Consumer unit feed / main tails more than 3m

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

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