Central Heating Boiler and Treadmill tripping MCB/RCD

Rod1971 · Nov 19, 2023

Good evening,

I have recently moved into a new home and I am having problems with the MCB/RCD tripping (Mem M6 Type 3 - 30mA). It intermittently trips when I switch on the Central Heating on and sometimes after running the heating for a couple of hours. The heating is gravity fed, hot water tank on first floor and 2 tanks in the loft. I spoke to my plumber a few weeks ago and he suggested the pump or the 3 way valve was faulty and carried out a resistance test which read approx. 400 ohms. I was thinking it was the valve because the pump comes for the hot water as well but it does not trip MCB/RCD (I realise there will be a different loading due to more energy needed to pump the water around the microbore plumbing). I measured the resistance of the windings after it had tripped and it was still around 400 ohms. Oddly the heating seemed to trip the circuit more in the evenings and then it would switch on fine the next morning. I cannot link the trips to any other appliances / lighting being on. Sometimes the heating would be ok after resetting the circuit but other times I would need to wait until the next day.

I was planning to replace the valve until we plugged in the treadmill into one of the sockets for the first time in the new house during the week and that also tripped the MCB/RCD. The treadmill trips the MCB/RCD every time I switch it on, and just before the motor starts running.

Both the heater and treadmill run from the sockets connection in the MCB board in the garage (Memera 2000). The MCB/RCD only seems to connect to the sockets (upstairs and downstairs) and one other circuit which I cannot trace. Not all of the MCBs are labelled which is not very helpful. The sockets upstairs and downstairs lose power and are on the same MCB/RCD circuit.

I used the treadmill to try and find the fault because it trips every time I switch it on. I disconnected all of the plugs in the house, all appliances, the outside light (which had permanent wiring), the electric oven (removed red and black wire going into the switch), the boiler circuit at the 3A fuse, the hot water pump (LNE on the side) and the MCB/RCD still tripped. I then took the cover off the MCB box in the garage and individually removed all the neutral wires to the other circuits in turn but that made no difference to the tripping.

I removed the neutral wire which goes to the socket and measured the resistance using a multi-meter (I don't have an insulation tester but can prob get one) and it read open circuit. When I read the resistance in the kitchen on the sockets it read 19 ohms.

I have attached a few photos of the unit in the garage.

Can anyone please suggest what I can try next? First time poster so please go easy

I am assuming that the fault can only be on something connected to the MCB/RCD i.e. from sockets connection, esp as I removed each other circuit in turn in the MCB box?

Rod1971 · Nov 19, 2023

Sorry forgot to mention the MCB/RCB is rated at 32 Amps (Mem AB32SH).

ninemaverickbars · Nov 21, 2023

I am not an electrician. As someone with reasonable DIY experience, If I was faced with the same situation, I would isolate the supply, test for isolation and then make sure everything in the socket is properly terminated.

Beyond that, I'd get an electrician to diagnose and find out what the unknown circuit is for as you'll need to know eventually anyway.

Rod1971 · Nov 21, 2023

ninemaverickbars said:
I am not an electrician. As someone with reasonable DIY experience, If I was faced with the same situation, I would isolate the supply, test for isolation and then make sure everything in the socket is properly terminated.

Beyond that, I'd get an electrician to diagnose and find out what the unknown circuit is for as you'll need to know eventually anyway.

Thank you, it is a bit of a mystery. Today I disconnected the neutrals of all the other circuits and still had the same problem with the treadmill. I tried split the ring main because there seems to be one circuit for the kitchen sockets and one for rest of the house sockets, both connected on the RCD, but a bit unclear how they are wired. I might give it one more shot tomorrow but I will probably have to get an electrician out to check, I am getting a few recommendations. The central heating also tripped the RCD today after a few hours of running when we started switching on other appliances.

I also connected the treadmill to the cooker socket which is not RCD protected and it ran ok but tripped the RCD, so all sockets in the house were without power (apart from the cooker socket).

Maybe there is a fault with the treadmill , it ran ok in my old house but there was no RCD in the supply..

ninemaverickbars · Nov 21, 2023

Rod1971 said:
Thank you, it is a bit of a mystery. Today I disconnected the neutrals of all the other circuits and still had the same problem with the treadmill. I tried split the ring main because there seems to be one circuit for the kitchen sockets and one for rest of the house sockets, both connected on the RCD, but a bit unclear how they are wired. I might give it one more shot tomorrow but I will probably have to get an electrician out to check, I am getting a few recommendations. The central heating also tripped the RCD today after a few hours of running when we started switching on other appliances.

I also connected the treadmill to the cooker socket which is not RCD protected and it ran ok but tripped the RCD, so all sockets in the house were without power (apart from the cooker socket).

Maybe there is a fault with the treadmill , it ran ok in my old house but there was no RCD in the supply..

I'll be honest, the CU looks a mess to me. Because of that, I'd assume that other wiring might also be a mess, or incomplete.

My view on this sort of stuff is that it's just not worth saving a few quid to try to DIY in these circumstances.

I'll happily wire a CU myself, maybe asking a few questions along the way, but I don't have the experience or qualifications needed to rewire a CU installed by someone else, if that makes sense? It's not you that I'm questioning, when I suggest getting a professional in.

One strong indicator that DIY should leave well alone is that things aren't even labelled. If they have skimped on labelling, the simplest of tasks, what else might they have skimped on?

I think thats why you haven't had any qualified electricians respond to your original post, it's just too complex to answer your question on a forum and would potentially put you at risk.

Rod1971 · Nov 21, 2023

ninemaverickbars said:
I'll be honest, the CU looks a mess to me. Because of that, I'd assume that other wiring might also be a mess, or incomplete.

My view on this sort of stuff is that it's just not worth saving a few quid to try to DIY in these circumstances.

I'll happily wire a CU myself, maybe asking a few questions along the way, but I don't have the experience or qualifications needed to rewire a CU installed by someone else, if that makes sense? It's not you that I'm questioning, when I suggest getting a professional in.

One strong indicator that DIY should leave well alone is that things aren't even labelled. If they have skimped on labelling, the simplest of tasks, what else might they have skimped on?

I think thats why you haven't had any qualified electricians respond to your original post, it's just too complex to answer your question on a forum and would potentially put you at risk.

Thanks, good point and make sense. Since moving in a few months ago I have been fixing problems left over from when the house was built in the 90's and I still have various other issues still to sort (non-electrical).

Rod1971 · Nov 28, 2023

Rod1971 said:
Thank you, it is a bit of a mystery. Today I disconnected the neutrals of all the other circuits and still had the same problem with the treadmill. I tried split the ring main because there seems to be one circuit for the kitchen sockets and one for rest of the house sockets, both connected on the RCD, but a bit unclear how they are wired. I might give it one more shot tomorrow but I will probably have to get an electrician out to check, I am getting a few recommendations. The central heating also tripped the RCD today after a few hours of running when we started switching on other appliances.

I also connected the treadmill to the cooker socket which is not RCD protected and it ran ok but tripped the RCD, so all sockets in the house were without power (apart from the cooker socket).

Maybe there is a fault with the treadmill , it ran ok in my old house but there was no RCD in the supply..

I disconnected a couple.more.ci

Rod1971 said:
Thanks, good point and make sense. Since moving in a few months ago I have been fixing problems left over from when the house was built in the 90's and I still have various other issues still to sort (non-electrical).

Solved - I tried disconnecting a few more circuits that I missed while carrying out initial checks. The immersion heater (double pole so unlikely to be a problem, the earth lead was not connected properly so good job I checked it), the gas oven socket feeding the ignition and the mains powered smoke alarms.

I split the ring main downstairs and the treadmill tripped on both circuits. I also tried the treadmill on a plug-in RCD on the oven socket and it worked so I thought it was unlikely to be the treadmill. To double check I took the treadmill to a friend's house and it worked on the RCD protected circuit and RCD protected extension lead.

I connected the treadmill to the MCB/RCD and disconnected all other circuits. After all the testing I concluded it was the MCB/RCD which I read sometimes get a bit sensitive. This was replaced with a RCBO and the treadmill and more importantly the heating works.

To note, I switched off the main switch on the CU when handling the circuits and double checked that this was no supply present with a multi meter.