Voltage Testers: NCV Pens, Two-Pole Testers, and Socket Testers Explained
The complete UK guide to voltage testing tools. NCV pens from £8-13, two-pole testers, socket testers, and why you need more than one.
Your electrician finishes the second fix, you pay the invoice, and a week later you plug in the toaster and nothing happens. Dead socket. You call the electrician back but they're on another job for a fortnight. If you'd spent two minutes with a socket tester before signing off, you'd have caught it on the day. Worse scenario: you drill into a wall during snagging, hit a cable you assumed was dead, and discover it very much wasn't. A voltage tester that cost less than a takeaway pizza could have prevented that.
Voltage testing is really three different jobs that need three different tools. Most guides treat them as one thing. They're not, and confusing them is dangerous.
The three tools you need to understand
There are three distinct types of voltage testing tool, each designed for a specific job. You don't necessarily need all three, but you need to understand what each one does and, critically, what it can't do.
NCV pen (non-contact voltage detector) is a pen-shaped device you hold near a cable or socket. It detects the electromagnetic field around a live AC conductor through the insulation, without touching bare wires. It beeps or lights up if voltage is present. Costs £8-13. Quick and safe for initial checks.
Two-pole tester (also called a voltage indicator) has two probes that make physical contact with conductors. It measures the actual voltage between two points. This is the tool that proves a circuit is dead before you touch anything. Costs £50-90. Required by HSE guidance (GS38) for safe isolation.
Socket tester plugs into a 13A socket and checks the wiring is correct. Three LEDs indicate whether live, neutral, and earth are connected properly. The tool you want for snagging after second-fix electrics. See the comparison table below for typical costs.
| NCV Pen | Two-Pole Tester | Socket Tester | |
|---|---|---|---|
| What it does | Detects live AC cables without contact | Measures voltage between two conductors | Checks socket wiring polarity and earth |
| What it can't do | Prove a circuit is dead. Detect DC. Work through metal conduit | Quick non-contact scanning. Requires access to conductors | Test anything except 13A sockets. Detect all fault types |
| When you need it | Before drilling into walls. Quick check if a cable is live | Before any electrical work. Proving dead at the consumer unit | Snagging walk after second-fix electrics |
| Typical cost | £8-30 | £50-90 | £8-63 |
| Skill needed | None | Basic electrical awareness | None |
NCV pens: what they do and where they fail
An NCV pen works by detecting the electric field that radiates from a live AC conductor. You don't touch the wire itself. Hold the pen near a cable, socket, or switch, and if there's AC voltage present, it lights up and beeps.
That's genuinely useful. Before you drill into a wall, sweep the area with an NCV pen and a cable detector. Before you pull a light fitting down, check the cable feeding it. Before you start stripping wallpaper around a socket, confirm it's not live.
But NCV pens have real limitations that most product descriptions don't mention.
An NCV pen cannot be used alone to confirm a circuit is dead. HSE guidance GS38 is explicit: non-contact detectors should only be used for "identifying live equipment, not for proving that it is dead." A false negative (the pen says dead when it's actually live) can happen with shielded cables, metal conduit, and armoured cable. If you rely on an NCV pen alone before touching wires, you're gambling.
Why false readings happen. The pen detects the electromagnetic field through insulation. Metal conduit or armoured cable shielding blocks that field. The pen reads "no voltage" because it can't see through the shield, not because the cable is dead. False positives happen too: a dead cable running parallel to a live one can pick up enough capacitive coupling to trigger the pen. Professional electrician forums are full of accounts of both failure modes.
NCV pens also cannot detect DC voltage. That rarely matters in domestic settings, but it's worth knowing if you're working around solar panel wiring or battery storage systems.
Two-pole testers: the tool that actually proves dead
A two-pole voltage tester has two probe leads. You touch one probe to one conductor and the other to a different conductor. It measures the potential difference (voltage) between those two points and displays the result on LED indicators or a digital display.
This is the tool professionals use for safe isolation, the formal procedure for proving a circuit is dead before working on it. HSE guidance GS38 and the standard BS EN 61243-3 govern this. It's not optional for anyone touching electrical conductors.
Why you can't just test one wire
Voltage is a difference between two points. It's not a property of a single conductor. A neutral wire that's become disconnected can "float" at the same potential as the live conductor. Test live-to-neutral and you'll read zero volts, which looks dead. It isn't. The circuit is still connected to the mains supply; you just can't see it from that single measurement.
This is why the prove-dead procedure requires testing multiple conductor combinations: live to earth, live to neutral, neutral to earth, and between all line conductors. Miss one combination and you can miss a fault that could kill you.
The proving unit: why it matters
A proving unit is a small device that outputs a known voltage. You test your voltage indicator against it before you start work and again after you finish. Why? Because if an LED on your indicator has failed (blown), it could show "no voltage" on every test. You'd think the circuit was dead. It wasn't. The tester was broken.
The prove-test-prove sequence works like this:
- Test your voltage indicator against the proving unit. All LEDs should light up across the relevant voltage ranges.
- Use the indicator to test the circuit you want to work on. Test all conductor combinations.
- Test the indicator against the proving unit again. If it still works, your readings from step 2 are reliable.
If the indicator fails the second prove, every reading from step 2 is suspect. Start again with a different indicator.
Some two-pole testers are "self-proving," meaning they have a built-in test function that verifies the indicators work. The TIS 851 from Toolstation is one example. A standalone proving unit adds significant cost to your kit, so a self-proving tester is good value for occasional use.
CAT ratings explained
Every voltage tester has a CAT (Category) rating that tells you what type of electrical installation it's safe to use on. The rating isn't about voltage alone. It's about the fault energy (how much destructive current can flow if something goes wrong) at that point in the installation.
CAT II covers mains sockets and plug-in appliances. Domestic socket testing.
CAT III covers distribution wiring, consumer units (fuse boards), and fixed installations. This is the minimum for testing at a consumer unit during extension work.
CAT IV covers the origin of the installation, the point where the supply enters the building.
The important point: a CAT II tester used in a CAT III environment is genuinely dangerous. Transient voltage spikes in distribution wiring can exceed what a CAT II tester is designed to handle, potentially causing the tester to fail violently. Always match the CAT rating to where you're testing.
Socket testers: the snagging tool
A socket tester is the simplest of the three. Plug it into a 13A socket and the three LEDs tell you whether live, neutral, and earth are wired correctly. All three LEDs lit in the correct pattern means no fault detected.
These are the tool for snagging after second fix electrics. Walk around the extension with a socket tester and check every single socket. It's surprisingly common for sockets to be missed during second fix, either not connected at all or wired incorrectly. Finding a dead socket the day the electrician is still on site costs nothing. Finding it three weeks later costs a call-out fee.
Basic three-LED socket testers cannot detect all fault types. Electrical Safety First (the UK's leading electrical safety charity) warns that simple socket testers are "often inappropriately used as the sole means of checking whether a socket-outlet is safe." They can miss earth-neutral reversal and cannot verify that the earth connection is actually effective (just that one exists). For a proper sign-off, your electrician should be testing with calibrated instruments. The socket tester is your first-line check, not the final word.
The Kewtech Kewcheck 103 is the standout in this category. See the buying recommendation below for full specs. It goes well beyond the basic three-LED pattern, and the circuit-locating function is particularly useful for identifying which breaker feeds which socket.
What to buy
If you're managing an extension build (recommended kit)
You don't need professional-grade equipment. But you do need to cover the three use cases.
NCV pen: LAP PEN01 from Screwfix. 50-1000V AC detection, CAT IV rated, LED and audio indicators. 4.7/5 stars from 66 reviews. This is the budget pick that actually works. Use it before drilling into walls and for quick checks during the build. See the budget summary table below for pricing.
Socket tester: Kewtech Kewcheck 103 from Screwfix. Checks 14 wiring conditions and has a circuit-locating buzzer. It costs a little more than the cheapest option, but that premium buys you detection of fault types the basic tester misses entirely. This is what you'll use during your snagging walk.
That covers the tools you'll personally use during a managed build. See the budget summary table below for total costs.
If you want a two-pole tester as well
Most homeowners managing a build won't need a two-pole tester because they won't be doing safe isolation themselves. Your electrician carries their own. But if you want one for peace of mind, or if you do any electrical work yourself (even changing a light fitting), a two-pole tester is the right tool.
Entry level: Martindale VT12. Voltage range 12-690V AC/DC, continuity testing, GS38 compliant with probe caps included. BS EN 61243-3 certified. CAT III 690V / CAT IV 300V. Two-year warranty. This is the tester that professional forums consistently recommend as reliable and good value.
Mid range: Fluke T90 from Screwfix. 12-690V AC/DC, IP54 rated (dust and splash proof), CAT II 690V / CAT III 600V. Fluke is the default professional brand. The T90 is the entry point into their range.
Premium: Fluke T110 from Screwfix. Adds a built-in torch (genuinely useful in dark consumer unit cupboards), vibration alert, and better ergonomics. Five-star reviews.
Professional electricians on forums consistently say: "Where it comes to the most vital test for your safety, don't look for a budget item." For NCV pens and socket testers, budget options work fine. For two-pole testers, the mid-range options in the comparison table are where the reliable products start. Below that, you're compromising on the tool whose job is to stop you getting electrocuted.
Budget summary
| Kit Level | What You Get | Cost |
|---|---|---|
| Minimum (snagging only) | LAP NCV pen + Kewtech 103 socket tester | ~£32 |
| Recommended (build management) | LAP NCV pen + Kewtech 103 socket tester | ~£32 |
| Full kit (hands-on electrical work) | Fluke 1AC-II NCV + Martindale VT12 two-pole + Kewcheck 103 socket tester | ~£103 |
| Professional grade | Fluke T110 two-pole + proving unit kit | £120-290 |
How to use each tool properly
Using an NCV pen
Hold the pen with the sensor tip (usually the narrower end, marked with an indicator) near the cable or socket you want to check. Within 10-25mm of the conductor is usually enough. If the cable is live, the LED lights up and the pen beeps.
Before you start, test the pen on a socket you know is live. The pen lights up. Good, it's working. Now go test the cable or socket you're checking. If it doesn't trigger, test the known-live socket again. If the pen still works on the known-live source, your target cable is probably dead. This is the "live-dead-live" protocol, and it's the minimum you should do even with a quick NCV check.
Don't rely on the pen near metal conduit, trunking, or armoured cable. The shielding blocks the electric field. Don't rely on it for cables buried deep in thick walls. And never use it as your only check before touching bare conductors.
Using a two-pole tester
Switch off the circuit at the consumer unit. Lock off the MCB (miniature circuit breaker) if possible, using a lockout device or tape and a warning label.
Prove your tester first: touch both probes to a proving unit or a known-live source. All indicators should respond. Then test the circuit you've isolated:
- Touch probes to live and earth. Should read zero.
- Touch probes to live and neutral. Should read zero.
- Touch probes to neutral and earth. Should read zero.
All three must read zero before the circuit is confirmed dead. Then prove the tester again on the proving unit. If it still works, your readings are reliable.
Using a socket tester for snagging
Plug the tester into each socket in turn. Check the LED pattern against the chart on the tester (or the instructions). All three LEDs lit in the standard pattern means correct wiring. Any other pattern indicates a specific fault: reversed live/neutral, missing earth, open neutral, or other conditions depending on the tester model.
Test every socket. Test every USB socket if your build includes them (some socket testers won't check USB outputs, only the 13A connections). Flip every light switch. It sounds tedious but it takes less than an hour for a typical extension, and it's far cheaper than a return visit.
Alternatives
A multimeter can measure voltage, and your electrician probably carries one alongside their two-pole tester. But a multimeter requires you to select the correct range, connect probes to the right terminals, and interpret a digital reading. For voltage detection specifically, an NCV pen is simpler and a two-pole tester is safer. The multimeter is a general-purpose tool; the voltage tester is a specialist one.
If you already own a multimeter and understand how to use it, it can substitute for a two-pole tester in some situations. But it won't have GS38-compliant leads unless you've specifically bought them, and it doesn't have the visual LED ladder that makes a two-pole tester intuitive to read.
Where you'll need this
- First-fix electrics - testing existing circuits are dead before your electrician connects new cables to them
- Second-fix electrics - verifying circuits are dead at the consumer unit before connecting faceplates and switches
- Snagging checklist - testing every socket and switch is live and correctly wired after installation
These tools appear across all stages of any extension or renovation project where electrical work is involved. The NCV pen is useful from demolition (checking existing cables before removing walls) through to final snagging.
Safety notes
Electricity kills. People die from mains electricity contact in the UK every year. Never work on any circuit without first confirming it is dead using an appropriate tester. An NCV pen alone is not sufficient for this. If you are not confident in safe isolation procedures, leave all electrical work to a qualified electrician.
If you're managing a build rather than doing electrical work yourself, your two key uses are the NCV pen (before drilling into walls) and the socket tester (during snagging). Neither requires you to touch any wires or open any electrical enclosures. They're safe for any homeowner to use.
The two-pole tester and safe isolation procedure are for people who are comfortable working with electrical circuits. If that's not you, that's fine. Understand what the tools do so you can ask your electrician the right questions and verify they're following proper procedures. A good electrician will have a two-pole tester and proving unit as standard kit. If yours doesn't, that's a red flag.