House RCD tripping - Upgrade to RCBO

[gblades]

My partner has an old farm house which is a listed building. The wiring was redone about 25 years ago and the consumer unit has a single RCD which supplies all the MCB's.

We have had issues with the RCD tripping out on occasion. There are a few oil heaters around which means it is difficult to test every item in turn as they may only have a fault when hot/cold or the thermostat switching in/out. The type of construction of the hose also means damp behind sockets is another thing to consider.

We tried switching off the power to the outside lights when we were last having problems however the MCB's are only single pole so if there is a neutral to earth fault outside power can still go along the neutral outside to earth and trip the MCB that way.
We got an electrician in last weekend to disconnect the two outside circuits entirely. That seemed to fix it as we turned everything on and no issue. However we later went upstairs and turned on the 20W table light and it tripped. Eventually we found we think it is one of the oil heaters as it hasn't tripped the last few days since we stopped using it.

Anyway thats the background and it is becoming a real pain as we have had this issue twice this year. So one plan is to upgrade the consumer unit.
Can someone with knowledge of the latest wiring regulations tell me if this setup is permitted and if not what additions would be required.

Enclosure only consumer unit.
Isolator switch.
Individual DP RCBO's for each circuit.

That way if there is a fault only that circuit is tripped and it completely isolates the circuit as it is double pole.

Thanks

 

[JohnW2]

Can someone with knowledge of the latest wiring regulations tell me if this setup is permitted and if not what additions would be required.
Enclosure only consumer unit.
Isolator switch.
Individual DP RCBO's for each circuit.
That way if there is a fault only that circuit is tripped and it completely isolates the circuit as it is double pole.

You might be able to get a CU enclosure with a Main Switch in it. As you have obviously worked out, it does not seem to be possible to buy CUs pre-populated with RCBOs.

DP RCBOs are uncommon, usually expensive and usually 'double width'. You probably do not need DP ones since, with all RCBOs, a neutral-earth fault on one circuit would not (other than in exceptional circumstances) result in any of the other RCDs tripping.

This would, of course, be very much 'notifiable' work, which would considerably add to the cost if you did it yourself (assuming you are competent to do so). If you intend to employ am electrician, he/she could advise on the materials, and could probably get them cheaper than you could (and would provide a warranty).

Kind Regards, John

 

[gblades]

Thank you for the reply.
I was looking at the likes of https://www.screwfix.com/p/wylex-16a-32ma-dp-type-b-mini-rcbo/8541x and these are about the same price as the better makes of regular SP RCBO's. Yes you can get cheaper but it is probably only going to add say £100 to the overall cost.

Thinking about it I agree with it not causing any other RCD's to trip. I still had in the back of my mind the existing unit with the master RCD but the new one would only have an isolator.

One additional thing is that due to the age of the property etc... it does have an earth rod outside the house and the supply to the house is quite long so I would suspect if a few things are on then there could be a noticeable voltage difference between neutral and earth. Thinking someone might turn off the breaker and then go outside and up a ladder to change an outside light a DP isolator certainly has an appeal at least for the two outside lighting circuits.

 

[fluorescence]

This is notifiable work & circuits will need testing etc for the work to be completed safely. You've already admitted there's a problem somewhere. I'd recommend just getting an electrician in the first place to do everything including buying the board, yes they'll add a bit of mark-up to the price but they'll know what board is decent and will provide any warranty work on it afterwards, should it be needed.

You don't need the expense of DP RCBO's IMO & getting a pre-populated board is often a better & cheaper idea.

 

[JohnW2]

Thank you for the reply. I was looking at the likes of https://www.screwfix.com/p/wylex-16a-32ma-dp-type-b-mini-rcbo/8541x and these are about the same price as the better makes of regular SP RCBO's. Yes you can get cheaper but it is probably only going to add say £100 to the overall cost.

Despite the title of that Screwfix listing, I'm far from convinced that it is a DP one. In any event ....

Thinking about it I agree with it not causing any other RCD's to trip. I still had in the back of my mind the existing unit with the master RCD but the new one would only have an isolator.

Quite. As I said, I don't tyhink you would really need DOP RCBOs.

One additional thing is that due to the age of the property etc... it does have an earth rod outside the house and the supply to the house is quite long so I would suspect if a few things are on then there could be a noticeable voltage difference between neutral and earth. Thinking someone might turn off the breaker and then go outside and up a ladder to change an outside light a DP isolator certainly has an appeal at least for the two outside lighting circuits.

Under normal circumstances (i.e. in the absence of faults), the length iof the earth connection will not affect the neutral-earth potential difference (since no current will be flowing in the earth) - but, yes, there would probably be a noticeable difference between neutral and earth, particularly when the installation (and other installations in the vicinity) was heavily loaded - but (again in the absence of major faults) that would not be a large enough potential difference to present a danger (and no different from the situation when one changes an indoor light bulb with a TN-S or TT (as yours) electricity supply. If you were concerned, you could always turn everything off (with the CU's main, DP, switch) when you changed light bulbs!

Kind Regards, John

 

[JohnW2]

You don't need the expense of DP RCBO's IMO & getting a pre-populated board is often a better & cheaper idea.

As you know, I agree that he doesn't need DP RCBOs and have commented on it being notifiable work. However, as for your latter comment, as I said (and the OP has presumably discovered) it seems difficult or impossible to find CUs pre-populated with (even SP) RCBOs.

Kind Regards, John

 

[fluorescence]

As you know, I agree that he doesn't need DP RCBOs and have commented on it being notifiable work. However, as for your latter comment, as I said (and the OP has presumably discovered) it seems difficult or impossible to find CUs pre-populated with (even SP) RCBOs.

Kind Regards, John

Fully populated RCBO boards are becoming quite common now-a-days. Any decent wholesaler will have stock on the shelf or will be able to get one next day

 

[EFLImpudence]

Despite the title of that Screwfix listing, I'm far from convinced that it is a DP one.

I suppose we can't tell from here but - going by the stated 32MA[sic] rating it is not suitable or written by someone unreliable.

 

[JohnW2]

Fully populated RCBO boards are becoming quite common now-a-days. Any decent wholesaler will have stock on the shelf or will be able to get one next day

That's interesting (and, I suppose, not surprising) - I was going by what I see 'advertised', which very rarely includes any RCBO-populated ones. TLC certainly doesn't currently 'offer' any (mind you, nor any MCB-populated ones other than BG).

Kind Regards, John

 

[ban-all-sheds]

Despite the title of that Screwfix listing, I'm far from convinced that it is a DP one.

It's SPSN.

How would a "DP" RCBO differ?

 

[ban-all-sheds]

I suppose we can't tell from here

We can look at Wylex's catalogue....

 

[JohnW2]

I suppose we can't tell from here but - going by the stated 32MA[sic] rating it is not suitable or written by someone unreliable.

Indeed - and even the part number as quoted by Screwfix (SFNHXS1B16) does not, as far as I can make out, seem to be known to Wylex (although it is similar in format to Wylex part numbers)!

Edit: I wonder if the initial 'SF' relates to Screwfix?

Kind Regards, John

 

[JohnW2]

We can look at Wylex's catalogue....

We could - if it were there. See my recent post.

 

[bernardgreen]

.
Other suppliers of RCBO equiped consumer units are available

 

[DetlefSchmitz]

Without checking the part numbers these look like the things to me: https://www.electrium.co.uk/media/20171130143632_0_224019 Wylex SNC RCBO (Web).pdf