Thoughts on replacing switched fuse with small consumer unit

Irrespective of the DNO requirements I think you will struggle to get even a small garage CU in there. There is also the issue of discrimination as an MCB in the CU is unlikely to discriminate with the MCBs in the house CU. In the past I have not managed to find a CU that will take more than a 30A fuse carrier and I have had to fit external switch fuse. I think you will have to tap into the T&E somewhere and possibly fit an external enclosure.

On a separate issue is the T&E protected during the run in the house? Usually when used to supply CUs I see it stuffed in cavities and run under capping outside zones with no physical or RCD protection.

What's the current fuse rated at? 60A 80A 100A? I suspect you'll struggle to find an MCB for a CU that'll match that - I don't think I've ever seen anything above 50A in a CU. Larger MCBs are available, but I suspect are intended for larger DBs rather than domestic CUs - you'd have to check with the appropriate manufacturer.

+1 for the issue of reducing discrimination by using an MCB upstream of MCBs/RCBOs - it would be a shame to arrange things such that a single fault could black out the entire main house, where the chances of that happening were much less likely before.

I do agree that a single enclosure would be a much neater solution - but would prefer HBC fuses to MCBs. Unfortunately there aren't many consumer unit manufacturers that support that approach (that was one of the issues I attempted to address in the fictional "Concept" CU range). I did resort to putting a couple of DIN-rail mounted 22x58 fuse carriers, some chunky terminals and an isolator in an enclosure to solve a similar problem in my home, but with now BS 7671 insisting that doing so means that the installer takes all the responsibilities of a manufacturer under BS EN 61439 makes that approach much more of a challenge these days).

 Guess the "proper" solution would be to introduce a 2nd enclosed space right next to the exiting meter box (another meter box is often used) and stick a set of Henleys and switchfuses in there.

   - Andy.

AJJewsbury said:

 Guess the "proper" solution would be to introduce a 2nd enclosed space right next to the exiting meter box (another meter box is often used) and stick a set of Henleys and switchfuses in there.

I agree, but it does appear that the meter box might be inset into the wall rather than surface-mounted. If not, definitely a second enclosure.

For me, it is tails to a second enclosure, henley blocks and and new switch-fuse installed there, and the existing one moved across.

(If that isn't a smart meter, the supplier might have a job squeezing one in.)

All, thanks for the thoughts so far.

I hadn't thought about availability of suitable circuit breakers or that fuses enable better selectivity for the down stream consumer unit. 

I agree the T+E feeding the consumer unit is probably unprotected.  Seems to be the case for about 80% of the houses built in my area in the last 20 years. CU in the middle of the house with 5 to 8 meter cable run through building voids with unprotected cable (tails or T+E) and in most cases no additional fuse either.

The cables tend to be clipped, maybe with proper clips but quite often just with nails bent over. I have tried pulling a a couple of cables through in the past with no luck. Generally now I just note it down as I can't do much about it without some significant damage to the house.

I have to agree that in my mind an additional flush meter box is the best solution, I have been avoiding this path as I get in to working out how to comply with building regs for installation of the box. Time to bite the bullet and work out how to do it.

Building regs show something very similar to this

https://www.wtknowles.co.uk/wp-content/uploads/Knowles-KM01023-Electric-Recessed-Meter-Box-IOM.pdf 

Which looks incredibly difficult to achieve in a retrofit sittuation.

I gather many just cut a hole and put the box in, the good guys put a small lintel in, like this.

https://www.iqbuildersmerchant.com/catnic-meter-box-lintel-114950-p.asp 

And I don't think anyone worries about putting any sort of protective membrane around it and can't think of a reasonable way to attach it anyway.The alternative of running a damp proof course from the inner leaf of the wall is again far from easy with access issues and disruption caused.

If it were my house, I'd want a lintel, but regardless, there needs to be a brick pier between the two holes.

Alan B said:

I have to agree that in my mind an additional flush meter box is the best solution,

Any particular reason for not fitting a surface mounted meter box? You could then put it quite close to the flush one (possibly above it so it is out of the way? Or around the corner <3M away?) move the switchfuse into it to keep DNO happy and have your garage unit or whatever.

There won’t be a lintel above a meter box in a cavity wall, but there should be a cavity tray.

Or loads of spray foam at the shallow end of the quality control - I'm pretty sure that there are suppose to be cavity stops at windows and doors too, but a very large fraction seem to be just stuck in until it oozes like tooth paste and then trimmed flush.  In many ways the easy but slightly cop-out solution is to find someone who does brickwork for a living to make the aperture for you, as the job then reduces to the use of two pens - a large marker pen to show where you want the hole, and a second smaller one to sign the cheque. Nowadays the second pen can be replaced by BACS....

A similar thought occurs when you are too old to dig trenches for SWA any more.

Mike

Or loads of spray foam at the shallow end of the quality control

Cavity trays aren't to close the cavity as such, but to ensure that any water that might drip down inside the cavity gets directed outside before it meets any horizontal barrier that might allow the water to track or splash onto the the inner leaf.  Thus they need to slope downwards to the outside (usually aligned with weep vents) to work.

As they're next to impossible to install after the wall is built, I reckon they usually just get left out in later work.

   - Andy.

Regards meter box installation, definitely needs a cavity tray to direct any water, and there will be some, that runs down the back of the face brickwork out.

But a lintel is not required.

nhbc-standards.co.uk/.../