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Today I heard a moderately loud "bang" sound whilst in the house, similar to someone dropping a heavy book, and the upstairs sockets all lost power (sockets has its own breaker). I noticed that the breaker had tripped (not the RCD) and after unplugging all devices, the breaker turns back on fine.

After plugging all the devices back in, everything is working normally - testing each individually. The upstairs has not typically very much load (two laptops and a printer at the time), and no-one else was around at the time.

Is this a concerning sign? Is there anything else I should be checking? I've looked at all the sockets and there is no visible damage. I've tried plugging in a low power device (LED Lamp) to each, and it all seems fine. Location is the UK, house was built ~2014.

David Wheatley
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SomeoneLostPower
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    A bang and a breaker tripping together usually not good. Possible both just happen near same time and one has nothing to do with the other, but odd. Think I would have someone check the circuit/house just for peace of mind. There was nothing happening outside(lighting storm)? – crip659 Dec 02 '21 at 11:39
  • This is the UK - lightning storms are pretty rare (no, no storm). The sound and the loss of power didn't happen near the same time, it was simultaneous. There was a wind "storm" a week ago, but nothing else mentionable – SomeoneLostPower Dec 02 '21 at 12:20
  • Winds can damage the power company's wires going to the building if in the air. Good breakers should only trip if overloaded or a short(bang?) in circuit. Should have power company check their wires for loose connections. – crip659 Dec 02 '21 at 12:31
  • The UK (at least where I am) uses almost exclusively underground cabling for residential distribution. Definitely no overhead wires to us! – SomeoneLostPower Dec 02 '21 at 12:32
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    Had something similar once and it was a nail through the cable under the floorboards. Everytime that board was stepped on it blew the breaker... – Solar Mike Dec 02 '21 at 12:33
  • Was it windy? Do you have overhead power entry to the building or is it buried ? – Criggie Dec 02 '21 at 21:50
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    +1 for location. It often helps! – Tim Dec 03 '21 at 08:56
  • Also check devices that you don't think are on the circuit - perhaps turn it off again and check all your sockets with a lamp, plus check that hard-wired stuff still functions (smoke alarms, burglar alarm etc.). Sometimes funny things can be done in wiring, like putting the garage on the upstairs circuit or mislabelling circuits and electricians sub-contracting to house-builders don't always follow the regs like they should – Chris H Dec 03 '21 at 11:42
  • @SomeoneLostPower true for the last bit in urban areas. Storms in the Cotswolds can cause glitches in Bristol from HV lines arcing, and plenty of rural properties are fed by their own overhead lines – Chris H Dec 03 '21 at 14:56
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    In about 4-5 days time, your nose will lead you to the location of the (by then very) dead rat that chewed through a wire, shorting it. – PcMan Dec 03 '21 at 16:27

3 Answers3

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Very difficult to diagnose. If there were other people in the house at the time you could ask them if they did anything to cause it.

Otherwise ....

The bang itself, what you heard, may have been an electrical arc caused inside (or worse, outside) a junction box. They do sound a bit like a book being dropped.

It may have been caused by damaged wires, foreign objects, physical damage, or other problems. The arc itself may have altered the problem so it doesn't immediately arc again. It may have improved the situation, for example, a stray bit of metal inside a junction box shorted the terminals, and was either incinerated or sent flying well out of harm's way. Or it may have worsened the situation, for example, damaged insulation on a cable exposed to a nail or pipe, and the arc moved the cable away from the metal object but also further damaged the insulation, which now is a bigger problem waiting to happen.

This is really hard to diagnose but here's what I would do:

  1. Think about any human activity during the prior week. Was any electrical thing added, removed, relocated, repaired, etc? Was anything hung, screwed, nailed to any wall, ceiling, or floor? That will provide priority for the search.
  2. Breaker popped but not RCD. That SHOULD mean there was a live-neutral short, not a ground fault. Not guaranteed but it may narrow the search a bit. The problem is likely to be in a junction box where live and neutral are both exposed, OR a nail clear through a cable shorting live to neutral.
  3. If there was any nail, screw, etc added recently ... start there. Be careful, it may be live (now touching hot but not neutral).
  4. If you've ever had squirrels/mice/rats in your attic inspect as much as practical for cable damage. If you have an open crawl space under your house, inspect exposed cables for rodent damage. Within reason (and with gloves and goggles) push aside insulation to inspect cables. Rodents love nesting behind insulation.
  5. Open up all the wall sockets, ceiling roses, spur fuses. If there was any human activity recently, start with those locations and then extend to everything on that breaker. If the arc was in a box it should be obvious from the soot, but look for other signs like foreign objects, loose cables, overcrowded boxes, etc.
  6. While each box is open turn the power back on and carefully use electrician's pliers to wiggle all the cables entering the box. Wear goggles. You're trying to provoke the short circuit.

If none of that reveals the problem it may be hidden inside the walls. There isn't much else that can be done. Change the batteries in your smoke detectors.

jay613
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    You probably can't rule out rodents either. – Duston Dec 02 '21 at 14:34
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    +1 for the smoke detector tip! – NL_Derek Dec 02 '21 at 15:37
  • And if the smoke detector is > 10 years old, replace the complete assembly. – spikey_richie Dec 03 '21 at 09:52
  • After those steps, think about DIY jobs etc. since last winter. If something has made delayed contact between L and N, it could be due to seasonal movement – Chris H Dec 03 '21 at 11:43
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    If it's an upstairs circuit, then it may run through the loft, which is where you're most likely to find rodents. – Simon B Dec 03 '21 at 11:57
  • However, the design of twin-and-earth cable (earth is between the L/N conductors) *should* mean that a rodent chews through to earth before the other L/N conductor, and that should trip the RCD. It should also be the case with nails through the cable etc. Junction boxes should also have L/N joins either side of earth connections. – Andrew Leach Dec 03 '21 at 12:11
  • @Andrew true, though it's not completely impossible to imagine a failure mode (including a staple through L+N, missing E, that circuit not being on the RCD as it presumably should be - which is worth checking in itself) – Chris H Dec 03 '21 at 12:27
  • @AndrewLeach good point re UK "twin and earth" cable construction. Still if a nail or screw was recently driven into a wall I'd prioritize that for investigation. I'm unaware whether ALL UK cable is constructed with earth firmly molded between two conductors. In the US that's relatively rare. [At least some](https://imgur.com/a/tHMjzzv) UK wall boxes are built so live and neutral terminals are in proximity. – jay613 Dec 03 '21 at 12:47
  • @jay613 the vast majority of modern UK domestic wiring is twin&earth constructed like that. The other options still permitted are more for commercial setting. While that socket design isn't uncommon, (i) the L and N are further apart than it looks, with N offset away from the camera compared to L, by more than the lateral offset, and (ii) even if a cable isn't screwed down properly, getting it to leave its own terminal is hard enough, let alone to reach another terminal. Far from impossible, but highly unlikely unless it's been worked on recently (e.g. loosened for decorating) – Chris H Dec 03 '21 at 13:13
  • Added rodents to answer, thanks to @Duston and SimonB. – jay613 Dec 03 '21 at 13:51
  • I'd skip straight to #5: open stuff up. Especially these "spur fuses" you're taking about. – Mazura Dec 03 '21 at 23:35
  • @Mazura Anyone reading this will hopefully be able to prioritize these ideas based on their own circumstances. The spur fuse is a junction box with a fuse in it where wiring transitions (spurs) from one size, suitable for the circuit's breaker, to a smaller size wire protected by that fuse. There is nothing about it that makes it more likely to be the cause of OP's problem. If the fuse were blown, lights would not work, it would be obvious. – jay613 Dec 04 '21 at 02:06
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Another thing to check: water where there shouldn't be water. Wet electrics can short with a bang, generating enough heat to dry them (mostly steam pressure expelling some of the water) so the breaker will turn back on.

Also check devices that you don't think are on the circuit. Perhaps turn that circuit off again and check all the sockets in the house with a lamp. plus check that hard-wired stuff still functions (smoke alarms, burglar alarm etc.). Sometimes funny things can be done in wiring, like putting the garage on the upstairs circuit, or mislabelling circuits, and electricians sub-contracting to house-builders don't always follow the regs like they should.

Chris H
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UK power circuits are (with certain exceptions), wired in a ring with a 32A breaker.

Each plug has either a 13A or a 3A cartridge fuse (other values are available, but discouraged).

It's quite unlikely that a problem in one of your loads would trip the breaker, rather than blow the plug fuse. It's even more unlikely that the total load exceeded 32A by enough to trip the breaker.

So it's likely that the ring suffered a live to neutral short which has either cleared itself or even broken one of the wires, opening the ring, while still leaving all the sockets powered in one direction or the other.

This could have been caused by a loose wire or water in a socket or rodent damage.

I suggest:

  1. Switch off the main breaker
  2. Test with a known working device, voltage detector, etc. each socket before ...
  3. Open each socket and check for loose or missing wires (each terminal should have two wires in a ring circuit).
  4. The earth wires should have green/yellow sleeving added by the electrician (the wire is bare in twin and earth cable), otherwise it can arc to the live terminal.
  5. Check for water and for soot smudges which might indicate the location of the arc which tripped the breaker.
  6. The next tests - loop impedance and insulation resistance - are best left to a qualified electrician.

I have personally experienced water ingress in a 15A circuit which removed 2cm of the outgoing live wire!

grahamj42
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  • IDK what a 'ring' is, but start checking every outlet, +1. – Mazura Dec 03 '21 at 23:32
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    @Mazura The Wikipedia article https://en.wikipedia.org/wiki/Ring_circuit explains reasonably well. – grahamj42 Dec 03 '21 at 23:42
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    "Ring circuits may continue to operate without the user being aware of any problem if there are certain types of fault condition or installation errors. This gives both robustness against failure and a potential for danger." - I thought "Switch off the main breaker" was for *extra* safety. It's an absolute requirement. – Mazura Dec 04 '21 at 00:02