I'm in the UK, and my house has old aluminium wiring for the wall sockets. I think it's a ring main.

Over the last couple of weeks, either the RCD or a circuit breaker would trip every few days. Eventually the RCD refused to turn back on. The emergency electrician, who was only allowed to give immediate advice to power off the circuit breaker, said that there must be a dead short somewhere in the wiring of the house. The fact that the circuit breaker would sometimes trip (and not just the 30mA RCD) means it must be quite a severe problem. He said that the aluminium wiring in the house would have to be replaced, which would be very expensive. Electricians I've talked to so far don't even want to do that kind of work if there is anyone living in the house.

I tried using a multimeter on each wall socket to check whether there is an amount of resistance (rather than an open circuit) across the live and neutral connections. Obviously, this was done with the circuit breaker off. I was able to find that there was around 10 MΩ displaying temporarily on the multimeter, but it kept changing every few seconds between showing a figure around 10 MΩ and showing "1-" indicating an open circuit.

Is this the multimeter malfunctioning, because it is at the limit of its capability? What could account for the resistance figure constantly changing? I'm hoping to be able to isolate the short to one room/area of the house, but I can't do that if I can't trust the reading on the multimeter.

  • You'll only be able to isolate it to one ring that way, though you already know which ring it is… it's the one that keeps tripping. If the main switch is tripping, but not one ring, I'm not sure where to start, except to switch off the suspect ring & see if the main trips whilst in otherwise regular use.
    – Tetsujin
    Oct 11, 2022 at 13:34
  • @Tetsujin I assume there is a ring for each of the 4 circuit breakers connected to the RCD? Can't I disconnect the ring in two places and hunt down the fault that way? Oct 11, 2022 at 13:42
  • If you break the ring you change the loading… you'd have to break it in two places to totally isolate one segment, but that would mean the rest of the ring is now a spur, meaning your already iffy aluminium cable is being subjected to higher loads.
    – Tetsujin
    Oct 11, 2022 at 13:44
  • @Tetsujin I only meant to imply that I'd break the ring for the purpose of isolating the fault to one section of the ring Oct 11, 2022 at 13:45
  • "Higher loads" only if there are high loads. Hopefully during this testing OP will give his grow lights (or whatever) a rest. :).
    – jay613
    Oct 11, 2022 at 13:49

3 Answers 3


I would "divide and conquer" by doing things first that are less effort and provide more information, and eventually doing the harder things.

  1. Swap the breaker with another identical one in your CU. If the problem follows the breaker, just replace it and hopefully you're done. Possibly there is a problem in your ring that caused the breaker to fail, but you'll know that if the new one starts failing slowly like the old one did.
  2. Make a list of all the sockets that don't work. Draw a map and imagine how a ring would be arranged, if it really is a ring. Remember there might be some spurs so your map may not be perfect. But it's better than nothing.
  3. Open all the sockets and inspect them inside for obvious problems. Loose wires, stray bits of metal, broken plastic.
  4. Disconnect the socket in the middle of your map, leaving both sides of the ring disconnected and also disconnect one side of the ring at the CU,test it, then switch sides and test it. Hopefully in this test you will find that on one side the breaker still breaks, and on the other side half your sockets come back to life. Label all of them "good side" or "bad side" on your map.
  5. Reconnect the removed socket. Pick a new socket somewhere in the middle of the "bad side" of your map, and repeat the whole of step 3 with that one. At the end of this step you'll have a bigger good side and a smaller bad side. Rinse and repeat.
  6. Eventually, hopefully, you'll get stuck where you have two good sides which means it was something in the socket's junction box that went wrong ... and you can diagnose that. If not, you'll end up with two sockets that are adjacent in your map and BOTH produce a "bad side" in the direction of the other one. Then the problem is buried in the wall between them.


The above deliberately and repeatedly provokes what might be a short circuit. You might cause the failure of your breaker. You might even start a fire. Ok, that would require both the branch and main breakers to fail in the On position, so is very unlikely, but it's there. It would be better to diagnose this with a meter if you can. So consider these steps first:

The short might be to Ground so you might not detect it by checking live-to-neutral with your meter. Disconnect both sides of the ring in the CU and use your meter to measure each side resistance to both neutral and ground. Hopefully you'll find a low-resistance path and then you can run the above test sequence using your meter rather than the breaker to decide between good side and bad side.

  • Thanks for the advice! I guess the implication is that using a multimeter is not the right approach... but I'm not sure why. Is it just simply not a high-enough power device for this purpose? Oct 11, 2022 at 14:06
  • 1
    The meter is a better approach than the "keep blowing the breaker" one. Don't give up on that. I added some tips at the end to try additional things with the meter. Also added a new step 1 ... maybe just a bad breaker.
    – jay613
    Oct 11, 2022 at 14:19

Respect, don't fear, aluminum

First and foremost, don't be afraid of the aluminum wires. North America had a bad time with small aluminum wires (but not large ones for some reason), but did the hard research to find out why they failed. Two rules rose to the top.

  • The aluminum wire must land on terminals actually designed and rated for aluminum wire. The most reliable are what I call a "lug terminal" which has an aluminum body, a hole to put the wire in, and a setscrew to clamp the wire.
  • The screw torque must be set correctly with a torque wrench to the manufacturer specification - yes, even on the small stuff! The eye-opener here was extensive testing of master electricians that showed they were extremely bad at setting torques by feel.

Europeans and British positively love lug terminals, but the key is making sure your lug terminals have an aluminum body. Those are "universal donors" which cheerfully accept both aluminum and copper, due to thermal differences (i.e. aluminum's larger expansion rate works favorably when it's the lug and the wire is copper. The reverse is a disaster, and that's what you must avoid.)

Of course most people simply grab any socket or switch they can get, and don't use a torque screwdriver, which creates the risk with aluminum.

So become a master of identifying either receptacles and switches rated for aluminum wire, or finding a Europe-legal aluminum-body lug splice connector such as North America's Alumiconn, which accepts both aluminum and copper and allows you to "pigtail" to copper-only devices.

Failing to torque to spec, and improperly rated terminals, both have the same potential bad effect: a "series arc fault" meaning normal usage current is jumping across a small gap, making a very hot arc. Your overcurrent device won't detect this, and neither will the RCD. However the North American experience is that properly rated terminals and torquing to spec takes care of this.

It's not the wiring in the walls

Often when you can't find the problem in areas you can see, you start thinking the problem is in areas you can't see. Try your best to put that out of mind. Damage inside walls is unlikely unless there has been recent construction or nailing into walls. The problem is overwhelmingly likely to be inside accessible points such as junction boxes - this is largely the purpose of junction boxes.

The thing about UK ring mains is they are typically 32A or 7680 watts (4 times the capacity of American 120V/15A circuits)... which means they can power many rooms - a large part of the house. So that means quite a lot of hunting.

To identify which things are on that circuit, turn it off.

RCD trips have totally different reasons than overcurrent

You actually are displaying two very different symptoms here.

An overcurrent or short circuit trip is indicated when a plain breaker trips. Breakers don't have a way to tell you which one occurred.

An overcurrent trip is simply due to over-use of the circuit with too many appliances running. This can be determined by looking at the nameplate data of every appliance which makes an appreciable amount of heat, and totaling up the amps of the various loads.

A short-circuit trip is caused by a hard dead short between a live wire and a neutral or earth wire. This could be happening inside an appliance, but it will generally blow the fuse in the appliance plug before blowing a 32A breaker. So more likely it is in the house wiring somewhere (again almost certainly in a junction box). Typically this may be a frayed live wire contacting a frayed neutral wire, or a bare earth wire contacting a live wire or live terminal.

I understand you have an RCD module which is an RCD only, and not also a circuit breaker monitoring overcurrent/short circuit. Having that functionality separate is very valuable, or at least having indicators on the device to show the reason for the previous trip.

The RCD has a completely different operating principle. As you know, electric current flows in loops. The "outbound" current should always be exactly the same as the "inbound" current. The RCD compares those two currents and assures they are the same within 30mA. If it's not, some current is going AWOL and may be shocking someone or starting a fire.

This can be a tiny amount of leakage from live to earth or other earthed point like a water pipe. Or it can be leakage from neutral to earth, and that's something most people don't think about! However, neutral-earth leakage only arises when loads are active on the circuit. And depending on the nature of a neutral-earth fault, it may only arise when significant loads are active on the circuit.

Now the nice thing about UK installations is that for the overcurrent or short circuit trip, we can cross off plug-in appliances. They all have fuses in the plug which will blow first. So you can focus on each switch and receptacle in the ring main. And simply work your way through them, one at a time, pulling them out, inspecting them for deterioration and to assure the terminals are aluminum rated, and torqued to specification. Or, pigtail them off of an aluminum-rated splice.


One of the main problems with smaller gauge aluminum wires is the connections become loose over time and/or connections become corroded.

All connections would need to be checked for tightness with a torque driver, and probably cleaned and the use of a special paste for aluminum non aluminum connectors if wanting to keep the aluminum wires.

A better idea would be to replace with copper wires, this might be a DIY job if local regulations allow.

  • UK law is quite lenient on allowing you to electrocute yourself [certainly compared to gas work, which you are not allowed to touch at all]. You can even get away with a partial rewire without hitting building regs - warwickdc.gov.uk/info/20375/building_regulations/1140/… If you're rewiring, swap to copper. It's more expensive but it doesn't set fire.
    – Tetsujin
    Oct 11, 2022 at 13:39
  • Thanks for taking the time to answer. The main part of my question, however, is about how to figure out what part of the house has the wiring that is failing. The circuit controlled by the circuit breaker that trips is connected to every single socket across 3 floors of the house. I'd like to be able to figure out which section of wiring has the fault. Oct 11, 2022 at 13:41
  • You mean one ring is three floors?? That's dodgy. One ring, one floor is the usual; sometimes even subdivided further.
    – Tetsujin
    Oct 11, 2022 at 13:42
  • @Tetsujin I think it's normal in the UK for all wall sockets in the entire house to be on one ring. There was a loft extension where some of the sockets in the loft were also connected to that ring. Oct 11, 2022 at 13:43
  • 1
    @Tetsujin North America about the same, but only for single family homes by the owner. Rentals, multi family places is a no-no except by license electricians.
    – crip659
    Oct 11, 2022 at 13:43

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