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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.

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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 2 '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 Dec 2 '21 at 12:20
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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 2 '21 at 12:33
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    +1 for location. It often helps!
    – Tim
    Dec 3 '21 at 8:56
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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 3 '21 at 16:27
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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.

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    You probably can't rule out rodents either.
    – Duston
    Dec 2 '21 at 14:34
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    +1 for the smoke detector tip!
    – NL_Derek
    Dec 2 '21 at 15:37
  • And if the smoke detector is > 10 years old, replace the complete assembly. Dec 3 '21 at 9:52
  • 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 3 '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 3 '21 at 11:57
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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.

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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!

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  • IDK what a 'ring' is, but start checking every outlet, +1.
    – Mazura
    Dec 3 '21 at 23:32
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    @Mazura The Wikipedia article en.wikipedia.org/wiki/Ring_circuit explains reasonably well.
    – grahamj42
    Dec 3 '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 4 '21 at 0:02

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