New boiler tripping RCD

[TheGovernor]

How soon after you adjusted the thermostat did it trip?

Very quickly, within a few seconds I'd say...

 

[TheGovernor]

It is hard to isolate the problem without test gear. Last house had loads of things on one RCD and could run for 2 years no problem, then trip 10 times in a week, no fault found and another 2 years trouble free.

As a result this house all RCBO which is a RCD and MCB combined, so any fault limited to one circuit.

As an electrician I have three tools to help me, or at least should have.
1) Insulation tester - this puts 250/500/1000 volt depending on selection between two points and it measures the resistance, this is the main tool, but failed to find any fault with it in my own house.
2) The RCD tester - this normally puts 15 mA, 30 mA, 150 mA on the RCD to show it will not trip too soon, will trip at 30 mA and will trip in 40 mS at 150 mA all tests done with both positive and negative half cycles so 6 tests in all.
3) Clamp on ammeter - this is used to see what back ground leakage there is during normal operation, should be less than 9 mA, I have never done this test, as don't have a meter which will go down to 9 mA.

The problem is where there is a neutral to earth fault, as this can point to the wrong item, in the main one thinks the fault is some heavy current user, where really it is some light current user and the heavy current user has caused the voltage between earth and neutral to increase and so cause the trip.

I hope the BG Homecare electrician has now found your fault, however you must ask yourself is it worth changing the set up and using all RCBO's rather than a RCD shared by many circuits? When I put two RCD's on the last house, you could not get single width RCBO's, so I had no option, loosing a freezer full of food just before I moved, made me fit all RCBO's to reduce the risk, and when my roof leaked, I lost just a few sockets which I could do without.

He's here now, he's got the fuse box in bits and is testing various sockets.

Luckily he observed the fault, although not in the way I have, boiler running and heating,he flicked the upstairs light circuit on and it tripped, then as he was switching other things on, the downstairs lights to Off, it tripped again.

The common thing is the heating is on, so I don't know if it's really an issue with the boiler/controls or just that they are exposing a general problem I have.

And yes, to answer your question, I'd be quite happy to get a split RCBO, it was in my mind a few years ago but didn't get around to it, so I may well do that anyway assuming we get to the bottom of this issue. If I did, I'd probably be looking for the boiler and controls to be on its own circuit rather than running off the kitchen...

 

[TheGovernor]

Minor update, no obvious cause, he said that there is a difference in insulation readings in parts of the house, so in some cases he seeing 999M Ohm and in others 1.6M Ohm, although he says the 1.6 is still a pass...

He said the RCD is tripping at 28ms so that's fine... so currently not much further forward.

His current guess is that we have a faulty junction box somewhere under the floor, I'm not so sure but can't really argue!

I've got Vaillant coming out tomorrow to look at the boiler itself, my main worry is I get caught between the electricians saying it isn't their side and Vaillant saying it's not theirs

 

[Deleted member 267285]

Only other one I’ve had (just thought of it), have you hung anything up recently, a photo or mirror perhaps? This happened to me, and the spark was blaming the boiler, and I said it can’t be the boiler, as it’s happening if she charges her phone on the hallway socket (boiler was in the kitchen). Turned out she’d hung something and punctured the cable.

 

[TheGovernor]

Only other one I’ve had (just thought of it), have you hung anything up recently, a photo or mirror perhaps? This happened to me, and the spark was blaming the boiler, and I said it can’t be the boiler, as it’s happening if she charges her phone on the hallway socket (boiler was in the kitchen). Turned out she’d hung something and punctured the cable.

No, nothing like that... as I say the most recent change was the boiler, although it was working okay for a couple of weeks... I'm mystified at the moment, my hope now is that the Vaillant guy comes out and says it's a faulty PCB or something...

 

[ericmark]

999M Ohm and in others 1.6M Ohm, although he says the 1.6 is still a pass...

That is correct, but when one sees an unusual reading even if within limits it points to a fault.

 

[TheGovernor]

That is correct, but when one sees an unusual reading even if within limits it points to a fault.

Yeah, his guess is a failing junction box... so he said see what Vaillant say tomorrow if no joy then he'll come back and start pulling up the floor to find it... assuming there is one. The other thing we realised through all the testing is that the entire back of the house (Lounge, Master Bedroom, Second Bedroom) is all on the Kitchen Circuit... so there are loads of appliances etc hanging off that circuit. Essentially rather than having a upstairs and downstairs circuit we've got a front of house, back of house split... so my other route of putting an RCBO in will actually also require some rewiring into the bargain!

His notes on the paperwork just say Low IR on kitchen/house sockets...

Feeling a bit deflated now, hopefully the Vaillant guy will cheer me up..!

 

[Terrywookfit]

Looking at the pipework suggests a recent change to a Y plan, (Which needs a H/W off wire).
I have often seen the neutral wire used for this purpose and a borrowed neutral from any convenient source nearby.
Try feeding the heating from a non RCD source to see if fault persists.

 

[ericmark]

My house also split front/back with sockets, which is good, as if the RCD can't be reset you can run extension leads without them going up/down stairs.

 

[TheGovernor]

Looking at the pipework suggests a recent change to a Y plan, (Which needs a H/W off wire).
I have often seen the neutral wire used for this purpose and a borrowed neutral from any convenient source nearby.
Try feeding the heating from a non RCD source to see if fault persists.

That's a bit tricky in my case because I don't think I have any sockets that are not protected by the same RCD... I only have the single RCD protecting all circuits, and the kitchen circuit has loads of appliances on it due to where the lounge etc are. So I think I'd need to get an electrician round to run a new cable from the fuse board to the heating spur...

I don't believe the fitter did any electrical work at all in the airing cupboard, and he reused the supply from the old boiler, so electrically speaking everything is as it was before he arrived...

 

[Terrywookfit]

Where is the programmer ?

 

[TheGovernor]

Where is the programmer ?

Directly next to the boiler, in the same circuit...

Vaillant guy just been, he replaced the PCB purely to rule it out, he doesn't believe it is that.

He did disconnect the pump briefly while we tried to make it trip, which it didn't, so his best guess is the pump, even though it all runs okay etc, I'm just going to get that done to rule it out...

 

[JohnD]

That's a bit tricky in my case because I don't think I have any sockets that are not protected by the same RCD... I only have the single RCD protecting all circuits,

have you got a separate cooker supply circuit?

 

[JohnD]

That is correct, but when one sees an unusual reading even if within limits it points to a fault.

when I moved into my house, the ground floor (excluding kitchen) socket circuit had more earth leakage than the others. The utility room was damp and had an old washing machine and fridge in it, and a socket behind the sink. The sockets were full of builders dust and grit.

the circuit improved to normal on replacement of the old appliances and sockets.

I didn't track down which one was the source.

 

[TheGovernor]

have you got a separate cooker supply circuit?

The cooker is on a separate MCB, but protected by the same RCD...

I was thinking back to last week, at one point I changed the settings on the pump because I was getting vibration on the new cylinder, I set it to Auto (it's a Grundfos Alpha+) which is when I *think* sometime later we started getting these trips... is it possible that changing it to Auto which would generally make it run slower, has damaged it in some way? I'm definitely getting it swapped regardless, but could it have run dry somehow or got so hot (due to running more slowly) insulation has failed? So even though it works I'm getting this weird intermittent tripping?

I'll be honest if it turns out to be a failing pump I'll be quite happy as it's rubbish not trusting the heating system!