OK I now I am a complete idiot but I just drilled through a mains cable. Big flash then the RCD trip switch kicked in and put all the power in the house out. However, when I put the power back on everything works no circuit trips? So I put an insulated screwdriver in the drill hole to check and everything trips again, the end of the screwdriver also got pretty melted. Reset trip switches and everything works fine.
I am having someone with a brain, unlike me, look at it tomorrow - an electrician. But is it safe to leave the power on overnight - the lead is below a floorboard but can not be accessed without a fair bit of work - skirting off etc hence the wait for the man with a brain. I am concerned that something may be smouldering away under my floorboards or if that was the case would the RCD/MCB kick in?
Thanks
PS Try to limit the comments on my stupidity and about how cheap pipe/cable detectors are. my wife wants to kill me as it is - take pity on me!
If you are that concerned then just turn the MCB off that powers this cable. Use extension leads to power essential equipment e.g. fridge/freezer if it is that circuit. The RCD or MCB should kick back in if there is a short but if it was my house I would turn the power off to that circuit for one night.
-- Adam
adamwadsworth@(REMOVETHIS)blueyonder.co.uk
PS We have all done it. The rules are to damage the CH cable on the coldest night of the year, the lighting circuits just before nightfall and the ring main just before a film you have wanted to watch for ages is due to start
Oh dear. You can get an RCD trip by connecting earth and neutral together but since you had a flash then it is live and either neutral or earth or both.
I don't think that it's a good idea to leave the cable powered, especially since it is hidden away. This would be a fire risk.
Do you have any way to identify which circuit the cable is other than poking a screwdriver into it? If you have a mains testing screwdriver you could put the tip *near* the cable and you *may* get an indication that it is live. Then you could selectively turn breakers off until you find which it is.
If you can't do that, then you should turn the power off for safety I think.
Regarding the other issue - flowers or a nice dinner, but not today. :-)
Think the best one along these lines is that when a chap drilled through the cable all the power got cut,he couldn't connect the fact that with no power the mains drill would no longer work, he then took the drill to bits to "fix" it couldn't put it back together so had to buy a new drill., (was on Britain's worst DIYer).
I have tried to isolate by using a mains testing screwdriver but there are 4 or 5 mains cables running near each other so the screwdriver does not give a clear answer.
I have noticed something odd in my consumer unit though, my MCBs are the type that go to the mid way mark when they trip, you then have to push them all the way down and all the way back up to reset. The one for my cooker is just all loose and floppy although the cooker is still working, could this be a knackered MCB but one that is still letting power through?
Take heart from the fact that I did the same thing about 20 years ago in a previous house. And we didn't have RCD's in them days.
It was a very useful lesson - subsequently I've always (without fail) checked for cables buried in the wall and take special care if there are sockets on the wall, or on the far side of the wall.
And I am always conscious if the drill appears to suddenly change in terms of physical feedback which could suggest it has stopped drilling brick and is now drilling something which isn't brick.
PoP
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Oh boy done that twice in my life! Once on that side of the Atlantic, in that instance it was fairly obvious what we (I) had disrupted; the lights in all the bedrooms! So we removed the fuse for that circuit and kept an eye on it until we went to bed. We did toy with the idea of turning off 'everything' overnight but decided against. The second time was on this side of the Atantic where we (I) cleverly drilled through a wooden floor joist at angle while completing this wood framed house; and ..... fssst! Splat! Into 115/230 volt house wiring. Fortunately everything was exposed and it only took about ten minutes to run a new piece of cable; minus the time spent cursing myself! Cheers. Terry.
I didn't do this, but had to fix an identivl situatin in a rented place
- I was pianting it an FLASH as you desribe. Emuslion pain in old screw hole with plug...
The electrician who came (via a call to teh letting agency) simply chased the wire out, and crimped on a new bit. We wrapped it all in insulation tape and I plastered it back where it came from.
from my experience of such circumstances, Damage to drill bit/screw/driver is far vorse than cable damage.
With RCD cable damage is often nothing more than slight loss of sleve.
chase out onough wall to get a few turns of good insulation tape around mising sheath then plaster back in. I'd be inclined to patch up existing cable than cut out and crimp a new piece in.
Oh, FFS! The original poster has had fault currents coursing through this little stretch of the cable not once, but twice. The first time he's had "a big flash"; the second time the fault current was enough to do damge to the end of his tool (a screwdriver, before you ask). That amount of energy released before his protective devices kicked in strongly suggests a hefty live-to-neutral flow, rather than a benign neutral-earth short or even a quickly-cleared live-to-earth. Assuming a modern installation, the main lengths of the cables ought to be OK, as the protective devices are specced to act quickly enough to limit the temperature rise in the conductors to below PVC-melting/softening point. *BUT* at the point of the short, the insulation and immediate conductors *will* be damaged, and it's irresponsible not to cut out the cable in the immediate area of the drilling and extend with a nice new bit.
With a bit of luck it might be possible to run a new length of cable to the nearest fitting in one direction without too much disruption, and make the join to the old cable in the other direction either by crimping or in a new wallbox fitted with a blanking plate, to act as a permanent reminder when next drilling, or maybe a wall-clock connection point ;-) Failing that, replacing the run of cable between the two existing fittings it serves makes for a decent job.
Patching up the injured cable with "a few turns of good insulating tape" is an approach worthy only of the spur-wearing mob...
The insulation is likely to be melted locally around the damage and the conductors will not be at their original condition even if the damage had only been slight. At best the continuity in the cable is going to be unreliable, and at worst it is a fire risk. Considering that the cable is also hidden and going to be hidden behind decorations that will be inconvenient to replace.
Cutting in a piece of cable with crimps is a 5 minute job. Making good the decorations will take longer.
It's silly bodging a job when it can be done properly and safely.
|Stefek Zaba wrote | ... in a new wallbox fitted with a blanking plate, to act as a | permanent reminder when next drilling, or maybe a wall-clock | connection point ;-)
"We'd always wondered why the previous owners had a Smiths Selectric Sun-Ray wired into the cooker circuit"
Well It is all sorted now. Basically, there was enough slack in the cable to cut out the damaged portion and fit a 30/40 Amp junction box to join the two sides of the cable. It was the old feed for the cooker which now just feeds
3 sockets as we have a gas cooker.
I was suprised by this whole thing because I was drilling and screwing along the joists as I have done to floorboards in the past. However, when the board was lifted you could see that someone has previously cut a fair size hole in the joist from its top i.e. u shaped and fed a few mains cables through.
Is/was this normal practice? I always thought following the joists was quite safe, I am begining to wonder if any of the other boards I have screwed down over the years, about half the house, may have screws perilously close to wiring. Although where possible I have always tried to look and work out the routing of wiring piping etc. I just got carried away yesterday as it was one of the last boards along the line of the joist.
Do you mean a junction box with screw terminals? If so, they are not supposed to be used in concealed places according to the Wiring Regulations - there needs to be access for repair. I believe that the theory is that screw terminals are considered to be liable to coming loose over time. Hence the discussion about using crimps. These are permitted for concealed joints.
Again the Wiring Regulations come into play. Cables are not supposed to be installed in notches in joists. The requirement is that they should be at least 50mm from either surface, so the convention is to drill through the centre of the joist and thread the cables. If there is a need to go nearer the surface, then the cables must be covered with a substantial shield - typically a thick steel plate which must then be earthed.
In any case, it is not a good idea to notch joists if at all possible because it does weaken them.
It's possible in an older house they simply re-used a notch made for either older cables or now redundant gas pipes etc. However, such a notch would normally be in the middle of a floorboard so it may still be nailed or screwed safely. If not, I'd put a steel strap across the notch *and* mark the floorboard clearly with the position of the cable.
the circuits might have been tested for loop impedance and insulation values but the wiring was certainly not 'tested' - i.e. search for buried junction boxes and notched loits.
Super; hadn't realised it was in the floor rather than in the wall; that's better for accessibility. Which is both a practical requirement (you did replace the floorboard nails with screws when you put them back down, didn't you? Makes lifting them next time *so* much easier; *and* gives you another opportunity to put a screw right through the cable you only just saw. Been there, done that, saw the flash, felt a total prat, still remember it though it was just about 20 years ago!) - and (breathlessly recovering back to the "both" where we started this sentence ;-) a requirement of the Regs when using junction boxen. I claim that under a screwed-down floorboard is accessible within the meaning of the Regs here, though I'll admit that it rather depends on the floor covering - vinyl you can peel back to reveal a nice hardboard with pipe and cable runs clearly labelled and access hatches is one end of the spectrum, while gert big sheets of marine ply, screed, and big-ass slate tiles would be quite the other ;-)
Ah, you *have* been screwing the boards back - good man! Joist notching
*was* normal practice, judging by the couple of older houses I've lived in and worked on, and comments in older installation guides. Except when I didn't, I usually played safe by putting any screws to replace lifted boards into, or pretty damn close to, the existing nail holes. These days notching's frowned upon, both because it weakens the joist, and because it makes it too unsportingly easy for Sod to guide your drill bit to them, as you discovered ;-) The current On-Site Guide says it's OK for cable to go
*through* joists; holes a minimum of 5cm away from top and bottom surface, and (for structural reasons) close neither to the centre nor the edges of the span. It's also OK to run the cable in earthed steel conduit, which provides good mechanical protection; the conduit can go in notches in the top of joints, and again there's guidance on max depth of notches (no more than 1/8th the depth of the joist) and position (away from center of span, not right close up to edges either). As a third alternative, it says you can try to provide "protection sufficient to prevent penetration of the cable by nails, screws and the like", and immediately pooh-poohs its own suggestion by saying that it's "difficult to meet" that requirement for protection! Hunky steel plate (2-3mm thick or more) is sometimes fitted over existing joist notches as a nod towards this "mechanical protection" requirement, but by the time you've done the chiselling around the notch to install and secure such plates, you could probably have rerun the cable through holes deeper down the joists anyway.
Hope that helps, and glad to hear you got a proper job done on the damaged bit of cable...
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