Ex-Certified, But SIL-Ready? What IEC TS 60079-42 Asks of Your Safety Device

By Reza Bagherylooieh, Technical Partner, NMi Group

A device is installed in a hazardous area. The nameplate reads Ex ia IIC T4, the loop parameters have been checked, and the equipment suits the zone and gas group. For many projects, that closes the hazardous-area question.

But if the same device is part of a safety function, another question is still open: will that function work when it is needed?

Two safety questions, two different types of evidence

Explosion protection and functional safety address different risks. Ex certification focuses on preventing effective ignition sources. Functional safety asks whether a defined safety function can achieve the required risk reduction. Passing one does not automatically prove the other.

Consider a 4–20 mA pressure transmitter that freezes at a normal reading while the real pressure continues to rise. Its Ex ia certificate can remain valid, yet if that transmitter is part of an overpressure shutdown, the hidden failure may prevent the safety action.

In functional safety, the failure that matters most is the one nobody sees coming: a dangerous failure that no diagnostic detects. That rate — λ_DU — drives how often a safety function fails to act on demand, alongside diagnostic coverage and proof-test interval.

When IEC TS 60079-42 applies

IEC TS 60079-42:2019 addresses a narrower case: an electrical safety device used to reduce the likelihood that a potential ignition source from Ex equipment becomes effective.

The key question is therefore not “is there a SIL function in a hazardous area?” It is: does this safety function control a potential ignition source from the Ex equipment?

The Technical Specification does not generally cover gas detection, inerting or ventilation used to prevent an explosive atmosphere, explosion mitigation, or purely mechanical control devices. Its focus is the ignition source in the Ex Equipment Under Control (EEUC), and the safety function must act before that source becomes effective.

How much risk reduction is required?

IEC TS 60079-42 links two things: how the potential ignition source can arise, and the target Equipment Protection Level (EPL). From these, the standard gives a minimum Risk Reduction Factor (RRF) for the safety device.

For example, if an ignition source can become effective during an expected malfunction, a target of Gb/Db requires at least RRF 10, while Ga/Da requires at least RRF 100.

The table also makes an important limitation clear. Some combinations are not permitted, so a safety device cannot simply compensate for an unsuitable Ex design. The Ex equipment and the safety device have to be considered together.

Simple or complex? A key decision for the assessment route

IEC TS 60079-42 distinguishes between simple and complex safety devices.

A simple device has well-defined failure modes and predictable behaviour under fault conditions. Its required RRF may be supported using FMEA and dangerous hardware failure-rate evidence.

A complex safety device falls outside that definition; devices using software or microprocessor control are not considered simple. A complex device must comply with an applicable functional safety standard, so for many electrical or electronic devices IEC 61508 becomes an important part of the evidence.

Identifying the correct route early can have a major effect on assessment effort. It avoids applying a full complex-device approach where a simpler evidence route is appropriate, and it prevents late discovery that additional functional-safety evidence is needed.

A practical example is thermal protection of an Ex eb motor. During overload or stall, motor temperature can rise until it becomes a potential ignition source. A current-dependent protection device disconnects the motor before the temperature limit is exceeded. The Ex assessment identifies the ignition risk; the safety-device evidence demonstrates that the protective action is dependable enough.

Where EN 50495 fits

EN 50495:2010 addresses a closely related subject for safety devices associated with explosion risks, and is currently listed as a harmonised standard under the ATEX Directive. IEC TS 60079-42 brings related thinking into the IEC 60079 series, but it is a Technical Specification and should not be treated as a direct replacement. The applicable route should be selected for the device and the target market.

Four questions to ask early

Do not leave the Ex and functional-safety evidence to meet for the first time at the end of a project. During design, four questions are especially useful:
• What potential ignition source is being controlled, and what is the defined safe state?
• What target EPL and RRF are required?
• Is the safety device simple or complex under IEC TS 60079-42?
• Are the failure-rate, diagnostic, proof-test and interface assumptions consistent with the Ex assessment?

How we can help

NMi Group and ExVeritas bring ATEX, IECEx and IEC 60079 expertise. On IEC TS 60079-42 projects we can help you:
• Identify the potential ignition source and the required RRF.
• Select the simple or complex assessment route.
• Review key interfaces and available failure evidence.
• Align the Ex assessment with the safety-device evidence.
• Identify technical and documentation gaps before formal assessment or customer submission.

The earlier these questions are addressed, the easier it is to avoid late redesign and assessment delays.
An Ex certificate remains essential. But when a safety device is relied on to control an ignition source, there is a second question to answer: can the required safety function be demonstrated?

Going deeper

Watch our free webinars, Functional Safety in Hazardous Areas: IEC 61508 / IEC 61511 & Explosion Protection in Practice
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We are expanding our support around functional safety and explosion protection, and want to hear what manufacturers are facing in practice. If IEC TS 60079-42, SIL evidence and Ex certification are coming together on your project, contact us at NMi.nl or ExVeritas.com.