House is around 40 years old, detached 4 bed with double width garage (the only requirement ) and we extended about 5 years ago, building above the garage, utility room and "family room" (which is now an enlarged kitchen). Builder did all the new wiring and put in a new consumer unit with two RCDs, one of which is causing grief with nuisance trips.

The troublesome RCD protects 5 circuits with MCBs:

1. downstairs lighting (6A, all others are 32A)
2. sockets in the extension
3. sockets in lounge/hall/front beds/loft
4. sockets in util room and kitchen
5. main oven

Circuit 2 powers the attic room plasma TV and PS4, in the study a laser printer and modem/router and distribution box, plus computer gear (desktop PC and LCD monitor, now running off a UPS due to the frequent RCD tripping) plus another TV
Circuit 3 has another plasma TV and receiver, satellite tuner, with a spur off through the outside wall to an outside lean-to "greenhouse"
Circuit 4 has all the appliances on: fridge/freezer, combination microwave/oven, dishwasher, kettle, toaster, washer/drier, water softener, induction hob, extractor fan

A trip can sometimes be provoked by simply opening the door on the combination microwave/oven (so that the light might come on) regardless of whether it is on or not (it's always on for the clock, but not always microwaving or heating). Other times we might get a random trip and the RCD can be reset no problem, it might go hours, days or weeks before another trip. Sometimes a reset is followed within a minute by another trip, and this can happen several times in a row, often after 5 or 6 resets, it finally is OK. Other times it has tripped and no amount of resetting has worked, we've even had to leave the kitchen circuits off and run an extension lead from the lounge to power the fridge.

It did seem that tripping was more common in wet weather, and when the electrician came round, we did suspect that the outside greenhouse (power via the lounge) was the cause, with a lighting circuit inadvisably running of the ring main (ahem, sorry about that) which has been know to get wet. However, even though the greenhouse is isolated via the switched spur, we're still getting these nuisance trips. I could get an electrician around again and pay another £30 or whatever but all the testing last time showed no problem and of course it will never trip when the electrician is there.

The builder suggested swapping out the MCBs for RCBOs, I could buy 5 for £130 (or buy fewer, perhaps just for kitchen) but even if I manage to figure out which circuit is causing the problem, it won't fix anything other than keeping the other circuits going when the faulty one trips. Can I replace MCBs with RCBOs myself or do I need a qualified electrican? Do I need to notify building control?

What else can I do to test? Is there any testing I can myself do or get done by others on the appliances (like the combination microwave/oven) or other stuff that's plugged in?

Surely I don't have to get half the house re-wired do I?

I’m no electrician (though I was qualified to 16th Ed). What I’ve found, the hard way, with RCDs is that contact (grounding) with neutral can trip the RCD even if live is isolated. I’m not sure the mechanism there (neutrals are common?). Are you sure that “isolating” the greenhouse light spur has actually isolated it or just the live?

Alternatively, how about a slightly flaky cooker switch? They tend to be dual-pole and breaking one before the other can cause the RCD to trip. If that switch is tired, it might be triggering the RCD.

Just guesses based on experiences. Ultimately it might be worth getting the circuits tested by someone with a Megger because some insulation might be breaking down somewhere. Cheaper than getting it rewired unnecessarily

The greenhouse spur just isolates live I think, it's not a dual pole switch IIRC, but you could be on to something there.

I've never heard of Megger (just looked it up). The electrician that came before did do some testing but I don't know what he did and didn't find anything, the greenhouse only came up as a possible because there was evidence of a junction box getting wet, it was a dry day and testing came up with nothing.

All appliances on the ring main are always live, the dual pole switches are inside cupboards at the back (rather than all over the walls so not immediately accessible) and are never touched.

It could just be a flaky MCB. I have recently replaced one for my shower unit which decided to trip once a week or so.
Replaced it like for like and all is well.

It could be, but there's 5 of them tied to that RCD and I don't know which one (or more) might be faulty. MCBs are cheap, RCBOs less so.

Before the extension the CU only had MCBs with no RCDs, but I did install RCD protection on two spurs that went to outside sockets and outside brick lighting. Those RCDs are no longer used, I could put one in place of the fused spur for the greenhouse. Longer term of course I need to weather-proof the wiring in the greenhouse.

From experience, both MCBs and RCBOs seem to age and start tripping too easily.

Personally, for the cost of the 5 MCB (they are 2-3 Euros each over here), I would just swap them out and, if nothing else, rule them out.

Maybe, but these are only 5 years old AFAIK, but you're right, MCBs are cheap

Hi Dave, If this were me I would get a spark in to do an EICR (Electrical Installation Condition Report) as part of this process he will do end to end testing and earth leakage testing which should highlight the cause of the nuisance tripping. Also swapping out mcbs for rcbos isnt a straight swap ... i cant imagine why it wouldnt be notifiable work as you would be altering every circuit in the house.

Notifiable work in England




Like-for-like replacement isn’t notifiable but modifications might be - it’s not clear from above.

BTW - I’d power the lighting in the greenhouse using a solar-powered light unless you’re a hard-core winter gardener. I’m a fan of solar outside lighting. No faff with RCDs (which you should have on circuits outside the home), no outside mains etc. I’ve used Philips MyGarden lights for the drive and front door and solar security (dog) light from Costco.