Foval Learning Institute
Courses / Home Repair and Maintenance / Lesson 14

Before you drill: cables, pipes and what's in the wall

70 min

Listen: this lesson as a conversation

Two hosts talk the lesson through. The voices are synthetic; the script was written from this lesson and checked against it, and asserts nothing the lesson does not.

In this lesson you will learn to
  • Apply the before-you-drill routine to a real wall: check what the wall is made of, scan with the power on, switch off and check, then drill with eyes and lungs covered
  • Explain the limit of a cable detector, that it finds only a cable carrying current, and say what a clear result does and doesn't show
  • Decide when what you find (a cable, a pipe, a material that might be asbestos) means stop, and who to call

Electrical Safety First (ESF), a UK charity that campaigns on electrical safety, asked 1,000 electricians about DIY work they'd been called to put right. It reports: "The most common mistakes people make when attempting to undertake DIY were cutting through power leads, drilling into wiring, or trying to repair an electrical item while it is switched on."1 ESF does not give the survey's date, and the list is what electricians reported, not a measure of how often DIY goes wrong. But a wall hides what's in it, and this lesson is about the second of those three: what to do before the bit goes in.

Before you pick up a tool

This course is education, not a trade qualification, and not legal advice about your own home. It teaches no gas work and no wiring. If you smell gas in Great Britain, open doors and windows, turn the gas off at the meter if you can do it safely (not if the meter is in a cellar or basement), and call 0800 111 999; in Northern Ireland the number is 0800 002 001. In the US, leave straight away and call 911 or your gas utility once you're outside. In either country, don't light a flame or touch an electrical switch. Isolate the water or power before you open anything up, and if a breaker, RCD or GFCI keeps tripping, leave it off and call an electrician. Which jobs the law leaves to a registered or licensed trade differs between England, Wales, Scotland, Northern Ireland and each US state, and it changes.

The box at the top says to isolate the water or power before you open anything up. A wall you are about to drill is the nearest this lesson comes to that, because it has cables in it that you can't see. Drilling is the one job in this course where the wiring is right in front of you, and your hands still stop at the front of the consumer unit or panel, as lesson 5 set out. Two facts from there matter more here than anywhere. CPSC, the US Consumer Product Safety Commission, says panel boxes "are usually electrically live, even with the main circuit breakers turned off".5 And HSE's guidance for people who work on electrical systems does not recommend "non-contact devices such as 'volt sticks'" for proving anything dead.6

The detector, and what it cannot see

A cable and pipe detector is sold for this. This course found no official UK or US consumer page on detectors, so it uses a maker's guidance for its own devices.

Predict first

Before the steps: a detector scanned over a wall on a bright afternoon, with every light off and nothing plugged in, shows green everywhere. Is the wall clear of cables?

Show the answer

No. Step 6 below is why: Bosch says a cable needs current to be picked up, so a cable with nothing drawing on it gives the detector nothing to find.2 The scan has told you nothing about cables on circuits that had nothing switched on, and it has not cleared the wall.

Bosch, about its own digital detectors, gives these steps:2

  1. Take off rings, bracelets and watches, and put your phone out of reach, or they can confuse the reading.
  2. Hold the whole device flat on the wall.
  3. Put your other hand flat on the wall, about 20 to 30 cm away, for grounding. Bosch says "This is especially important when looking for power lines", and warns that "If there is insufficient grounding, for example, due to insulating footwear, or if you are standing on a conductor, it won’t be possible to detect live cables."
  4. Move it across the wall with even pressure, along one line at a time.
  5. "The device needs to be moved over the same place several times to detect whether there is something behind the wall."
  6. Switch on electrical items, such as a light or a vacuum cleaner, while you scan. "A cable must have a current in order to be picked up by the device."
  7. Then: "for your own safety, after detection you should switch off the power supply completely before you start drilling."

Bosch doesn't say which items. Since a cable needs current to be found, this course's reading is to switch on things on the circuits likely to be in that wall: the room's lights, and a lamp in the nearest socket.

Step 6 carries the mechanism of the whole thing. On Bosch's account, a detector of this kind needs current in a cable to find it. A cable carrying no current, because nothing on it is switched on or because its breaker is off, gives it nothing to find. Bosch's own summary of a green light is "Nothing was detected and you’re good to go", and of a yellow one, "If the light turns yellow, the result is inconclusive and you need to check the spot again."2 Read the green line beside step 6: green means nothing carrying current was found where you scanned, not that there's no cable. Your detector may work differently; its own instructions decide.

Where cables run

Cables often run straight up, down or across from sockets and switches. For UK homes, Electrical Safety First, a UK charity, puts it as a requirement: buried cables are to run horizontally or vertically from sockets and switches.1 This course didn't read the wiring standard behind that line, so it can't tell you that every cable in a given wall follows it, and ESF's page says, just before it, "A quality cable detector can help track buried cables before you start work."1 This course read nothing on where cables run in American walls. So in either country, treat the wall above, below and beside any socket or switch as the likeliest place for a cable, but not as the only one.

Power off, and the check that it is

ESF: "For any DIY near electrical wiring or power supplies, switch off the power at your fuseboard and, where possible, only use battery-powered tools." And: "To test that the power is off before beginning non-electrical DIY, plug an appliance in or try switching on lights."1 Elsewhere the same page writes "fuseboard (consumer unit)", so the two words mean the same box. That is ESF's test for non-electrical work such as drilling, and it is the one this course gives you. It is not a way of proving a circuit dead for work on the wiring, which lesson 5 left to electricians: that needs a proper tester and training, as HSE's guidance for electrical workers describes.6 ESF is a British charity, and this course read no American page on checking the power before non-electrical work; in a US home, trying the lights is the course's reading of ESF's test, not an American body's advice.

The main switch, which lesson 1 found and lesson 5 explained, is the front of the box and it is yours to switch. Everything behind the cover still is not, and CPSC's warning that a panel is usually live with the main breakers off still stands.5

If you hit something

If you hit a cable anyway, stop, and leave that circuit off. A damaged cable in a wall is wiring, and this course's line is Call someone, an electrician, whether or not anything tripped. ESF's advice on electrical work is the same: "Any electrical work should be left to competent professionals."1 If anyone has had a shock, First Aid and CPR lesson 2, "Danger, the call, and recognising cardiac arrest", has what comes first: the power off before you touch them, then the call.

If you hit a gas pipe, lesson 9, "Gas: the smell and the valve", has what National Gas says to do in Great Britain: call the emergency number day or night, even if no gas seems to be escaping. In Northern Ireland the number is the one in the box at the top of this lesson, 0800 002 001. This course read no Northern Ireland page on a struck pipe, so its reading is to follow lesson 9's Northern Ireland steps, which put that call first. In the US, this course read no utility page on a pipe hit inside a house either. Its reading is to treat it as a leak: leave, and call 911 or your gas utility once you're outside, as the box at the top says. Don't try to deal with the pipe: SoCalGas, a US utility, says "DO NOT attempt to control the leak or repair a damaged pipe or meter."11 Lesson 9 has the rest of each country's steps.

If you hit a water pipe, the stop tap or main shut-off valve that lesson 1 found is the first move, and if the water is reaching sockets, switches or the consumer unit, lesson 1's "When the water reaches the electrics" has the course's rule for going near the box.

A material that might be asbestos

In Great Britain, HSE lists "drilling into a wall" among the activities that can disturb asbestos, and says any building "built or refurbished before the year 2000" may contain it, while "Buildings constructed after 2000 are unlikely to contain asbestos."3 Its guide says that if you come across any material you think may contain asbestos, "stop work immediately".3 That guide is written for people at work; stopping is also what this course tells a householder. In the US, EPA says: "Generally, you can't tell whether a material contains asbestos simply by looking at it, unless it is labeled. If in doubt, treat the material as if it contains asbestos and leave it alone", and on its list of don'ts: "Don't saw, sand, scrape, or drill holes in asbestos-containing materials."4

A textured ceiling, an old board that looks like plasterboard (drywall), anything in a pre-2000 British home or an older American one that you are not sure of: don't drill it. That is a stop, not a judgement call, and lesson 19, "Asbestos, lead paint and radon: what not to disturb", is where the course teaches what comes next. That stop is this course's line, built on HSE's and EPA's advice. What the law says about asbestos in your own home, and who may work on it, is lesson 19's, with the legal label it needs.

A shelf on the Isle of Wight

The whole routine, on one job. A couple want a shelf on the living room wall of their house on the Isle of Wight, built in 2004. The wall is plastered, there's a double socket about a metre to the left of where the shelf will go, the shelf needs three fixings, and they've borrowed a digital detector and have a cordless drill.

  1. The date. Built in 2004, so on HSE's line the house is unlikely to contain asbestos.3 The wall is plain plaster, with no textured coating. No reason to stop here.
  2. Scan with the power on. Rings and watches off, the detector flat, a hand on the wall, the lamp on that socket and the room's lights switched on, several passes over the shelf's position, following the detector's own instructions.2 The line straight above the socket gets extra care; the shelf's fixings are well to the right of it. They mark what the detector finds.
  3. Power off. The main switch at the consumer unit, then ESF's check: the lights don't come on.1 With the mains off, a corded drill would have nothing to plug into, so ESF's "battery-powered tools" is also the practical choice.
  4. Eyes on. Eye protection, as lesson 13, "Tools, dust and fumes: a small kit used safely", set out.
Predict first

Step 5 is the dust. The wall's surface is plaster, and HSE's construction dust sheet puts gypsum, the stuff of plasterboard, under "general" dust, not silica. Does HSE's silica advice have anything to say about this job? Think about what the drill goes through.

Show the answer

It may well. Plaster is only the surface. Behind it there may be brick or concrete block, and HSE lists those among the silica materials ("bricks, tiles, concrete and mortar").8 If so, the drill makes silica dust once it is through the plaster. Three holes is the "occasional short-duration drilling" row of HSE's table: a dust collector at the bit or an extractor on the drill where possible, and otherwise a respirator with a factor of 20, which in the UK means FFP3.7 The couple do not know what's behind the plaster, so they plan for the silica case.

  1. Dust. An FFP3 on, with HSE's pre-use check, and a dust collector or an extractor on the drill if they have one. Without either, HSE's table leaves the job to the respirator.710 No dry sweeping afterwards. HSE's wood dust sheet says not to dry sweep, and to clean up with a vacuum rated at least dust class M ("medium hazard"). That sheet is written for wood dust at work, and this course applies it to plaster and brick dust as its own reading.9
  2. Fixings. The fixings for that wall, by the fixing maker's instructions. This course read no source on fixings, so it gives none.
  3. Power back on, and the lamp works.

Putting up a shelf this way is Yours, on conditions: a scan with the power on, the power off before drilling, eyes and lungs covered, and a stop at anything that might be asbestos.

A blind in an older American house

This one has a wrinkle, and you are going to find it before you read it.

In a 1950s house in the American Southwest, a homeowner wants to put up a blind above a bedroom window. There's a light switch beside the window. The wall above the window has the same swirled texture as the ceiling. It is a bright afternoon, so the room's lights are off, and nothing is plugged in on that wall. She runs a borrowed detector along the line above the window, gets green all the way, switches off the main breaker, and picks up the drill.

Check yourself

At least two things are wrong. Find the two that matter most, say which source's words answer each, and see if you spot anything else, before you open this.

Show the answer

1. The scan. With the room's lights off and nothing plugged in, any cable to that switch, or running past the window, had no current in it. Bosch: "A cable must have a current in order to be picked up by the device."2 So green told her nothing about cables. A switch beside the window is also a reason to take extra care along the line above it, though the rule ESF describes is British and this course read nothing on how American walls are wired. The order is lights on, scan, then power off.

2. The texture. It is a 1950s house and a textured finish she cannot identify. EPA: "Generally, you can't tell whether a material contains asbestos simply by looking at it, unless it is labeled. If in doubt, treat the material as if it contains asbestos and leave it alone", and "Don't saw, sand, scrape, or drill holes in asbestos-containing materials."4 This is the stop, and it comes before the scan matters at all. The blind waits until she knows what the texture is, which is lesson 19's subject.

Also: she went straight from the breaker to the drill, with no check that the power is off (ESF's lights test, which in a US home is the course's reading), and nothing for her eyes or the dust (lesson 13). But the texture stops the job before either matters.

The order of the checks is the lesson: what the wall is made of first, because some walls mean stop before anything else; then the scan, with current in the cables; then power off; then the drill.

What people get wrong

"A detector that shows nothing means there's no cable." It means nothing carrying current was found. Bosch: a cable needs current to be picked up.2

"Cables always run in straight lines from sockets." Often, and for UK homes ESF says that is how buried cables are required to go. This course did not read the standard behind that, read nothing on American walls, and a detector is still the check.1

"Main switch off, so everything's dead." CPSC: a panel is usually live with the main breakers off. ESF's lights test is for drilling, not for opening anything.15

"It's only a small hole." EPA's don'ts include drilling holes in materials that may contain asbestos, whatever the size.4

Practice

One wall, on paper

Take 20 minutes over this, with a notebook. It is planning only. Don't drill anything for this exercise. The only thing you touch is paper. You can look at the front of your consumer unit or panel to see where the main switch is, but no cover, faceplate or fitting comes off, and no tester or volt stick goes near anything. If you rent, any real drilling waits until it is your home to repair or your landlord agrees.

Pick one real job that means drilling a wall. Write the plan in this lesson's order:

  1. The house's age and the wall's surface, and whether anything about it means stop.
  2. Where the nearest sockets and switches are, and the lines above, below and beside them.
  3. How you will scan: what you will switch on first, and how many passes.
  4. How you will switch off, and ESF's check that it is off.
  5. What dust the wall will make, and how you will deal with it in HSE's order.
  6. What you will do if you hit a cable, a gas pipe or a water pipe, with the number or the valve.
Check yourself

Check your plan. Did you decide about the wall's material before the scan, or after? Did you scan before switching off, or after?

Show the answer

The material first: a stop at anything that might be asbestos comes before any scan (HSE, EPA).34 Then scan with something on the circuit switched on, and only then switch off before drilling. The other way round, the detector has nothing to find (Bosch).2

For your plan

To the stop list: any material you think might be asbestos when you're about to drill, and a cable or pipe hit while drilling. To the shut-off map: where you've found cables and pipes in walls you've scanned. And for the repair write-up: if your one repair involves drilling, this lesson's order is the plan.

Connections

Lesson 1, "How a house works, and where it switches off", found the main switch and the stop tap that this lesson relies on. Lesson 5, "Electricity: what trips, and why", explained why a silent volt stick doesn't prove a wire dead, and why the box stays shut. Lesson 13, "Tools, dust and fumes: a small kit used safely", gave the dust, the masks and the order of the controls that the shelf used. Lesson 9, "Gas: the smell and the valve", has what to do if the bit finds a gas pipe. First Aid and CPR lesson 2 has what to do for someone who has had a shock.

Next, lesson 15, "Ladders and working at height", takes the drill up a ladder. Lesson 19, "Asbestos, lead paint and radon: what not to disturb", teaches what happens after this lesson's stop.

Go deeper

Sources

  1. Electrical Safety First, "Advice for DIY-ers". Read: full text, re-read 25 and 26 September 2026. A UK charity that campaigns on electrical safety; the survey's date isn't given.
  2. Bosch DIY, "How to drill without any nasty surprises". Read: full text, re-read 25 and 26 September 2026. A maker's guidance for its own detectors.
  3. Health and Safety Executive, "Introduction to asbestos safety", parts 1 and 2, and "Asbestos: FAQs". Read: full text of parts 1 and 2, re-read 25 September 2026; the FAQs in full. Written for people at work.
  4. US Environmental Protection Agency, "Protect Your Family from Exposures to Asbestos", last updated 25 June 2026. Read: full text, re-read 26 September 2026.
  5. US Consumer Product Safety Commission, Publication 5133, "Preventing Home Fires: Arc Fault Circuit Interrupters (AFCIs)", dated R042012. Read: full text.
  6. Health and Safety Executive, Electricity at work: Safe working practices, HSG85, third edition, 2013. Read: the whole PDF extracted; "Proving dead" paragraphs 53 to 55 read. Workplace guidance, used here only for what it says about non-contact testers.
  7. Health and Safety Executive, Construction dust, Construction Information Sheet No 36 (Revision 3), first published 06/13, revised 03/20. Read: full text, 26 September 2026. Written for employers.
  8. Health and Safety Executive, "Cancer and construction: Silica". Read: full text, re-read 25 September 2026.
  9. Health and Safety Executive, Wood dust: Controlling the risks, Information Sheet No 23 (Revision 3), first published 11/12, revised 06/22. Read: pages 1 to 3, 6 and 7 (the risks, the law, respiratory protection), 26 September 2026. Written for woodworking employers.
  10. Health and Safety Executive, "All you need to know about disposable dust masks". Read: full text, re-read 25 September 2026. No date shown; written for agriculture.
  11. SoCalGas, "Natural Gas Leaks". Read: full text; re-read 25 and 26 September 2026. A US utility, about its own customers.

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