A fail-safe actuator protects people, product and plant, so it is certified as a safety-related system rather than sold on performance alone. This final paper maps the compliance campaign for an electric valve actuator with a nickel-metal hydride fail-safe reserve: the IEC 61508 functional-safety standard and its safety-integrity-level methodology (against which leading electro-hydraulic fail-safe actuators are certified, for example to systematic capability 3 and use in SIL 2/3 loops), the EN 15714 product standard for industrial valve actuators, the IEC 60529 ingress and IEC 60068 environmental qualification that an IP68 field device requires, and the IEC 61951-2, IEC 62133-2 and UN 38.3 evidence for the NiMH cells. It shows how the reserve's guaranteed trip at end of life and minimum temperature becomes a testable safety requirement, how state-of-health diagnostics support the proof-test interval, and why a battery partner that supplies reliability data and complete documentation is essential to a credible SIL claim.
IEC 61508 is the generic functional-safety standard for electrical/electronic/programmable systems, under which a safety function is assigned a required safety integrity level (SIL 1 to 4) from a risk assessment and then engineered and documented to meet it. Real fail-safe actuator ranges illustrate the achievable levels: electro-hydraulic spring-return actuators such as a leading Skilmatic range are assessed to IEC 61508-2 with systematic capability suitable for SIL 2 and SIL 3 duty, and constant-force spring fail-safe units are offered in SIL 3 versions.
For a battery-backed actuator, the reserve is a subsystem in the safety channel, and its failure modes - loss of capacity, raised internal resistance, an open connection, charger failure - must be identified, their rates quantified, and dangerous failures either made safe by diagnostics or counted against the probability of failure on demand. The NiMH supplier's cell reliability, ageing and failure-mode data therefore enter the SIL calculation directly; without credible data the integrity claim cannot close.
IEC 61508 distinguishes dangerous detected failures, which diagnostics handle, from dangerous undetected failures, which a periodic proof test must reveal. For a fail-safe reserve, automatic partial-stroke tests and continuous state-of-health monitoring maximise diagnostic coverage and lengthen the allowed proof-test interval, while a full proof test periodically confirms a complete trip at worst-case condition.
The certification evidence includes the response when the reserve falls below trip capability - a defined diagnostic alarm and, where required, a controlled safe action - and demonstration that a single fault (one weak cell, one lost charger channel) does not silently remove the safety function. NiMH's measurable, gradual degradation and tolerance of frequent diagnostic cycling make high diagnostic coverage achievable, which is a concrete argument for the chemistry in a safety-related design.
Industrial valve actuators are addressed by the EN 15714 series, which covers electric actuators (EN 15714-2) and their interfaces, basic requirements and test methods, sitting alongside the valve standards (such as EN 736 and the EN 593/EN 1171 valve families) and process-industry application guidance. These standards define the actuator's rated output torque, duty classification, mechanical interfaces and the tests that demonstrate it can deliver its rated operation - including operation from its fail-safe reserve.
Type testing therefore includes a demonstrated fail stroke on battery power: the actuator is taken through the required number of trips with a reserve aged and cooled to worst case, confirming it develops the rated torque through break, running and seat phases and reaches the declared safe position within the required time. This is the test that converts the sizing work of the design paper into certified performance.
Field actuators are qualified to demanding environmental levels: IEC 60529 ingress protection commonly to IP68 for buried, flooded or washdown locations; IEC 60068 sequences for cold, dry heat, damp heat, vibration, shock and temperature cycling; and corrosion resistance for coastal or chemical environments. These tests are run with the reserve fitted and, where relevant, after long standby so they reflect the real aged condition.
For the NiMH pack this means demonstrating trip capability after thermal cycling and vibration, no leakage or venting under enclosure sealing, and safe behaviour across the rated temperature span including cold trip at the minimum. The charger's behaviour across temperature is verified at the same time. Passing these tests with the production pack proves the reserve is an integral part of an IP68 field device rather than a bench-only design.
The cells carry standardised evidence. IEC 61951-2 defines performance - capacity, charge retention, endurance and internal resistance - supplemented with high-rate discharge, long-standby readiness and wide-temperature data specific to a reserve that must deliver a high-current stroke after years of float. IEC 62133-2 covers safety under controlled charge, forced discharge, external short circuit, vibration, shock, free fall, thermal abuse, crush and pack-level protection faults, demonstrating the pack remains safe even under the fault conditions a functional-safety analysis imagines.
UN 38.3 supplies the transport test summary for shipping cells and spare reserve packs worldwide; NiMH's non-lithium classification simplifies delivery of spares to remote pipeline and process sites. Beyond these, the supplier provides the reliability and failure-rate figures, ageing curves and lot traceability the SIL calculation needs - evidence a generic battery vendor cannot supply and a specialist NiMH manufacturer can.
The complete technical file integrates the IEC 61508 safety requirements specification, failure-mode analysis and SIL verification with the reserve's failure-rate and diagnostic-coverage data; the EN 15714 actuator type tests including worst-case fail strokes on the NiMH reserve; IEC 60529/IP68 and IEC 60068 environmental results; and the IEC 61951-2, IEC 62133-2 and UN 38.3 battery dossier. The unifying principle is that the stored-energy reserve is a certified, monitored element of the safety function, not a purchased afterthought.
A battery partner that supplies matched high-rate NiMH cells with the full standards evidence, reliability data for the SIL case and engineering support for worst-case trip and self-test design removes the least predictable component from that file. With that backbone, an actuator maker can deliver a fail-safe electric actuator that genuinely moves the valve to safety - through several controlled strokes where the process requires, after years of standby, in the cold, and with a demonstrable, certifiable integrity level that process-industry customers can rely on.
Weijiang Power designs and manufactures sealed nickel-metal hydride cells and matched industrial packs for remote, off-grid and safety-related equipment, and supports OEM partners with IEC 61951-2 performance files, IEC 62133-2 safety evidence, pulse-load characterisation, wide-temperature testing and charger/pack co-validation. Tell us your duty cycle, peak current, temperature envelope, autonomy target and the standards your product must meet, and our engineers will specify a cell-and-pack combination that protects runtime, reliability and service life. Review the range on the products page.
