Consumer unit feed / main tails more than 3m

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?

Ah, but who says it's always going to be identical. DNO fuses aren't necessarily a constant - within my memory the 'norm' has gone from 60A to 80A to 100A and back to 80A (at least in these parts) . With the coming demand for EVs and heat pumps tomorrow might soon be different again. It's not uncommon for DNOs/suppliers to be asked for upgrades on existing installations - so what's the DNO/supplier to do then? At the moment if you ask for an upgrade to 100A they look at the visible bit of the meter tails up to the CU or whatever and if they're less than 25mm² they say no. Simples. So what happens if the tails disappear into the building and re-appear at one (or more) CUs miles away? The DNO then can't be sure that their change won't make thing less safe than it was before without having to do a lot of faffing about on the consumer's side which they're usually very disinclined to do.  With domestic customers they can't just say "we'll do what you ask, on your head be it" - as any court will say the professional DNO/supplier owes a duty of care to the 'ordinary' customer who the courts won't expect to necessarily understand the subtleties of CCC or ADS. So they invent an administrative procedure that avoids the problem for them.

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"

As an aside, one at least does say exactly that (I'll try to dig out which one) - they offer no agreement at all and their documentation explicitly says to treat the situation as if the meter tails were unprotected and apply BS 7671 434.2.1, which as it happens gives a result very similar to what the other DNOs would agree too.

   - Andy.

Maybe someone can explain this? 

Just a nice simple 3m rule will do me!

Not a standard I'm familiar with and I can't see the context, but my guess is that I think they're saying that if you have one protective device protecting two difference c.s.a. cables, that the total length that can be protected varies depending on proportions of the lengths of the two types of cables - as added together they need to meet the maximum permissible loop impedance.

For instance, say your protective device can protect either 10m of a 'thick' cable, or 5m of a 'thin' cable (before increasing loop impedances mean that disconnection times get too long), then you could have:

0m of 'thick' cable feeding 5m of 'thin' cable, or

5m of 'thick' cable, feeding 2.5m of 'thin' cable, or

10m of 'thick' cable, feeding 0m of 'thin' cable, or

anything in between, so long as the proportions still add up to 1 - so 25% of your thick cable allowance means you can have at most 75% of your thin cable allowance, or 33% vs 67% or 99% and 1%.

  - Andy.

It is just ohms law made incredibly complicated by an unnecessary nomograph.. Basically how far along you can slide line tap with a length of thin cable along a fatter distribution cable and achieve the same PSSC at the ends M,N and B. It is the street main and house radial situation.

It deserves a prize for being a really, really poor explanation, and I rather wonder if the authors did not grasp the simplicity of the point either.

Which document is it from ?

Also of course short circuit faults are something of an ideal fantasy - in reality all faults have some voltage drop, if they did not there would be no energy dissipated at the point of fault, and no flash no heat and no sound.

Mike.

It’s from Annex43D of IS10101 2020, effectively just an informative to the equivalent of Chapter 43 in BS7671 although the Irish Standard refers to it as Part 43! I vote it as the worst attempt at an informative explanation in either standard. It looks like it was taken from IEC 1246/08, whatever that is. I didn’t get it until AJ explained so perhaps the authors didn’t intend the readership to extend to duffers like me! 

Flippin ek, everybody does seem to make things very complicated. I have just worked in a 60s bungalow. The meter is in an outside cabinet on the attached garage wall, remote from the consumer unit in the bungalow. In the garage is a 60 Amp fuse contained within a metal clad Wylex switch fuse. The run from garage switch fuse to the bungalow consumer unit is about 6 metres. The run is in 16.00mm2 T&E cable run above the ceiling. It is fully visible in the garage. This installation has had no problems with this arrangement over the years and continues to operated perfectly.

This is the type of switch fuse used, although it is if an older design.....

Wylex 108M 60 Amp Metal Switchfuse Isolator Single Phase | eBay

Z.

Therefore - why does every installation not require a switch fuse?

Edit - this is in reply to AJ's previous post but it will not appear in the correct position.

There is no electrical reason for it. Of course as the tails lengthen the resistance rises and the PSSC falls, until eventually the fuse would not clear if there was a fault at the far end.The justification is one of the boundary of responsibility. Actually when DNO fuses do fail it is nearly always due to stuff happening beyond the 3m distance.

But that is equally true of DNO fuse, or a customers local one.  Personally I'd prefer things if a customer side switch or switch fuse after the meter was fitted in pretty much all installations, as it provides a safer approach for in-CU working where the temptation is to not pull the company fuse and to just be careful, but in terms of ADS it makes no difference, unless it was actually an RCD or perhaps RCBO.

M.

Slightly different tack a few miles down the road!