IEC 60079-11 — Ex i Intrinsic Safety Standard Explained
IEC 60079-11 is the standard governing intrinsically safe (Ex i) equipment. Intrinsic safety is the protection type of choice for instrumentation and control systems in hazardous areas — sensors, transmitters, analysers, and their associated wiring and control equipment.
The Intrinsic Safety Principle
Ex i works by limiting the electrical energy in the circuit to levels below the minimum ignition energy of the surrounding atmosphere. Even if a spark or high temperature occurs (due to a fault or open circuit), the energy involved is too low to ignite the surrounding gas.
Ex ia vs Ex ib vs Ex ic
| Type | Fault Count | Zone |
|---|---|---|
| Ex ia | 2 independent faults | Zone 0, 1, 2 |
| Ex ib | 1 fault | Zone 1, 2 |
| Ex ic | No fault | Zone 2 only |
Entity Parameters
IS circuits are designed using entity parameters — Voc (open circuit voltage), Isc (short circuit current), Ca (capacitance), La (inductance) — that must be matched between the hazardous area apparatus and the associated apparatus (barrier or galvanic isolator). These parameters are specified on the Ex certificate and must be verified during installation.
Intrinsically safe circuits must be segregated from non-IS circuits — separate cable trays, separate conduit, different coloured cable (typically blue for IS circuits). A dedicated IS earth bar must be provided, earthed independently of the general building/plant earth.
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