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I have fairly solid walls in my apartment (plaster?), no drywall, so when I want to hang something or put in a screw, I need to drill a hole and first put in one of those small plastic 'sleeves' to hold the screw in place. (For what it's worth, I live in Europe, so it may be my conditions are slightly different from yours).

I was wondering, how do I avoid hitting an electrical wire when I do this. I asked an electrician who did work here, and he said 'good question', but that he didn't really have an answer other than to steer clear of vertical and horizontal lines from fixtures, plugs, etc. Is there any way to detect where the lines are running? Is there anything similar to a 'stud detector' for electrical wires? I suppose it can't be that hard, since people drill into walls frequently.

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    More important than the wall covering (drywall, plaster over drywall, etc.) is what's supporting that wall covering. Do you have poured concrete walls, brick walls, cinder block walls? Knowing what's behind the plaster will help a lot in determining how to find the wiring and plumbing. Don't forget, you don't want a drill bit going through a water line, either. Plumbing locations are often more obvious, but, especially in a multi-floor dwelling, there's still no guarantee.
    – FreeMan
    Sep 20 at 16:23
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    For hanging pictures or even putting those 1" or so anchors, you really don't need to sweat it. Sep 20 at 19:15
  • @FreeMan, thanks -- I think it's brick, but I'll check.
    – Cerulean
    Sep 21 at 23:40

7 Answers 7

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Start by searching online for "cable detector" - there are single & multi-task devices, live cable, pipe-work, stud wall.
If you have literally no clue what may be in there, check for all three.
If you have no idea what the substrate is, but know it cannot be stud wall [nothing sounds as cheap as stud wall when you knock on it;)) then obviously you will get false positives should you check for stud. Interior vs exterior wall might be block or brick vs reinforced concrete.

Example search at B&Q, UK big box store.

These things are not infallible, though they tend to err on the side of caution - false positive rather than negative, which is generally safer.
The visual guide of "is there a socket or light switch in line, horizontally or vertically, from here" is a good start, though.
Sensible cables runs go vertically up or down from sockets/switches, or occasionally horizontally - but you can never be 100% certain some previous DIY-er didn't make up their own odd route, literally & figuratively cutting corners.

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Depends on your local electrical codes.

In El NEC countries, and probably Canada, Code requires that any electrical cables and pipes be at least 1-1/2" below the finish surface, or else have a "guard plate" made of at least 1/16" steel to resist drills, screws and nails. As such, if everyone followed Code it is safe to use shorter fasteners than 1-1/2" as long as you STOP when you hit unexpectedly hard material.

Note that steel pipes are their own guard plates. PVC and PEX are not.

A sufficiently determined person can drill right through a nail plate or steel pipe, but they'll know they're doing it, and they'll have no one to blame but themselves LOL.

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There are stud detectors that also detect electrical wires and plumbing pipes. Have not used them so will need to check them out yourself.

There are also detectors for just electrical wires, but think they might cost more for the usual odd time you might need one.

People like to save money, so wires tend to run in straight lines, up/down, side to side of a room. Your electrician has a good general idea for looking.

Depending on the location, wires sometimes need to be an inch and a half from the wall surface(for stud type walls) or be protected by a steel plate.

Undoing face plates of nearby outlets/switches might show the direction the wires go.

Should find out what is behind the plaster, in case you want to put up heavier shelves that will need better anchoring than just in plaster.

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As others have mentioned:

  • better stud detectors can detect AC

  • You can also buy a specialized AC detector

  • You can also buy a special cable tracer. This works by putting an AC voltage on a DC wire.

  • In the US, it is code to put a metal plate on studs when a a pipe or electric wire goes through it. You should feel it when drilling.

  • Drill slowly and check as soon as you get through the plaster -- if you have a sufficiently (and not particularly) large hole, you can put a borescope in and take a look around.

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    borescopes are underutilized given how cheap they are now. a 1/4" hole push it into the wall cavity has have a look around! Sep 21 at 3:39
  • That assumes there is a wall cavity, Sep 21 at 3:39
  • you can get a cell phone borescope for <$50. I've used them to diagnose plumbing issues. As for cables: they are always in wall cavities.
    – gbronner
    Sep 21 at 13:17
  • Most of this advice is only applicable to hollow walls - drywall. In the UK only new builds use drywall. older buildings use brick, block, concrete etc, depending on age. UK code doesn't demand metal shielding on wiring drops, only plastic. Older houses won't have anything at all.
    – Tetsujin
    Sep 21 at 18:55
  • I'm a bit unclear how you'd hide an unshielded set of wires in a brick or concrete wall. In the US, we tend to use cheap 2x4s with drywall over it as well as incredibly fast drills and romex (NM) wire, which is soft enough to quickly 'catch' the sharp drill and damage the wires inside. If you are drilling through concrete / brick/ block, you are usually using a smaller and less sharp drill, and the whole process is much slower and you can stop and look around. So less of an issue.
    – gbronner
    Sep 21 at 21:03
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For what it's worth, I live in Europe, so it may be my conditions are slightly different from yours

Or it may be your conditions are massively different. Building and wiring practices are something that vary massively round the world. The impression I get is there are lots of people on this site who know how things work in north America, some (like myself) who know how they work in the UK, but very few who know about how things work elsewhere.

There are a few things that can help, none of them are perfect.

  1. Know the regulations/standards/codes for your country on where wires are or are-not allowed to be installed. This isn't perfect, not everyone follows the rules but it's a good first pass.
  2. There exist both metal detectors that aim to detect any metalwork in the wall and detectors that aim to detect live wires explicitly. Nether are perfect.
  3. Remove accessories and observe how the wires enter the box. This is likely to be very informative on solid walls, less so on hollow walls were cables may run behind the box before entering it.
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Get a cable detector - inexpensive ones run for 10$ or less.

As you already know, the common (correct) way to lay cables is horizontally/vertically from outlets, so if you intend to drill anywhere nearby one of these lines, start by tracing the wires from there to check they don't run in your direction (and to get familiar with how the device works).
You can try mapping whole room and note it for future use. If the detector cannot see a cable, try turning the light on. Of course check the place you are drilling into, in case somebody ran a cable through there.

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Use a drill stop or a piece of tape on the drill bit, and only drill through the thickness of the plasterboard. That's 10mm where I am. YMMV.

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  • OP has no plasterboard.
    – Tetsujin
    Sep 21 at 18:56

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