Cable Detectors: What They Find, What They Miss, and How to Use One Before Drilling
The UK guide to cable detectors. Why wall scanners miss dead circuits and plastic pipes, which walls defeat them, and what to buy from £20 to £120.
A drill bit hitting a live 230V cable inside a wall trips the RCD (the safety switch in your consumer unit) if you're lucky. If the cable isn't on an RCD-protected circuit, or the RCD is faulty, you get an electric shock. Hitting a gas pipe is worse. Every year in the UK, around 60,000 cable strikes are reported, and that's just the ones utilities hear about. A cable detector costs less than a takeaway for two. Not owning one before you drill into a wall is a gamble with terrible odds.
What it does and what it doesn't
A cable detector (also called a wall scanner or pipe and cable detector) is a handheld device you press flat against a wall. It senses what's hidden behind the surface: electrical cables, metal pipes, and sometimes timber studs. You move it slowly across the wall, and it beeps or lights up when it detects something.
Here's the part no guide tells you clearly enough: most consumer cable detectors only detect live electrical cables. They sense the electromagnetic field that a cable carrying AC current produces. Turn a circuit off at the consumer unit, and that cable becomes invisible to the detector. It looks identical to an empty wall. This is the single most dangerous limitation of every detector under about £300 and it means you cannot rely on the detector alone.
The other blind spot is plastic. Modern UK homes use plastic pipes (push-fit systems like HEP2O and PEX) for hot and cold water. Standard cable detectors sense metal, not plastic. If your house was built or replumbed since the 1980s, there are almost certainly plastic pipes in the walls that no consumer detector will find.
What a cable detector can do is give you one layer of protection. Combine it with knowing where cables should run (the prescribed zones from BS 7671), turning off circuits before drilling, and logically tracing pipe routes from visible fixtures. No single method is foolproof. Using all four together makes hitting something very unlikely.
How cables should be routed: prescribed zones
BS 7671 (the UK wiring regulations, Regulation 522.6.202) requires that cables buried in walls at less than 50mm depth must run in specific routes called prescribed zones. Before the 18th edition, these were called "safe zones." Understanding these zones tells you where cables should be, which is often more reliable than what your detector tells you.
Two types of zone exist.
Corner and ceiling zones. A 150mm-wide strip runs along the top of every wall (where it meets the ceiling) and a 150mm-wide strip runs down each internal and external wall corner. Cables should be within these strips. These zones apply to every wall, whether or not it has any electrical fittings on it.
Accessory zones. From every socket, switch, or other electrical accessory, cables should run vertically to the ceiling or floor, and horizontally to the nearest corner. The zone extends 150mm either side of the accessory's centre line.
If you're drilling more than 150mm from the ceiling, more than 150mm from any corner, and not directly above, below, or beside any socket or switch, cables should not be present in a properly wired home. "Should" is doing heavy lifting in that sentence. Older properties, DIY wiring, and extensions done without proper supervision regularly break these rules. The detector is your backup for when the wiring doesn't follow the regulations.
Types of cable detector
Three detection technologies exist. Most consumer devices combine two of them. Understanding which your detector uses tells you what it can and can't find.
Capacitive (AC detection). Senses the electromagnetic field around a live cable carrying alternating current. This is how budget and mid-range detectors find electrical wiring. The limitation: it only works when the cable is energised. A dead circuit, a disconnected cable, or a cable on a tripped breaker is invisible. Detection depth: typically 40-50mm through plasterboard.
Inductive (metal detection). Senses the presence of metal, whether it's carrying current or not. Finds copper pipes, steel conduit, cable clips, nails, joist hangers, and the actual copper conductors inside cables (live or dead). The limitation: it finds everything metallic, which means false positives from nail heads, plasterboard screw lines, and steel lintels. Most mid-range detectors include this mode alongside AC detection.
Radar (microwave pulse). Sends a microwave pulse into the wall and reads the reflection. Can detect changes in material density, so it finds wood studs, plastic pipes, voids, and cables regardless of whether they're live. The limitation: cost. Radar detectors start at around £290 (Bosch D-Tect 120) and go up to £750. They're overkill for a homeowner.
The walls that defeat detectors
This is where every product review site fails you. They test detectors on clean, modern plasterboard-on-stud walls and report the results. Most UK homes don't have those walls. The wall construction determines whether your detector works, and several common UK wall types cause serious problems.
Foil-backed plasterboard. Used for thermal performance in many extensions and new builds. The foil backing is a continuous metallic layer across the entire wall. Your detector's metal-sensing mode will light up everywhere, making individual cable detection impossible. The only workaround is a radar-type scanner (£290+), or switching to AC-only mode and accepting you'll miss dead circuits.
Dot-and-dab plasterboard. The standard way to finish blockwork internal walls in UK construction. Plasterboard is stuck to the blocks with blobs of adhesive (Gyproc Dri-Wall adhesive, typically). Those blobs show up as objects on your detector. A wall done with dot-and-dab has a regular grid of adhesive blobs across the surface, and most detectors read them as potential cable locations. You end up with false readings across the whole surface.
Lath and plaster. Found in pre-1940s homes. Thin timber laths nailed across the studs, covered with plaster. The variations in plaster thickness and the metal nails in every lath create noise that consumer detectors can't filter out. Forum users consistently report these walls as the hardest to scan: "detectors only work well on standard thin wallboard, lath and plaster gives them significant problems."
Solid brick or masonry. External walls on most UK homes are solid or cavity brick/block. The detection depth through 100mm+ of masonry exceeds the range of every consumer detector. If you're drilling into an external wall from outside, your detector won't reach the cables even if they're there. Scan from the inside face where the cables are closer to the surface.
How to use a cable detector properly
The technique matters more than the price of the device. A £45 Bosch Truvo used correctly outperforms a £120 Bosch GMS 120 waved vaguely at a wall.
Step 1: Calibrate away from everything. Hold the detector flat against the wall in an area you know has nothing behind it. Away from sockets, switches, corners, and the ceiling strip. Press the power button and let it calibrate. Some models do this automatically (the Truvo auto-calibrates on power-up); others need you to hold a button. If you calibrate over a cable, the detector sets that signal as its baseline and everything else looks clear. This is the most common beginner mistake.
Step 2: Test on a known cable. Before trusting it on an unknown area, scan near a light switch or socket where you know a cable exists. The detector should indicate. If it doesn't, something is wrong: the batteries are low, the wall type is defeating it, or the circuit is off. Don't proceed until you get a confirmed detection on a known location.
Step 3: Scan slowly in a grid pattern. Move the detector horizontally across the area where you plan to drill, keeping it flat against the wall. Then scan vertically. Cover an area at least 200mm wider than your drilling zone on all sides. Move slowly, about 100mm per second. Fast scanning misses cables, especially at depth.
Step 4: Mark every detection. Use a pencil to mark where the detector indicates. Scan the same area from a different direction to confirm. A real cable will show up consistently. A false positive from a nail or adhesive blob will often disappear on the second pass from a different angle.
Step 5: Turn off the circuit. Even after scanning, isolate the nearest circuit at the consumer unit before drilling. Use a voltage tester to confirm the socket or switch nearest your drilling location is dead. Belt and braces.
What to buy
Three tiers. The jump from budget to mid-range is worth it. The jump from mid-range to pro is only justified if you're doing sustained renovation work across multiple rooms.
Budget: £20 – £30 range. The Draper Combined Metal, Voltage & Stud Detector (around £20 at Toolstation) and the Stanley S160 (around £28) sit here. They work on modern plasterboard-on-stud walls and give you basic live-wire and metal detection. Detection depth is limited to about 27mm. Fine for hanging a shelf. Not reliable enough for serious renovation work where you're drilling dozens of holes. The Stanley is the better of the two.
Mid-range: £43 – £60 range. The Bosch Truvo is the clear recommendation. Around £43 at Screwfix, £48 at Toolstation. It auto-calibrates, differentiates between metal and live wire with separate LEDs, and detects ferrous metal to 70mm, non-ferrous to 60mm, and live cables to 50mm. It doesn't detect wood studs, which is a deliberate design choice (Bosch's separate stud finders handle that). For a homeowner managing an extension project, this is the right tool. Buy this one.
Pro consumer: TBC range. The Bosch Professional GMS 120-27. Adds wood/stud detection to 30mm, increases metal detection to 120mm, and has an IP54 weatherproof rating. It's a better tool, but the live-wire detection depth is the same 50mm as the Truvo. The extra money buys you deeper metal detection and stud finding, not better cable detection. Worth it if you're renovating an entire house. Unnecessary for a single extension project.
Radar scanners (£290+). The Bosch D-Tect 120 at £290 – £336 uses radar pulses and detects plastic pipes, wood, and cables through difficult wall types. A completely different class of tool. The only option that works reliably on foil-backed plasterboard. If you have foil-backed boards and need to drill multiple holes, hiring one for a day is smarter than buying.
Hiring professional equipment
For groundwork near buried utilities, or if your walls are defeating consumer detectors, a professional CAT and Genny (Cable Avoidance Tool and Signal Generator) is the right answer. These are what utility companies use. The Genny injects a signal into a specific cable or pipe, and the CAT traces it. They can detect dead cables, estimate depth, and find things no consumer scanner will.
Hire rates for a CAT and Genny pair run about £55 per day or £110 per week (plus VAT) from tool hire companies like Wellers or National Tool Hire. That's less than the cost of hitting one water pipe. If your extension involves any digging near existing utilities, or if you're chasing cables through walls with foil-backed plasterboard, a day's hire is cheap insurance.
The trick nobody tells you: photograph before you board
The most reliable cable and pipe detection technology costs nothing. During your extension build, before the plasterboard goes up, photograph every wall from multiple angles. Capture every cable route, every pipe run, every junction box. Get close-ups with a ruler or tape measure for reference.
Those photos become your definitive map of what's behind every wall. No detector will ever be as accurate as a photograph taken before the wall was closed up. Store them somewhere permanent, not just on your phone. Email them to yourself, put them in cloud storage, or print them and keep them with the house deeds.
Where you'll need this
- First fix electrics - scanning walls before drilling to avoid hitting existing cables or pipes when running new circuits
Cable detectors are used at any stage where someone drills into a wall or floor, across any extension or renovation project. Before hanging kitchen wall units, before fixing battens for plasterboard, before running new cable routes, before fitting a bracket for a boiler flue. Any time a drill bit goes into a surface with something behind it, scan first.