wondering about this incoming before the distibution board

welshdragon79 · Jul 24, 2023

whilst watching many a video in the middle of reading the many books on the journey to becoming a qualified sparky, i notice in a setup, that seems to have multiple main switches and fuses before the distribution board, as i am far from being qualified, let alone experienced, i was wondering if anyone could shed some light on the set-up, more so on the switch/fuse to the bottom right of the picture, as it seems extra, but i have no idea as to why.

[ElectriciansForums.net] wondering about this incoming before the distibution board

Taylortwocities · Jul 24, 2023

Very simple. The maximum length allowed for the meter tails connection to the consumer unit is 3 metres.
If it is more than that a fuse must be provided to provide over-current protection.
You can’t have 10metre tails only protected by the supplier’s fuse that is in the service head.

welshdragon79 · Jul 24, 2023

Taylortwocities said:
Very simple. The maximum length allowed for the meter tails connection to the consumer unit is 3 metres.
If it is more than that a fuse must be provided to provide over-current protection.
You can’t have 10metre tails only protected by the supplier’s fuse that is in the service head.

ah, makes sense now, this is only the second time ive seen this set-up, first one being in my mother's bungalow - a 60's build completely re-wired including a new incoming cable all the way back to the overhead line/pole,

oscar21 · Jul 24, 2023

There was a thread on the Screwfix forum about this a while ago, one poster was going nuts saying the KMF or whatever wasn't necessary, others loved winding him repeatedly saying it was.

I can kind of see his point though, the cutout has a 100A fuse and the KMF might do as well so technically any one of them might blow first in an overload or short circuit scenario. I don't really see what value it adds apart from obedience with the rules.

welshdragon79 · Jul 24, 2023

oscar21 said:
There was a thread on the Screwfix forum about this a while ago, one poster was going nuts saying the KMF or whatever wasn't necessary, others loved winding him repeatedly saying it was.

I can kind of see his point though, the cutout has a 100A fuse and the KMF might do as well so technically any one of them might blow first in an overload or short circuit scenario. I don't really see what value it adds apart from obedience with the rules.

in the case of my mother's home, i think the original distribution board was right at the end of the building in the garage ( in what is now an en-suite bedroom) and the only place to put the new one was daed centre of the building which on the shortest, iagonal route would be around 4 mtrs, but which i assume is 5-6 meters in totall, going off topic - a friend's (retired sparky) friend (a doctor) has had his house recently re-wired, but none of the cabling burried in the wall put through conduit, both of us thinking it should have done, but the sparky doing the work insisting that there's no need for conduit...

oscar21 · Jul 24, 2023

Dont really need tubing or capping in the wall, plastering over the cable directly is accepted, I always use capping though for a couple of reasons, one its easy to fasten the capping to the wall instead of trying to get a fix on hundreds of clips and two, its a lot easier to replace the cable when someone with half a brain screws or drills though one.

another thing about long tails is if they go through the fabric of the building then they need RCDing as well unless you can get them more than 50mm deep everywhere, so unless you want to start messing about with time delayed RCD's and all that its easier to use a steel wire armour submain instead of tails.

Taylortwocities · Jul 24, 2023

oscar21 said:
I can kind of see his point though, the cutout has a 100A fuse and the KMF might do as well so technically any one of them might blow first in an overload or short circuit scenario. I don't really see what value it adds apart from obedience with the rules.

A properly designed installation would have a lower- rated fuse in the KMF than that in the service head. Otherwise there will be no discrimination.