32

One of the double sockets in our garage needed replacing (damaged after a washing pole hit it) - a pretty trivial job that I'd done before. So I flipped the MCB for the ring main, used a non-contact voltage tester to check nothing was there, and set to work. I disconnected the old socket fine, and was lining the cables up to fit into the new socket when... ow.

My finger bridged the live and earth pins, and suffice to say the live wire was very much alive. Bar a few finger spasms I was fine fortunately, and believe me I realise I was very lucky in that respect (I'm in the UK, so that was 240V.) Before finishing the job I then shut off the entire property's electrics - I was taking no more chances.

Further inspection confirmed two things I immediately suspected at this point - the MCB in the garage (installed before we moved into the house) was shutting off power via the neutral, not the live, and the voltage tester was dodgy (I had tested it on the same socket earlier and it seemed fine, further testing showed it presumably had a loose connection internally and only worked sporadically.) Needless to say it's now in the bin, and an electrician has been called to sort the MCB and check the rest of the house for similar issues.

Unsurprisingly this has shaken me up a bit - I would say I'm a rather safety conscious person, and in this case thought I'd done all the checks properly (including taping over the relevant trip switch to ensure no-one flipped it back on while I was working.) In some ways I can't help feeling I was unlucky that the MCB was wired by a cowboy, in combination with the voltage tester failing in the way that it did, at the time that it did. However, I'm not one to solely blame tools - clearly some, if not most of the blame here lies with myself also.

So in short, what should I have done differently - and was I primarily being stupid, or really quite unlucky?

Improve this question
12
  • 12
    I have a habit that if I'm sure that something is not live and I'm about to touch it, then before that I connect the live&earth pins with whatever (insulated handle) tool I have in hand. It's quick enough to do it by habit even in cases where you wouldn't pull out a tester, but if you're about to make a mistake then it shows you that without risk of life.
    – Peteris
    Commented Aug 17, 2014 at 9:03
  • 3
    If you read the directions that come with non-contact testers, it says to check it for proper operation on a live circuit. So what I do is push the button near a live circuit, and while holding it bring it to the circuit under test, then back to the live one.
    – Brad Gilbert
    Commented Aug 17, 2014 at 15:16
  • 7
    #1 Rule: The wire is always live.
    – Phonics The Hedgehog
    Commented Aug 18, 2014 at 2:12
  • 2
    I've been shocked too. All I was doing was taking the cord out of the socket so I could plug something else in but it was pretty difficult getting it out and somehow one of my fingers managed to touch the steel bit while it was still half plugged-in, the shock lasted for a good 4-5 seconds. I was really jittery (all over my body) for a few minutes, but other than the jittery feeling I felt absolutely fine. This has happened a few times over the years, but not in a long time.
    – jay_t55
    Commented Aug 18, 2014 at 10:38
  • 2
    As someone who has been through the same situation as Aeron (but shorter duration), I can confirm that, yes, it's unpleasant, and yes, it is frighteningly common, but no, it's not an instant fatality. Most likely, it didn't go deep enough to hit your heart, probably just the muscles of your arms. it's uncomfortable for a few minutes after it happens, but it doesn't kill if you're lucky.
    – Nzall
    Commented Aug 18, 2014 at 12:28

6 Answers 6

Reset to default
24

The paranoid way to use a non contact voltage tester is:

  • Test that outlet shows as live
  • Flip breaker
  • test that outlet shows as dead
  • Test a different outlet that should still be live

The last step is what would have saved you a shock here - its to check that your tester didn't die while you were flipping the breaker, and that you haven't done anything silly like forgetting to turn it on.

Though if it were a loose connection you still could be very unlucky and have it pass all those tests with the outlet still live.

Improve this answer
4
  • 4
    This is indeed the correct way of doing it. Verify the change, then verify the tool.
    – SQB
    Commented Aug 17, 2014 at 12:00
  • I would add one more step: STILL treat bare wires as if they are live and avoid bodily contact. The tester could be failing intermittently (as the OP suggested his was). The above test is definitely good but still not infallible.
    – pcdev
    Commented Feb 19, 2018 at 23:11
  • 2
    If I may offer some additional advice - once you've done all of these tests, your first form of contact with the wire should be with the back of your finger, preferably between the knuckle and the first joint while your hand is loosely balled into a fist. If you get a shock your hand will convulse away from the wire, and since your hand is already balled up your fingers can't easily catch on other wires and pull at them. It's also a relatively low-catch area of skin that heals quickly. Much better than getting shocked while pinching the bare wire.
    – Polynomial
    Commented Nov 20, 2018 at 15:23
  • I always hold the button down and bring it by a cord: beep. Keep the button down (and do not jar the NCV) and bring it to the suspect: beep (or no beep). Still having the button held down, bring it back to the cord: beep. If you jar it and get a beep, start over, which begins with first testing it while it's on. Then lastly, actually check it for potential with a solenoid voltage tester.
    – Mazura
    Commented Nov 1, 2019 at 2:06
16

Step 1

1. Test.

Step 2

2. Confirm.

enter image description here

3. Do work.

Step 4

4. Check your work.

Always test the tester. (Preferably with a different tester.) NCV's should only be used to confirm an outlet should work, NOT that it is dead. They are also nice for locating CBs if you want to play the flip on/off game, though not to be used for confirmation of that either. Item 4, the outlet tester, would have alerted you to the live wire and it's incorrect wiring. It's a common courtesy in my crew to ask "Did you check it with a wiggy yet?" (item 2) for their own safety as they begin. We also never defer to each other, check it again yourself.

Once I have a box open I check all around with a NCV again. Most likely there are other wires from other circuits and depending on the situation those get shut-off too.

A note on NCVs:

They will sometimes pick up a strong enough induced current (from proximity to live wires) to give you a false positive. Again, non-contact voltage testers are best used to see if an outlet should be working, at a glance, NOT to confirm that it is dead.

More on Stack Exchange: Is a negative reading from a non-contact voltage detector sufficient?

Improve this answer
8
  • 1
    I agree with all of this but am wondering if they make a plug in tester for 240v outlet configurations used in the UK and do they have gfci protection for them?
    – user24125
    Commented Aug 17, 2014 at 3:03
  • @user24125 I have no experience with UK power. A quick search found this: earthingoz.com.au/online-store/accessories/uk-outlet-tester I never had one so nifty so I assume the GFCI 'test' function on the one pictured just shorts the wires the same way the outlets button does, so you can redundantly test the outlet. Check your local giant hardware store.
    – Mazura
    Commented Aug 17, 2014 at 3:45
  • 1
    The GFCI test button shorts the live conductor to the ground conductor through a resistor. The point of the resistor is to provide just enough unbalanced current to trip the GFCI, but not enough to trip the breaker. Mine has a resistor that reads just under 18K ohms, which works out to be about 0.006 amps.
    – Brad Gilbert
    Commented Aug 17, 2014 at 15:37
  • 1
    @user24125 I have a similar socket tester for UK sockets, a few quid from maplin years ago. It's handy for checking the polarity of sockets on campsite pitches - in France they're backwards at least 1/3 of the time, but if working in the power circuit I plug that in to the appropriate socket and can see what's going on as I walk up to it. I still check with another tool though (often a multimeter as soon as the wires are exposed). I'm lucky in that only the labelling is dodgy here, not the wiring.
    – Chris H
    Commented Aug 18, 2014 at 8:30
  • I wonder what people would think of an NCV design which, when a button was pushed, would emit a "smooth" tone in the absence of AC or a "warbly" one in its absence? Such a device could be made in such a fashion that a "smooth" tone would pretty well imply the absence of AC, and almost any possible failure mode would cause it to either warble or not emit any sound at all.
    – supercat
    Commented Oct 18, 2014 at 23:00
7

Unlucky to have come across a dodgy installation. What follows is a US experience, same voltage. We lived in a house dating from "Rural Electrification" so that might give you an idea of the age of the fuse panel. There was this coiled bit of Romex hanging off a nail in the eaves, bare clipped off ends, that was hot. After pulling all the fuses and the Range and Pump 220V fuse blocks, it still was hot. We had to call the power company to come pull the meter as it was wired for who knows what ungodly reason, into the input lugs to the fuse panel.

The experience shows why I prefer a standard contact voltmeter for verification, two different methods for cross-check. I love the non-contacts for ease of use.

Improve this answer
3

Always verify the non-contact voltage tester is working properly, before trusting it (this is true for any tool, I guess). Aside from that, it just sounds like you were unlucky.

This is why it's important to know what you're doing, before attempting any DIY project. The person before you obviously did not, and the end result could have been your death (luckily it wasn't).

Improve this answer
1
  • Agree completely - I like to think I'm more than comfortable with a variety of simple tasks, but would never proceed with something I wasn't confident with.
    – user24536
    Commented Aug 16, 2014 at 13:03
1

I use a multimeter and check all three pairs (live-neutral, live-ground, and neutral-ground) before doing anything. This lets me catch bad wiring, as well as things like a leak to ground that may not have a high enough voltage to trigger an NCV or other yes/no tester.

Improve this answer
1

and believe me I realise I was very lucky in that respect

Most people I've spoken to who have worked with electricity have had at least a few shocks from the mains or mains-level voltages in their lives (I must be up to at least 5 or so by now) yet only a handful of electrocutions are reported in the UK each year. I must conclude from this that electric shocks are usually not fatal.

Of course "usually not fatal" is not a good basis for a safety policy. You should still do everything you can to avoid further shocks.

So in short, what should I have done differently

Non-contact voltage testers should be viewed as indicative only.

The professional electrician approach in the UK is to use a dedicated two-pole voltage testing device, combined with a proving unit. The steps would be

  1. Either test the circuit is live before de-energising or prove the tester is good on a proving unit or a known live supply.
  2. De-energise and lock off the circuit if it hasn't already been done
  3. Test between ALL COMBINATIONS of conductors. i.e. on a single phase supply live to neutral, live to earth and neutral to earth.
  4. Use the proving unit or known live supply again to prove the tester works.

It's a tricky question what diyers should do though, the procedure used by electricians works well for them but the equipment is expensive and the procedure is not without risks of it's own. Personally I would say the most important aspect is testing between all combinations of conductors. Using a multimeter that hasn't been "proved" is obviously less certain than using a dedicated tester that has been proved but it's still a lot better than not testing at all.

I would say your other mistake is buying a property and not getting the electrics checked out. Dodgy DIY wiring is all too common and you want to know about it before it bites you.

Improve this answer

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.