Best Voltage Indicator & Proving Unit (GS38 Safe Isolation Kit)

Quick answer

For most UK electricians the sensible buy is a complete safe isolation kit: the Martindale VIPDLOK138 bundles the VI-13800 two-pole voltage indicator, the PD440 proving unit and a lock-off kit in one case, and it is the setup most safe isolation courses assume you own. If you want a numeric readout as well as simple indication, the Fluke T150 is the premium two-pole option. Whatever you choose, it should follow HSE guidance note GS38, and neither a multimeter nor a non-contact volt stick is an acceptable instrument for proving dead.

Why this is the one purchase you don't cheap out on

Most tool decisions cost you time or money when you get them wrong. This one can cost you your life, or someone else's. I spent 18 years in food-production plants, and the near misses I remember are almost never exotic: a panel with a second supply nobody documented, a borrowed neutral in a lighting circuit, a "dead" busbar that was backfed through a control transformer. Every one of those was caught by a boring, disciplined prove-test-prove routine with a proper two-pole tester.

The legal backdrop is the Electricity at Work Regulations 1989. Regulation 13 requires adequate precautions to prevent equipment you're working on becoming live, and Regulation 14 says you don't work on or near live conductors unless it's unreasonable to work dead and you have suitable precautions. Proving dead correctly is how you stand behind both. GS38 is the HSE's guidance on the test equipment itself, and the Electrical Safety First Best Practice Guide on safe isolation is the procedure most employers base theirs on. If your employer has a written procedure or a permit-to-work system, that comes first. Always.

Why a two-pole tester, not a multimeter

GS38 steers you firmly towards a dedicated, proprietary voltage indicator for proving dead rather than a multimeter, and the reasoning is exactly what you'd expect from anyone who's watched people work under pressure:

• A multimeter has ranges and functions to get wrong. Select ohms or millivolts by mistake and a live circuit can look dead. A two-pole indicator has nothing to set. You touch the probes on, it tells you.
• A flat battery on a multimeter looks like a dead circuit. Blank screen, no reading, crack on. Most dedicated indicators, including the VI-13800, indicate voltage without needing a battery at all. There's no failure mode where the tester silently shows nothing.
• Auto-power-off and auto-ranging add more ways for a meter to mislead you at the worst moment, including ghost readings from capacitive coupling on long parallel runs.
• Leads and probes. GS38 wants finger barriers, minimal exposed probe tips and fused or current-limited leads. Proper two-pole testers ship like that out of the box. Plenty of multimeter leads don't.

And to kill the other bad habit: a non-contact volt stick is a first-glance tool, nothing more. It can fail to indicate on screened or armoured cable, it depends on a battery, and it tells you nothing trustworthy about a conductor being dead. Use one to spot the obvious before you open a panel if you like. Never use one to prove dead.

What GS38 actually asks of your kit

You don't need to memorise the document, but these are the points that matter when you're comparing testers:

• Probe tips: maximum 4 mm of exposed metal, and preferably 2 mm or less, or spring-loaded retractable shrouds. Long bare tips are how you short across terminals and start an arc flash.
• Finger barriers on the probes so your hand can't slide down onto the tip.
• Leads: adequately insulated, robust, coloured to avoid confusion, and fused or current-limited where appropriate.
• The instrument itself: clearly marked, suitably rated for the environment. Check the CAT rating against where you actually work. Testing at the origin of a supply is a harsher place than a downstream socket circuit.
• Condition: inspect leads and probes before every use. Cracked insulation or a wobbly probe means the lead set gets binned, not taped up.

The voltage indicators worth buying

Martindale VI-13800: the default, and rightly so

The Martindale VI-13800 is the modern descendant of the old Drummond test lamps that generations of industrial sparks trusted, and it's the tester I'd hand an apprentice without hesitation. LED indication, AC and DC, no batteries needed for voltage indication, no switches, no ranges, nothing to configure and very little to break. It does one job and it is extremely hard to misread.

The honest downside is the same as the upside: it's an indicator, not a meter. No numeric readout, no resistance function. For proving dead that's a feature. For diagnostics you'll still carry your multifunction tester or a meter, and we've covered those separately in our multifunction tester guide.

Fluke T150: the premium two-pole

The Fluke T150 is the top of Fluke's T-series two-pole testers. You get LED indication backed by an LCD readout, continuity and resistance measurement, and a switchable load that helps collapse ghost voltages so you can tell a genuinely live conductor from capacitive coupling. Build quality is what you'd expect from Fluke, and it's a genuinely nice thing to use day in, day out.

Who shouldn't buy it: if you're domestic-only and you already carry an MFT and a decent meter, the T150 is lovely but you're paying premium money for functions you own twice over. A VI-13800 plus proving unit does the safety-critical job for a lot less. The T150 earns its keep for commercial and industrial engineers who live inside panels and want one tool in the pocket that indicates, reads and checks continuity.

Kewtech KT1780: the value pick

The Kewtech KT1780 is a solid GS38-style two-pole voltage tester at friendlier money. Kewtech gear has a good reputation among UK contractors for doing what it says without fuss, and if the budget is tight this plus a proving unit and lock-off kit is a perfectly respectable safe isolation setup. It's a sensible line on a first-year kit list, and it beats stretching to a premium tester and then "temporarily" skipping the proving unit.

The Fluke T6 question

I get asked about the Fluke T6-1000 constantly. FieldSense is genuinely clever: voltage and current readings through the open fork without breaking into the circuit, which makes it a brilliant fault-finding and survey tool in crowded trays and panels. But non-contact measurement is exactly what you don't want as your proving-dead instrument, and it depends on batteries and electronics to tell you anything at all. Buy one because it makes diagnostics faster. Don't let it replace a two-pole tester in your safe isolation routine.

Proving units: the half of the kit people skip

The routine is prove, test, re-prove. You confirm your tester works on a known source, you test the isolated circuit dead, then you confirm the tester again. That second check is what catches a lead that failed mid-job.

GS38 accepts a known live source for proving, and strictly speaking that's fine. In practice it means finding, opening and touching something live twice for every isolation, which is its own risk and a real nuisance in a factory where the nearest known source might be three fire doors away. A pocket proving unit removes the excuse.

The Martindale PD440 is the standard answer: pocket-sized, battery powered, and it exercises the indicator across its ranges rather than just tickling one LED, which matters because "the 230 V light works" doesn't prove the whole instrument does. It pairs naturally with the VI-13800 but it will prove other two-pole testers too. Whatever you buy, make sure the proving unit actually drives every indication your tester relies on.

Lock-off kits: securing the isolation

Proving dead is worthless if someone can re-energise behind you. A proper lock-off kit is:

• MCB and breaker lock-off devices in the various patterns to fit different toggle designs
• A padlock with a unique key, and that key stays in your pocket, not in the site office drawer
• A multi-lock hasp for when more than one person is working on the same isolation, so everyone fits their own lock
• Warning tags and labels saying who has isolated it, why, and how to reach them

This is where the Martindale VIPDLOK138 kit makes sense as a first purchase: indicator, proving unit and lock-off gear in one case, nothing forgotten in the van. In multi-contractor environments, follow the site's permit and lock-out procedure. In food factories we ran permits for anything beyond a simple local isolation, and I'd defend every minute that paperwork cost us.

Safe isolation, step by step

This is the generic process, aligned with GS38 and the Electrical Safety First best practice guidance. Your employer's written procedure and any permit-to-work system take precedence over anything on the internet, this article included. If you're isolating for something bigger, say a board change, plan it properly; our consumer unit upgrade guide covers that context.

1. Plan and identify. Confirm the circuit or equipment, and every source of supply to it. Drawings lie; look for second feeds, UPS supplies, generators, solar PV, control supplies from other panels. Get permission to isolate and warn anyone affected.
2. Isolate at an appropriate point using a device suitable for isolation.
3. Secure the isolation. Lock off with your unique-key padlock, keep the key on you, fit a warning tag.
4. Prove your tester on a proving unit or known live source, checking the indications you're about to rely on.
5. Test dead at the point of work. Single phase: line to neutral, line to earth, neutral to earth. Three phase: all ten combinations, each phase to the others, each phase to neutral, each phase to earth, and neutral to earth. Every one, every time.
6. Re-prove your tester on the proving unit. Only now is the circuit proven dead.
7. Then work. If you leave the job for any length of time, verify the isolation is still secure before touching conductors again, and never, ever rely on someone else's isolation without proving dead yourself.

Two extra cautions from experience: capacitors in drives and power-factor equipment hold charge after isolation, so allow discharge time and test for DC as well. And treat neutrals with respect; a borrowed neutral can bite on a circuit you've correctly isolated.

Verdicts

• Best for most people: Martindale VIPDLOK138. Everything in one case, nothing forgotten.
• Best premium tester: Fluke T150, for commercial and industrial engineers who want readout, continuity and ghost-voltage handling in one pocket tool.
• Best on a budget: Kewtech KT1780 plus a PD440 and a basic lock-off kit.
• Buy as a diagnostic extra, not for isolation: Fluke T6-1000.

If you're building a kit from scratch, the safe isolation set should be bought before almost anything else; see our first-year electrician tool kit for where it fits in the order of purchase.

FAQ

Can I prove dead with a multimeter? GS38 steers you away from it, and so would I. Wrong function selection, flat batteries and misleading ghost readings are all failure modes that a dedicated two-pole indicator simply doesn't have. Use the multimeter for diagnostics, the two-pole for proving dead.

Do I really need a proving unit, or is a known live source acceptable? A known live source is acceptable under GS38. In practice a proving unit is safer and faster, because you're not hunting for and touching live terminals twice per isolation, and it proves the tester across its ranges. For the money involved, buy one.

Are non-contact voltage sticks acceptable for safe isolation? No. They're an indication-only convenience for a first look. They can miss voltage entirely in screened or armoured cables and they fail silently with a flat battery. Never use one to prove dead.

What does GS38 say about probe tips? No more than 4 mm of exposed metal, and ideally 2 mm or less, or spring-loaded retractable shrouds, with finger barriers on the probes and fused or current-limited leads where appropriate.

How often should I check my test leads? Visually inspect them before every use, and prove the whole instrument before and after every dead test. Damaged leads get replaced, not repaired. A lead set is cheap; the day it fails silently is not.