Wall mounted thermostat trips out electic box
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Alistair diydoctor
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Wall mounted thermostat trips out electic box

Post by Alistair diydoctor » Wed May 25, 2022 12:09 pm

Hi I got an intermittent fault where if I turn my thermostat up or down It can sometimes trip out the old electric box. I should mention that I have the electrics going into a modernish trip box first of all then goes into an old fashioned main switch and this is the one that intermittently switches off the trip box seems unaffected.

stoneyboy
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Re: Wall mounted thermostat trips out electic box

Post by stoneyboy » Fri May 27, 2022 8:40 pm

Hi alistair diydoctor,
Presumably you old switch is one of the original RCCD models. If this is correct it’s time to replace it.
Regards S

Alistair diydoctor
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Re: Wall mounted thermostat trips out electic box

Post by Alistair diydoctor » Fri May 27, 2022 9:03 pm

Sorry I thought I had an old switch but I don't. I just have a modernish RCD unit and occasionally by turning the thermostat up or down it switches the main switch off and the trip switches are unaffected.

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Re: Wall mounted thermostat trips out electic box

Post by stoneyboy » Sat May 28, 2022 10:56 pm

Hi Alastair diydoctor,
Your problem is beyond remote diagnosis and you should get a qualified person in to identify the problem.
Regards S

ericmark
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Re: Wall mounted thermostat trips out electic box

Post by ericmark » Sun Jun 05, 2022 9:36 am

The RCD/RCBO/RCCD or what every other name it is given, was called an ELCB at one time, comes in many flavours, specially when looking at older units.

In the UK we have two types of supply, called TN and TT, with TN the supplier provides an earth, with TT the user has an earth rod, the use of plastic pipes for water and gas means the basic earth rod can not allow enough current to flow to cause a MCB to operate due to an earth fault. So back around the 1980's we started fitting ELCB's the early ones were voltage operated, and it was found they could all too easy be shorted out by accident so the ELCB-v was banned and the ELCB-c was used, this used an imbalance of current in and out to trigger them.

In the main we used 100 mA although one could get 300 mA and even on amp versions, these did trip the supply if there was an earth fault, but it was decided the human body can normally survive a 30 mA shock, so we started fitting 30 mA versions.

However the 30 mA trips far easier and it was soon realised having one trip for all was not going to work with most homes, first we went to using two with a split board, then the RCBO was produced, which is a MCB and RCD combined, so we used RCBO's on essential supplies and RCD's on the rest using what was called a high integrity board, and as the price of the RCBO's dropped we went to all RCBO boards, but with a TT supply they had to be double pole RCBO's.

So over the years the RCD protection has changed, and also the internals of the RCD, today we have types AC, A, F, and B where back 20 years ago we only had type S which was delayed and the no type S would often trip with spikes on the supply.

Today we fit a surge protection device (SPD) to stop these surges, and the modern RCD is not affected as much with spikes and surges, but in my old house where RCD's were fitted around 1992 resetting one could trip the other unless all MCB's switched off first, they tripped much easier than the new ones.

So a boiler which has a spark generator could trip the old type due to the spikes from the spark generator, old boilers had pilot lights. The diodes inside a three port valve can cause DC on the supply which can affect type AC RCD's today we tend to use type A.

You may still have one of the old voltage operated types. So there are so many reasons why yours can trip near impossible to work out remotely, and the earth - neutral fault can be very hard to find, as fault can be in toaster but when kettle is used the RCD trips.

With that last fault unplugging rather than just switching off can help, as switches often don't switch neutral, but often it needs either a clamp-on ammeter with a 0.001 amp resolution on the scale, or an insulation tester to locate faults, since both are expensive and need skill to use, the insulation tester uses 500 volt to test with, in the main not a DIY job, sorry to be bearer of bad news, but likely you need an electrician.

You can try posting pictures of the device which trips, we may be able to at least identify what it is, but seems likely you will need an electrician.

Alistair diydoctor
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Re: Wall mounted thermostat trips out electic box

Post by Alistair diydoctor » Sun Jun 05, 2022 11:38 pm

Wow! That reply is so above my head but thanks for your thorough breakdown. And reminded me of my neighbours problems with her RCD unit tripping out and various electritions from the council trying to figure out her problem I unfortunately don't have that free service having bought my property. I think the only thing I worry about with it happening to me occasionally is...Is it dangerous to carry on vert occasionally switching it back on? Or is is it something I need to fix now?

ericmark
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Re: Wall mounted thermostat trips out electic box

Post by ericmark » Fri Jun 10, 2022 3:50 am

Hard to say, I know old house my RCD's could not trip for 2 years, then have 2 weeks with 5 trips, and then no more trips for another 2 years and no faults found.

The basic problem was too much on one 30 mA RCD, which is why with this house I fitted all RCBO's.

Central heating was designed before the RCD age, and I know some boilers would not work if the earth was missing, this means they must have used the earth, which they should not do really, but clearly they did, I think the spark ignition used the earth, but I am no heating and ventilation engineer, this was a bit naughty of the manufacturers, but in the days before the 30 mA RCD they got away with it.

Also the three port motorised valve rectifies the AC causing DC to be superimposed on the supply also a bit naughty, as it can cause RCD's to freeze, but it has been done for years, likely still done, and unlikely the plumber checks to see if the RCD is a type A before fitting a three port valve.

The RCD is with a TN installation claimed to be secondary protection, the primary protection is the earthing system and the overload device, so in the main there is no real problem with a RCD tripping every so often other than the problem when the freezer defrosts.

But it would be wrong without testing for me to say it's OK don't worry, it can stay that way for years, but likely it could without any real problems.

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