What Ex ia means
Ex ia is an ATEX and IECEx protection concept defined in IEC 60079-11. It limits the electrical and thermal energy available to a circuit to a level that cannot ignite the surrounding hazardous atmosphere, not just under normal operation, but under any combination of two countable faults.
The designation breaks down as:
- Ex, explosion protection
- i, intrinsic safety protection concept
- a, the highest level (two-fault tolerant, suitable for Zone 0)
Intrinsic safety is fundamentally different from other ATEX protection methods. Rather than containing an ignition source inside a robust enclosure (Ex d) or eliminating sparking under normal conditions (Ex e), Ex ia ensures that the circuit itself cannot produce a spark or surface temperature capable of causing ignition, regardless of what goes wrong with the circuit.
Ex ia vs Ex ib vs Ex ic
| Level | Fault tolerance | Equipment Protection Level | Zone suitability |
|---|---|---|---|
| Ex ia | Two countable faults | EPL Ga / Da | Zone 0, 1, 2 (gas); Zone 20, 21, 22 (dust) |
| Ex ib | One countable fault | EPL Gb / Db | Zone 1, 2 (gas); Zone 21, 22 (dust) |
| Ex ic | Normal operation only | EPL Gc / Dc | Zone 2 (gas); Zone 22 (dust) only |
The fault tolerance requirement is cumulative. For Ex ia, the system must remain safe with any combination of two simultaneously applied faults. This makes Ex ia the only protection concept suitable for Zone 0, where a flammable atmosphere is present continuously or for long periods.
Ex ib requires safety under one fault condition and is the most common level in Zone 1 instrumentation. Ex ic applies only in Zone 2 and requires no fault tolerance beyond normal operating conditions.
How energy limitation works
A flammable atmosphere ignites when three conditions are met simultaneously: fuel, oxygen, and a sufficient ignition source. Intrinsic safety eliminates the third element by ensuring the circuit can deliver neither sufficient spark energy nor sufficient surface temperature to cause ignition.
Energy limitation is achieved by restricting the voltage, current, and power available to the field device. This is done through a safety barrier (or galvanic isolator) installed in the safe area. The barrier:
- Limits open-circuit voltage (Uo) to a safe level
- Limits short-circuit current (Io) via a resistor or current-limiting device
- Limits maximum power (Po) transferred to the hazardous area circuit
- Clamps transient voltages using Zener diodes (in Zener barriers) or optical/transformer isolation (in galvanic isolators)
Key parameters: Ui, Ii, Pi, Ci, Li
Intrinsic safety approval depends on matching the electrical parameters of the barrier (safe area) to the parameters of the field device (hazardous area). Both devices carry a certificate specifying these values:
| Parameter | What it represents | Constraint |
|---|---|---|
| Ui / Vmax | Maximum voltage the field device can accept | Uo of barrier ≤ Ui of device |
| Ii / Imax | Maximum current the field device can accept | Io of barrier ≤ Ii of device |
| Pi / Pmax | Maximum power the field device can dissipate | Po of barrier ≤ Pi of device |
| Ci / Ca | Internal capacitance of the field device | Ci + cable capacitance ≤ Co of barrier |
| Li / La | Internal inductance of the field device | Li + cable inductance ≤ Lo of barrier |
The cable parameters (capacitance and inductance per metre) must be accounted for in the loop calculation. A long cable run can consume a significant portion of the allowed capacitance or inductance budget, leaving insufficient headroom for the field device.
If the parameters do not match, for example, if the barrier's Uo exceeds the device's Ui, the combination is not certified and must not be installed.
System design and the safety barrier
An intrinsically safe system consists of three certified elements: the barrier, the cable, and the field device. Each must be approved, and the combination must be verified by calculation.
Two types of barrier are used in practice:
- Zener barrier, uses Zener diodes and fuses to limit voltage and current. Requires a certified earth connection (≤ 1 Ω). Inexpensive and widely used but requires careful earthing.
- Galvanic isolator, uses optical or transformer isolation to separate safe and hazardous area circuits. No shared earth required; easier to install in larger plants. Higher cost.
The entity concept allows mixing of certified components from different manufacturers, provided the parameter inequalities above are satisfied. The system integrator (the person who designs the loop) is responsible for verifying compatibility, this is not done automatically by the certification body.
Documentation requirements
Every intrinsically safe installation requires a Descriptive System Document (sometimes called a Safety Document or Loop Document). This document records:
- All components in the loop and their certificate numbers
- The barrier parameters (Uo, Io, Po, Co, Lo)
- The field device parameters (Ui, Ii, Pi, Ci, Li)
- The cable type, length, and cable parameters (capacitance and inductance per metre)
- The calculated total capacitance and inductance, showing compliance
- The gas group and temperature class of the installation
This document must be kept with the installation records and updated whenever any component is changed. Under UK DSEAR and EU ATEX Directive 1999/92/EC, the duty holder is responsible for maintaining these records throughout the lifetime of the installation.
Where Ex ia is and is not used
Intrinsic safety is the dominant protection method for instrumentation and process measurement in hazardous areas. It is well suited to:
- 4–20 mA current loop transmitters (pressure, temperature, flow, level)
- Thermocouples and RTDs
- HART and Foundation Fieldbus devices
- Solenoid valves (small, low-power types)
- Switches, proximity detectors, and position sensors
- Gas detectors and flame detectors with low power draw
Ex ia is not suitable for equipment with high power requirements. The energy limitation that makes intrinsic safety safe also limits what the circuit can power. Applications that cannot use Ex ia include:
- Electric motors (even small ones draw too much current)
- Electric heaters and heat tracing
- High-power lighting
- Large solenoid valves
- Power supplies to remote equipment panels
For these applications, alternative protection methods, Ex d, Ex e, or Ex p, are required.
Installation and verification
Intrinsically safe wiring must be kept physically separated from non-intrinsically safe wiring to prevent energy from a higher-power circuit being coupled into the IS circuit. IEC 60079-14 requires:
- IS wiring in separate conduit, trunking, or separated by a physical barrier within common trunking
- IS cables identified by light blue colour coding or labelling
- Dedicated terminals or terminal blocks (blue) for IS circuits
- Earthing of Zener barriers verified before energisation
Initial and periodic inspection requirements are specified in IEC 60079-17. The inspection regime (continuous, close, detailed) depends on the zone and the site inspection schedule. For Zone 0 and Zone 1 installations, a detailed inspection, including verification of loop parameters, is required during initial commissioning.