10 years ago we bought this house. A bit of a project, and we had it
fully rewired among other things.

5 years ago I had the horrible concrete sectional garage and "log cabin"
(shed) used as a garden office replaced with a nice new building.
Including of course a workshop. Wired up of course from scratch.

As the 10 years were up on the original electrical certificate, and the
ins co requires an up to date one (it's thatched) we've just had it
redone. Decided it would be easier to get the lot done, which we did.

The new sparky realised that there was no overload protection on the
feed to the garage. Board fuse, RCB, the the wire under the drive. The
other end has protection on each _circuit_ but diversity allows them to
add up to more than the wire could take.

Ah well, we now have a new RCBO in the main box feeding the garage.

Andy

On Thursday, 23 November 2023 at 16:26:04 UTC, Vir Campestris wrote:
> 10 years ago we bought this house. A bit of a project, and we had it
> fully rewired among other things.
>
> 5 years ago I had the horrible concrete sectional garage and "log cabin"
> (shed) used as a garden office replaced with a nice new building.
> Including of course a workshop. Wired up of course from scratch.
>
> As the 10 years were up on the original electrical certificate, and the
> ins co requires an up to date one (it's thatched) we've just had it
> redone. Decided it would be easier to get the lot done, which we did.
>
> The new sparky realised that there was no overload protection on the
> feed to the garage. Board fuse, RCB, the the wire under the drive. The
> other end has protection on each _circuit_ but diversity allows them to
> add up to more than the wire could take.
>
> Ah well, we now have a new RCBO in the main box feeding the garage.
>
> Andy

Why an RCBO rather than an MCB? I imagine the feed is almost certainly SWA
which doesn't need residual current protection.
John

On 23/11/2023 16:37, John Walliker wrote:
> Why an RCBO rather than an MCB? I imagine the feed is almost certainly SWA
> which doesn't need residual current protection.

It is indeed SWA.

The guy seemed to know what he was doing, and that's what he supplied.
The cost of the device is going to be trivial in the overall bill.

Andy

On Thursday, 23 November 2023 at 21:02:30 UTC, Vir Campestris wrote:
> On 23/11/2023 16:37, John Walliker wrote:
> > Why an RCBO rather than an MCB? I imagine the feed is almost certainly SWA
> > which doesn't need residual current protection.
> It is indeed SWA.
>
> The guy seemed to know what he was doing, and that's what he supplied.
> The cost of the device is going to be trivial in the overall bill.
>
> Andy

I wasn't thinking about the cost but the lack of discrimination at the far end.
John

On 23/11/2023 22:52, John Walliker wrote:
> On Thursday, 23 November 2023 at 21:02:30 UTC, Vir Campestris wrote:
>> On 23/11/2023 16:37, John Walliker wrote:
>>> Why an RCBO rather than an MCB? I imagine the feed is almost certainly SWA
>>> which doesn't need residual current protection.
>> It is indeed SWA.
>>
>> The guy seemed to know what he was doing, and that's what he supplied.
>> The cost of the device is going to be trivial in the overall bill.
>>
>> Andy
>
> I wasn't thinking about the cost but the lack of discrimination at the far end.

Oh, I see.

There's certainly a chance that if I have an earth leakage fault it will
trip in two places. That should be easily sorted out by turning off the
main breaker in the garage, then resetting the one in the house. If it
doesn't trip then it'll be in the garage somewhere. I can then try each
circuit in turn. Besides I've had earth leakage protection for 5 years,
and it hasn't tripped yet!

None of the garage circuits will take enough current to trip the breaker
in the house. It would take a big load on two or more to do it. Pretty
unlikely.

It's only about 5m between the two boards.

Intermittent faults will of course be a PITA. But then, they always are.

Andy

Are you saying you have two identical paths under the drive feeding the
Garage wired as one big feed.
Brian

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"Vir Campestris" <vir.campestris@invalid.invalid> wrote in message
news:ujnuen$1t919$1@dont-email.me...
> 10 years ago we bought this house. A bit of a project, and we had it fully
> rewired among other things.
>
> 5 years ago I had the horrible concrete sectional garage and "log cabin"
> (shed) used as a garden office replaced with a nice new building.
> Including of course a workshop. Wired up of course from scratch.
>
> As the 10 years were up on the original electrical certificate, and the
> ins co requires an up to date one (it's thatched) we've just had it
> redone. Decided it would be easier to get the lot done, which we did.
>
> The new sparky realised that there was no overload protection on the feed
> to the garage. Board fuse, RCB, the the wire under the drive. The other
> end has protection on each _circuit_ but diversity allows them to add up
> to more than the wire could take.
>
> Ah well, we now have a new RCBO in the main box feeding the garage.
>
> Andy

On 30/11/2023 09:54, Brian Gaff wrote:
> Are you saying you have two identical paths under the drive feeding the
> Garage wired as one big feed.

It used to be that the only overload protection for the garage was on a
per-circuit basis, and the total permitted load for all the circuits is
more than the cable could take. Also that apart from the board fuse
there was no protection against an overload fault in the garage feed
cable itself.

We now have, in order:

Feed into house
Board fuse
Main breaker box with overload and earth leakage protection
New breaker in main box going to old ELCB for the garage
Small breaker box in the garage.

An earth fault in the garage could result in multiple things tripping,
or a random one of three. But at least now it is protected against all
earth leakage and overload faults, wherever caused.

Andy