HSE Guidance Note GS 38 sets the standard for electrical test equipment. Proving units, voltage indicators, fused probes, the 4 mm probe tip rule, and the prove-test-prove procedure. This guide covers everything electricians need to know about safe testing.
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Written and reviewed by Andrew Moore, founder of Elec-Mate, against BS 7671:2018+A4:2026, IET Guidance Note 3 and the IET On-Site Guide.
GS38 is the HSE guidance note on electrical test equipment for use by electricians. It sets out how voltage indicators, test probes and leads must be designed and used safely — finger barriers, minimal exposed metal at the probe tips, fused leads and clear ratings. Proving dead means confirming a circuit is isolated using an approved voltage indicator, after first verifying that indicator works on a known live source or proving unit.
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GS 38 is a Guidance Note published by the Health and Safety Executive (HSE) titled "Electrical test equipment for use on low voltage electrical systems." It specifies the requirements for test probes, test leads, voltage indicators, and proving units used when proving circuits dead before work begins.
The guidance was introduced because electricians were being injured and killed by electric shock while testing circuits they believed to be dead. In many cases, the test equipment being used was inadequate — unfused leads with long exposed probe tips, multimeters set to the wrong range, or voltage indicators with flat batteries that gave a false "dead" reading. GS 38 addresses these specific failure modes by setting minimum standards for test equipment design and use.
Although GS 38 is guidance rather than regulation, it represents the accepted industry standard. The Electricity at Work Regulations 1989 require work on electrical systems to be carried out in a manner that prevents danger — and using test equipment that meets GS 38 standards is the recognised way of satisfying this duty when proving circuits dead. All competent person schemes (NICEIC, NAPIT, ELECSA), all training providers, and the HSE itself treat GS 38 compliance as the baseline requirement.
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A proving unit is a battery-powered device that generates a known voltage output to verify that your voltage indicator is functioning correctly. It is an essential piece of test equipment — without it, you cannot carry out the prove-test-prove procedure that is central to safe isolation.
GS 38 strongly recommends a dedicated two-pole voltage indicator for proving circuits dead. A two-pole indicator is preferred over a multimeter because it has fewer failure modes:
Popular GS 38 compliant voltage indicators include the Fluke T-series, Megger TPT, and Martindale VI-13800. Ensure your indicator meets the category rating requirements for your work (CAT III minimum for distribution-level testing, CAT IV at the supply origin).
The test leads and probes are just as important as the voltage indicator itself. GS 38 specifies detailed requirements for test lead construction:
Always carry spare fuses for your test leads. If a fuse blows during testing, investigate why before simply replacing it — it may indicate a live circuit.
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The probe tips are the most critical safety feature of your test leads. GS 38 specifies:
Many older test leads do not meet these requirements. Long, sharp, fully exposed probe tips are still commonly seen on site, but they are non-compliant and dangerous. Replace any leads that do not meet the 4 mm maximum and spring-loaded shroud requirements.
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Try the safety tools freeThe prove-test-prove procedure is the cornerstone of safe working on electrical circuits. It has three steps:
Use your proving unit to verify that the voltage indicator is working correctly. The indicator should clearly detect the voltage. If it does not respond, the indicator is faulty — do not use it.
Before approaching the load, secure the isolation device against inadvertent re-energisation. Fit a personal padlock to the isolating device and apply a warning label or tag at the point of isolation. Where you are not in full control of all isolation points, every point shall be locked off (OSG 12.5). BS 7671 Regulation 462.1 requires that isolation provisions allow the installation to be worked on safely — securing against re-connection is integral to satisfying this requirement.
Test at the load side of the isolation device — not the supply side. This is a critical spatial requirement: testing the supply side of the isolator will always read live and give a false sense of security. Test between all combinations at the load side and at the point of work: line to neutral, line to earth, and neutral to earth. If the indicator shows no voltage on any combination, the circuit appears dead. If voltage is detected, the circuit is still live — do not proceed.
Use the proving unit again to confirm the voltage indicator is still working correctly. If the indicator responds normally, the test result is valid and the circuit can be confirmed as dead. If it fails, the test result cannot be trusted — replace the indicator and repeat the entire procedure.
Isolating the incoming mains supply may not render all conductors dead. Modern installations increasingly include solar PV inverters, battery storage systems, EV chargers, or standby generators that can back-feed conductors even with the main switch open. GN3 Regulation 2.9 requires a multiple-supplies warning notice at distribution boards where alternative voltage sources are present. BS 7671 Regulation 462.1 requires that isolation provisions are provided for each source of supply. Always check for an alternative-supply notice before commencing work, and prove dead at the actual point of work — not just at the board.
BS 7671 Regulation 537.2.2 prohibits using semiconductor devices as the means of isolation. Variable speed drives (VSDs), soft starters, solid state relays, AFDD electronics, and smart switching modules are all semiconductor-based — none of them constitute a safe means of isolation, even when in an off state. A mechanical isolator (fused isolator, switch-disconnector, or MCB with visible contacts) must always be used as the isolation point before proving dead.
Maintaining your test equipment in good condition is essential for safety and compliance.
Elec-Mate inspection and testing training courses cover GS 38 compliance, test equipment requirements, and the prove-test-prove procedure in detail. The RAMS generator includes GS 38 references in every risk assessment and method statement it produces.
Elec-Mate's site safety tools help you manage test equipment calibration dates, PPE inspection records, and training certificates.
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