Consumer unit feed / main tails more than 3m

It's a rewire.

3m is not a BS 7671 rule, it's a DNO rule. Basically the cut out fuse is there to protect their network, not the wiring in the house. Yes, people usually use a switch fuse near the meter. If its a long run, then Zs and voltage drop need to be taken into account, although this is unlikely to be an issue in practice unless things are already marginal.

SWA is probably a good idea, especially as the cable won't be RCD protected and is likely to be within range of a 50mm nail at various points. It does mean that you'll need an adaptable box or similar to terminate the SWA near the switch fuse.

Could someone please explain the reasoning behind this requirement (never seen one - just do it) when it is perfectly acceptable to install an additional fuse of exactly the same rating as the DNO fuse.

In other words, how is a fault within the premises 'trained' to selectively blow the addition fuse but not the DNO one?

Despite it being said to be a DNO requirement, I have always thought it to be a misreading of 434, in particular 434.2.1, even though it does not apply in such an arrangement.

Bear in mind that 25 mm² SWA is not exactly bendy stuff!

Evening all

You need to have a quick squint at BS 7671 Regulation 433.3.1 and 434.3 to see what those regulations say about meter tails.

JP

Are you saying that the distributor disagrees "that their fuse affords protection to the part of the installation between the origin and the main distribution point of the installation" so an identical one must be fitted?

Essentially the DNO are saying that they'll only guarantee their fuse affords protection for the tails if they are less than 2.5m or 3m or whatever, and/or their route between meter and CU is visible, or whatever rules the DNO decides. Bear in mind that the DNO is (in principle) allowed to upgrade the head and/or the fuse rating.

There's no requirement for selectivity.

A good point Geoff. I remember reference to a “fault-free zone” being made in the busbar chamber days where you could lift say a set of 6mm2 tails of a 1000A busbar providing you didn’t exceed 3m before installation of your switch fuse.
Perhaps like the 6month RCD test, it’s just a number on the back of a fag packet!

wallywombat said:

Essentially the DNO are saying that they'll only guarantee their fuse affords protection for the tails if they are less than 2.5m or 3m or whatever, and/or their route between meter and CU is visible, or whatever rules the DNO decides.

I had no idea the DNOs were so powerful that they only have to say something to make it so.

I think that is my point. How do they manage to do that; i.e. limit the protection of their fuse to a short distance when the identical second fuse protects the whole length?

This is not a technical rule, it is an entirely administrative one. Of course if the DNO fuse will blow from a dead short 3m down the line, it will also blow if the fault path is made  a  few cm longer !! But, the DNO's legal team do not want responsibility for fusing cables that may be of any length and condition. Put the boot on the other foot and would the company fuse be the only protection for tens or hundreds of metres of cables to various outbuildings  and unknown routes and hidden joints across a rambling site and through the building fabric ? So an easy to remember rule is required. 
There is a coincidental  distance of 3m with some rules that are in BS7671, one about unprotected cables in things like busbar rooms comes to mind, but we are not talking about putting smaller cables on the 100A fuse without an MCB or something, so its not the same, (but DNO time switches on the board may be wired in something thinner) and  until recently you could just have easily said the same 3m distance occurs in rules separating  a bath and a socket, but it is not really that case either.

It is not wise to look for a technical reason the distance is not 1m or 10m, there really is not a good one.  In the days of the electricity boards, rather than separate DNOs and metering companies, in some blocks of flats etc the equivalent of meter tails spanning 2 or 3 floors was not unheard of. Nowadays  there is an extra row of fuse holders in the basement.

Mike