Main Electrics Isolator Switch

Deleted member 155212 · Aug 5, 2023

Hi, I have received conflicting advice about Main Isolator Switches and would appreciate forum member's help to plan a way ahead. Looking to the future I might need a 7.4kW/32A charging point and the garage isolator is only 60A.

1. Recently, during a kitchen refurb, double pole Mains Isolators were fitted in the Meter Enclosure - image 1. 25mm2 tails were fitted because the incoming power was upgraded to 100A.
2. Inside the garage, opposite the Meter Enclosure, the new tails are connected to an old MEM Isolator Switch - image 2. That Isolator switch has a ceramic re-wirable fuse and that fuse is only rated at 60A.
3. The armoured cable from that MEM Isolator Switch goes to a new, metal, split load Consumer Unit inside the house - image 3. Naturally that Consumer Unit has another Mains Isolator Switch.

The oldest part of the installation is the MEM Isolator Switch. I am having trouble finding a ceramic MEM re-wirable 100A fuse and am not sure that is the best solution anyway. So my questions are:

1. Do I actually need the MEM Isolator Switch in the garage if the Meter Enclosure and the Consumer Unit both have isolator switches?
2. Could that MEM Isolator Switch just be removed OR should it simply be replaced with a 100A cartridge type fuse OR a suitable 100A breaker?
3. One day I will need to fit a Charging Unit for a car and the location of the MEM Isolator Switch would be a good point to take a cable through the garage wall to the outside for that charging point. So what would that arrangement look like? Would it be a very small 'Consumer Type' unit with an isolator switch plus a 32A 'take off' connection for the charging point?
4. OR should the power for that charging point come from the new 'Consumer Unit' inside the house (i.e. the unit in image 3)? It seems odd to take the power all the way to the Consumer Unit in the house and then cable it all the way back to the garage.

Apologies for some of the terminology. Any help and advice would be greatly appreciated. Thanks for your time.

oscar21 · Aug 5, 2023

I'm guessing the MEM isolator is there because the run to the house consumer unit is more than 3 meters, if it is you need some kind of fuse in between the meter and the consumer unit. You coul dreplace the isolator with a garage type fuse box with several ways, the house being one of them but that would mean the house would be fed from something like a 63A MCB which isn't ideal, even a blowing light bulb could possibly trip it.

The best solution if you want to future proof it is to replace the mem isolator with a tail block,

[ElectriciansForums.net] Main Electrics Isolator Switch

from the tail block go to a new switch fuse isolator that would then supply the SWA for the house and you have spare ways in the tail block to connect a small consumer unit for the car charger and any other electrics in the garage.

pc1966 · Aug 5, 2023

As @oscar21 has said, if the run from the incoming supply to the CU is over 3m then it should have both a means of isolation (now coverd by the supply isolator) and some fault protection (e.g. fuse), as you are not normally allowed to rely on the DNO fuse (in the cut out before your meter) for those.

However, there might be another reason fo it being 60A rated before as that might be the limit of the SWA run to the house CU. In which case if being replaced by a more modern switch-fuse you would also need to limit it to whatever that cable is rated at as well as meeting the disconnection times needed, etc.

Splitting the supply to feed a garage CU that in turn has capacity for the sockets, lights, and an EV charger (without going through the house) is better for various reasons as already suggested.

But this is really not a DIY project! You would be best to get in a professional. It might be someone on this forum is close enough and can help you out.

oscar21 · Aug 5, 2023

Is the SWA new with the consumer unit upgrade or original, whenever I've seen those older isolators it usually feeds a piece of twin and earth to an old CU, not often a SWA. But if the charger is coming off the tail blocks then you have no reason to upgrade the supply to the consumer unit to 100A anymore, so can leave it at 60A if the SWA isn't big enough. I suppose you could also leave the older isolator there as well if everything complies which it should do if a new consumer unit has just been fitted.

Deleted member 155212 · Aug 5, 2023

The Forum has limited descriptions and the only one applicable was 'DIY' - there is no intention whatsoever to interfere with the arrangement and a qualified electrician will be used.
Good point about the tail block and that will help in all sorts of ways. I will have the SWA checked for rating and the Consumer Unit was fitted very recently.
The tail block will limit the power through the Consumer Unit so maintaining the 60A fuse at the Garage Isolator is a good option.
Thank you all for the valuable comments.

Deleted member 155212 · Aug 5, 2023

The revised arrangement - please check that I have fully understood the points made in the above threads. Obviously a qualified electrician will be used to carryout a detailed survey before any changes are made.