Ex i Intrinsic Safety — Complete Technical Guide
Ex i intrinsic safety is the dominant protection type for instrumentation and control systems in hazardous areas — sensors, transmitters, analysers, thermocouples, and their associated control equipment. Rather than containing or preventing sparks, Ex i limits the electrical energy in the circuit to levels incapable of causing ignition.
The Intrinsic Safety Principle
In a fault condition — open circuit, short circuit, or mechanical damage — an IS circuit generates energy levels below the minimum igniting current (MIC) of the surrounding gas. Even repeated sparking cannot cause ignition because there simply is not enough energy present.
Ex ia, Ex ib, and Ex ic
| Type | Fault Resistance | Zones | Typical Application |
|---|---|---|---|
| Ex ia | 2 independent faults | Zone 0, 1, 2 | Zone 0 and Zone 1 instruments |
| Ex ib | 1 fault | Zone 1, 2 | Zone 1 instruments and sensors |
| Ex ic | Normal operation only | Zone 2 | Zone 2 instruments |
Entity Parameters
IS circuits must be designed using entity parameters to ensure the total energy available in the hazardous area cannot cause ignition. The key parameters are:
- Voc — open circuit voltage of the associated apparatus (barrier/isolator)
- Isc — short circuit current of the associated apparatus
- Ca — maximum permitted cable capacitance in the IS circuit
- La — maximum permitted cable inductance in the IS circuit
The IS apparatus (sensor/transmitter) parameters must be compatible with the associated apparatus (barrier/isolator) parameters: Ui (IS apparatus) ≥ Voc; Ii (IS apparatus) ≥ Isc; Ci (IS apparatus) + Ccable ≤ Ca; Li (IS apparatus) + Lcable ≤ La. Incompatible parameters result in a system that is not certified as intrinsically safe.
IS Earth and Segregation
Intrinsically safe circuits must be completely segregated from non-IS circuits. Requirements include:
- Separate cable trays or conduit — minimum 50mm separation or physical barrier
- Blue cable colour or blue marking for IS circuits
- Dedicated IS earth bar — earthed independently from main plant earth
- IS terminals in control panels must be clearly labelled and physically separated
Reading about it is one thing. EX Academy puts you in the scenario — interactive inspection simulations, equipment selection decisions, and 1,185+ practice questions all referenced to IEC 60079.
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