Approved Voltage Indicators: GS38 and Choosing an AVI

An approved voltage indicator, usually shortened to AVI, is a two-pole voltage indicator that meets HSE Guidance Note GS38. It is the instrument used to prove a circuit dead as part of a safe isolation procedure.

The term matters because it is the language GS38 itself uses, and the language your training provider or assessor will use on course and assessment paperwork. If a checklist calls for an AVI, it means a GS38-compliant two-pole indicator, not any voltage-sensing tool you happen to have in the van. For the full sequence an AVI fits into, see our safe isolation procedures guide.

GS38 is HSE guidance covering the test equipment used on low-voltage electrical systems. It sets out what makes a voltage indicator suitable for the job, rather than leaving it to guesswork.

• ✓Finger guards on the probes, keeping fingers away from the live tip
• ✓Limited exposed metal at the probe tips, so accidental contact with adjacent live parts is far less likely
• ✓Leads and probes suitable for the equipment being tested
• ✓Two-pole operation: both probes must be applied together to give a reading

An approved voltage indicator is only trustworthy if you know it is working. Safe isolation requires proving the indicator on a known source, a proving unit, immediately before testing for dead, and again immediately afterwards.

Proving before the test confirms the indicator is functioning before you rely on it. Proving again afterwards confirms it was still working the whole time you were treating the circuit as dead. Skip the second check, and a fault developing mid-test could leave you believing a live circuit is safe.

Because the two instruments are always used together, most electricians buy an indicator and a proving unit as a matched pair rather than sourcing them separately, and increasingly as part of a kit that adds lock-off devices too.

Once you know what GS38 requires, the practical choice comes down to a handful of established names. Kewtech is the value benchmark: widely owned, straightforward, and the brand most newly qualified electricians reach for first. Martindale is the other specialist UK manufacturer, with its own long-standing range of indicators and proving units. Megger and Fluke sit at the premium end, both long-established names in wider test instrumentation whose voltage indicators carry the same reputation as their multifunction testers.

None of these is the wrong choice. All of them meet GS38 when specified as a two-pole indicator, and the decision mostly comes down to budget and whether you already own other kit from the same maker.

An indicator bought on its own cannot satisfy the safe isolation procedure, since it has nothing to be proved against. A kit pairing a voltage indicator with a proving unit is the simplest way to make sure you own both halves of the pair from day one.

Related Course

Level 2 Diploma (2365)

Safe isolation and the correct use of an AVI are covered on the diploma. Equipment is provided during training; most electricians buy their own kit once they start working.

View Course

If you already own one half of the pair, perhaps a proving unit inherited from a previous job, and just need to replace the indicator, or the other way round, here are both separately. For the leads and probes that wear out long before the instrument itself, see our electrical testing equipment guide.