house/shed supply arrangement

[SparkyTris]

IF

I have a house and a workshed/garage which are currently wired as follows:

incoming TNS header supplies two old "MEM" isolators (steel boxes with wire fuses and main switchgear).

One of these boxes controls the underground supply to the shed (small fusebox, 2xPIR and a socket).

The other box controls the supply to the domestic fusebox in the dwellinghouse, which distributes to the services.

Then,

Would I be well-advised to stick to this general arrangement for renovation (obviously installing RCD protection for the garage)?

or,

should I route the header straight into a split-load CU and take the garage supply from the RCD side?

or is it six and half-a-dozen?

also, what's the proper name for these steel boxes with a number of BS88 fuses and a switch? from memory, the switch is SPST - that's to say, the neutral is not switched in these boxes.

 

[plugwash]

are theese old mem switchfuses in good condition?

if so then you may as well just keep them in place and replace the fuseboxes in the house and outbuilding with modern CUs

saves you having to pull the service fuse too

 

[SparkyTris]

Thanks for that, Plugwash. Like everything else of empire-vintage, they are still working fine and probably have more life left in them than anything I can buy now. Believe me, I would *love* to leave those switchfuses in. However, the whole story is this:

the main header is in a cupboard in an "extension" to the dwelling (formerly the generator-shed); this is where the MEM switchfuses are located also. My mother & father now want this relocated in order to make an en-suite using the cupboard. So I am in the process of getting the header relocated to be on the external wall of the main dwelling.

So, although the locations will be slightly different, am I still OK to leave the general arrangement the same, or would I be better to go for the single split-loader?

 

[Big_Spark]

SparkyTris, these sound like Switched Fused Isolators, and in that case they are Double Pole Double Throw, ie, they break both live and Neutral on disconnection, except under fault conditions when only the phase will be disconnected.

I personally would bin them, and replace them with a 4 way enclosure containing One Double Pole Type C MCB for the House supply, and One 100mA RCD for the Garage supply. It would make the Job a lot neater, easier to work on in future and comply.

 

[fido]

I don't yet have power to my garage but I do have power to my workshop, which is a lean-to extension on the house with its only entrance door outside. I don't like RCDs so in the house I fitted a consumer unit with MCBs and a mainswitch. I have left the existing mini consumer unit in the workshop, which consists of a mainswitch and fuses for lights and sockets. The supply cable for the workshop has its own MCB in the main house CU so the wiring between the 2 boxes is also protected. The garage is remote from the house so that will have to be RCD protected when I get round to it.

 

[ban-all-sheds]

The garage is remote from the house so that will have to be RCD protected when I get round to it.

And so will any socket which might reasonably be expected to supply portable equipment outdoors, i.e. ground floor of the house and the workshop.

What do you mean by "I don't like RCDs"?

 

[fido]

I use a plug in RCD when using outside power tools etc. I don't like RCDs because they tend to cut off circuits unrelated to the one causing the unit to trip. I know you can use a split CU so that your freezer does not go off when a security light bulb blows and you are away on holiday, but they cause other problems. There are quite a few holiday homes in my area and many of them have problems with the electric cooker when they open up the house in the Spring. Dampness seeps into the elements, particularly solid hotplates, causing the RCD to trip when they are turned on. The RCD trips in well under a second so the element has no chance to heat up and dry out.

 

[ban-all-sheds]

So don't put the cooker on an RCD circuit. Unless you have a TT supply there is no need for it.


RCBOs would be better than relying on people always using a plug-in RCD...

 

[SparkyTris]

FWLEngineer - thanks for this advice, which I will take. Am I correct in thinking that the "4 way enclosure" you refer to could also be referred to as a 4-gang fusebox, the 4 gangs being needed for 2 double-width devices (the DP MCB and the RCD)?

I am also in a quandary about my equipotential zones. Along with the garage, there is an "annex" to the main-dwelling which is used as a guest bedroom with WC. It is not butted up to the main building but is accessed via 4 feet of covered alleyway. it has a radiator and running water. Can I make this part of the equipotential zone, or should I consider it as an outhouse (and therefore protect with RCD)?