Consumer unit feed / main tails more than 3m

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.

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!

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!

lyledunn said:

Perhaps like the 6month RCD test, it’s just a number on the back of a fag packet!

And the 5 s disconnection time.

Yes, it's the same 'rule of thumb 3 m' as the 3 m distance we discuss in Regulation 433.2.2, i.e. where there is a reduction in CSA and where a supply-side protective device does not provide protection against overload current, we can place the device providing protection against overload current up to 3 m from the reduction in csa.

That is not the same situation as the meter tails.

However, you appear to have confirmed my suspicion by misreading the regulation.

geoffsd said:

That is not the same situation as the meter tails.

Not sure whether you missed my point. This is why I said "it's the same" as the 3 m distance we discuss in Regulation 433.2.2.

This particular rule of thumb being in actual fact a way of putting a quantitative value on "so close that we could perhaps consider the risk of fault is negligible, provided the wiring system is suitably installed to prevent faults, and preferably available to see there is no damage"

gkenyon said:

Not sure whether you missed my point. This is why I said "it's the same" as the 3 m distance we discuss in Regulation 433.2.2.

But it's not the same. Meter tails are not unprotected against overload nor are a reduction in CCC.

Where 433.2.2 is appropriate, the 3m. rule only applies where fault current protection is inadequate and then the cable must also be "installed in such a manner as to reduce the risk of fault to a minimum, and it is installed in such a manner as to reduce to a minimum the risk of fire or danger to persons".

Yes I think you've missed the point I was trying to make. I'm not saying it's not the same purpose (i.e. "Regulation"), but it's root of the "rule of thumb", i.e. for 433.2.2, and the DNO determining what they are wiling for their device to cover off, is the same effective installation requirement installed in such a manner as to reduce the risk of fault to a minimum, and it is installed in such a manner as to reduce to a minimum the risk of fire or danger to persons

So for 433.2.2, installed in such a manner as to reduce the risk of fault to a minimum, and it is installed in such a manner as to reduce to a minimum the risk of fire or danger to persons equates to 3 m, and in the DNO requirement it equates to 3 m

gkenyon said:

I'm not saying it's not the same purpose (i.e. "Regulation")

So you are saying it is the same. It is not.

My point is that the DNO cannot control what their fuse does merely because they say it will only work for 3m. so another identical one must be fitted near it.

The tails (if compliant) are "covered off", if that means protected, by the DNO fuse against overload and fault current therefore the allowances of 433.2 (position of devices for protection against overload) and 433.3 (omission of devices for protection against overload) do not enter into it.

 geoffsd 

NO, I'm saying the "rule of thumb" is the "same rule of thumb", NOT that the requirements it's used to satisfy are the same.

The rule of thumb being that if you only have 3 m of cable, the chances of a fault or damage are limited. That's all.

You may want to interpret that I'm saying it's related to BS 7671 ... you'd be wrong there, although I accept the rule of thumb is used in cases there too