Ex Explained #4: Intrinsic Safety (Ex ia, Ex ib, Ex ic) – Not Enough Energy to Ignite

You'll see it marked as Ex ia, Ex ib or Ex ic typically on instrumentation, sensors, transmitters and control equipment throughout hazardous areas.

The Principle: Limit the Energy

For an explosive atmosphere to ignite, a spark or hot surface must deliver enough energy to start combustion. Intrinsic safety works by ensuring there is insufficient energy available, be that from the supply or from stored energy sources such as cells/batteries, capacitors, inductors and similar. Depending on whether it is Ex ia, Ex ib or Ex ic, the limits apply not just in normal operation, but under defined countable and uncountable fault conditions too.

If there is never enough energy present to create an ignition-capable spark or hot surface, the atmosphere cannot be ignited. No containment is needed, because there is no explosion pressure to contain.

Given the energy levels are strictly defined, the gas group is very important. Whether IIA, IIB or IIC, there are very tightly specified values for voltage, current, power, capacitance and inductance that have been tested and researched for many, many decades. Combine this with very strictly defined physical component, wire and track sizes, and the T-Class is also very well controlled.

This is why intrinsic safety is used for low-power equipment: process instrumentation, sensors, transmitters, gas detectors, and measurement and control circuits. It is not a concept you'll find protecting a large motor where the power levels involved are far too high. It does, however, appear in rotating machines in a sensing capacity, from vibration and temperature to position and speed.

The Levels: ia, ib and ic

Intrinsic safety comes in three levels, distinguished by how many faults the circuit must tolerate while remaining safe.

Ex ia remains safe with up to two countable faults applied and is designed for Zone 0, areas where an explosive atmosphere is present continuously or for long periods.

Ex ib remains safe with one countable fault applied and is designed for Zone 1.

Ex ic is safe in normal operation and is designed for Zone 2.

This makes Ex ia one of the few protection concepts suitable for the highest-risk areas, such as the inside of storage tanks and vessels.

It's a System, Not Just a Device

Here is a point that catches people out: intrinsic safety is a property of the complete system, not of a single piece of equipment.

A typical intrinsically safe loop has three parts: the field device in the hazardous area, an energy-limiting interface in the safe area (such as a shunt diode safety barrier or galvanic isolator), and the cable connecting them. All three matter. The barrier/isolator limits the energy that can reach the hazardous area; the field device is certified for the energy it can safely receive; and the cable's electrical parameters form part of the calculation, because cables store energy too.

Change any element, a different barrier, a longer cable run, an uncertified replacement device, and the loop's safety can no longer be assumed. The documentation describing the complete system is part of the protection.

Why This Matters for Repair

The repair standard, BS EN IEC 60079-19, treats intrinsically safe equipment differently from concepts like Ex db and Ex eb, and it is notably restrictive. As a simple example of the very restrictive methods used, the equipment may or may not rely on degree of protection (IP rating), where some merely need IP2X, but some may need IP54.

Many components should be replaced, not repaired. The standard covers equipment such as enclosures, terminations, soldered connections, fuses, relays, safety barriers, printed circuit boards, optocouplers, batteries, internal wiring, transformers and encapsulated components, and makes clear that not all of these can be repaired. Where repair isn't permitted, the answer is replacement with the correct part, as shown/listed in the schedule documents.

No reclamation of safety-critical components. No attempt at reclamation should be made on components on which intrinsic safety depends. This may be only a small percentage of components in a device, but without specific information, it may be very difficult to determine which components these may be.

Test after repair. Where equipment is repaired or overhauled, the work must be followed by testing of the dielectric strength of the insulation between the intrinsically safe circuit and the metallic enclosure. This may be waived under certain, specific conditions.

Keep the loop documentation intact. Because the protection belongs to the system, records covering the barrier, field device and cable parameters are part of what makes the installation safe. Repairs and replacements must be traceable against that documentation. The I.S. system drawings need to be kept and maintained by the site owner/operator, with changes to the system unambiguously marked and justified.

Where You'll Find Intrinsically Safe Equipment

Ex i protection is everywhere that process measurement and control meets a hazardous area: pressure, temperature, level and flow transmitters, gas detection, LED lighting, load cells, solenoid valve circuits, and handheld instruments. If it's low-power and it's measuring something in a hazardous area, there's a good chance it's intrinsically safe.

The Bottom Line

Flameproof manages the results of an ignition. Increased safety prevents ignition sources becoming active. Intrinsic safety presumes arcs, sparks and hot surfaces will happen, but prevents ignition by making sure there is insufficient energy to cause ignition. It's an elegant concept, but its protection lives in the whole loop, its certified components and its documentation, not in any single box. Treat every element of an intrinsically safe circuit as part of the protection, because it is.

What's Next?

In the next post, we'll look at Pressurisation (Ex p), a concept that protects equipment by keeping the explosive atmosphere out altogether.