrinsically safe circuits

Associated Apparatus is generally thought of as a stand-alone zone barrier or isolation interface (known colloquially as the barrier) but it applies to any equipment with intrinsic safety and non-intrinsic safety terminals as the barrier may be built in to other equipment e.g. distributed I/O cards or output from an alternatively protected Hazardous area device such as a thermocouple connected to an Ex d transmitter.

The Associated apparatus limits the power, voltage and current, out to the hazardous area and field equipment and does not allow for installation in the hazardous area unless another protection method (listed in EN 60079‑0) is also used

  1. 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.

Simple apparatus does not need to be certified and consequently does not require a certification label. However, it may require to be identified with a label so it can be easily recognised as simple apparatus rather than uncertified equipment in the hazardous area.

  1. Certified Equipment
    Equipment which has a notified body type certificate (Intrinsic Safety) for Atex Category 1 & 2 or for Category 3 a competent person’s certificate e.g. by the manufacturer or other certification body.[/notice]

  2. Cables
    Cables are not simple apparatus and do not require to be certified.
    Intrinsic safety has a very low requirement for cables, the minimum is a double insulation i.e. multiple cores and outer sheath. Armour or protective sheath is not required although may be required to comply with segregation, site or environmental requirements.
    Single cores cables are not allowed unless they are otherwise protected i.e. internally to an enclosure.

As Intrinsic Safety is a system concept each of the entities together in the the complete loop must be documented to confirm integrity in the form of a Descriptive System Document

Internal Intrinsic Safety

Occasionally intrinsic safety is used completely internally on equipment to go into the hazardous area. The most common use is on sparking (switches) or inductive components switches to obtain a non-sparking apparatus certificate. In this case there will be no external intrinsic safety connections and it will be evident only from the equipment certification and label e.g. II 3G Ex ic nA IIC T4 Gc

Portable Equipment

Of course intrinsic safety can also be used for portable equipment, the same principles apply but all part are contained in a single enclosure and the power limited within the battery assembly. As the power comes from internal source i.e.battery, the voltage is limited by definition and the current available will be limited by a resistor or , depending on battery type and size, internal battery resistance3.
If limited by an external resistor it would need to be built in the form of a failsafe assembly and encapsulation is often used.


For more detailed information please see refer to the specific Intrinsic Safety section.



  1. Is the highest level of protection when includes safety with 2 fault conditions i.e. Ex ia. 

  2. The 2014 edition of EN 60074-1 introduced the concept of Ex da, (EPL 'a') Flameproof technique for Category 1G. Only Gas detection sensors equipment can be certified Ex da. 

  3. For items using small batteries such as watches and hearing aids, the batteries inherent internal resistance may be sufficient to enable current limitation in conjunction with the fixed voltage to comply with intrinsic safety. 

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