Consumer unit

[JohnW2]

I was thinking more on the lines of them accepting or expecting their fuse should blow for faults on their 3 metre portion and if it also blows on a fault past that on towards the far end of the universe they are not concerned as they have done their bit.

Yes, I realised what you were thinking/saying, but am still not sure that I would/could really understand that thinking. You seem to be assuming that a fuse-blowing-fault in the cable beyond the 3 metre mark would be a significant 'risk' - but, as I said, II would imagine that a fuse blowing as a result of a fault anywhere in the cable(s) between meter and CU would be incredibly rare, regardless of length ...

... and, as I also said, even if the consumer's additional (probably 'redundant') fuse did blw, there's probably more than a 50% probability that theirs would also blow.

As I wrote to Harry, if they would probably deny and interest in, or responsibility for, (short) tails which were in an overtly dangerous condition,why should they be concerned about the protection of those tails?

Kind Regards, John

 

[Harry Bloomfield]

... and, as I also said, even if the consumer's additional (probably 'redundant') fuse did blw, there's probably more than a 50% probability that theirs would also blow.

All things being equal, if one of the fuses blew, in all likelihood, the second fuse will have been stressed, and needing to be replaced anyway.

 

[JohnW2]

All things being equal, if one of the fuses blew, in all likelihood, the second fuse will have been stressed, and needing to be replaced anyway.

That also may well be true but, in reality, I'm not sure that many people would think about that, let alone try to get it done.

Although I may be wrong, I strongly suspect that if one contacted a DNO and asked them to replace a fuse of theirs which hadn't blown, they would very probably say that it was unnecessary. It's not as if there is really any 'safety' sisue .... if their fuse has been 'stressed', if that has any ongoing effect it will presumably be to make it more likely to blow (perhaps at currents below its 'rating'), not less likely to blow when it should?

Kind Regards, John

 

[Murdochcat]

Yes, and it's not only EDF. DNOs seem to have invented their own set of rules and regulations, some of which make little sense to mere mortals.

Quite and hence why it makes sense to understand the rules and approach of the DNO(s) in your area

There is no one size fits all and as they are outside the scope of BS 7671 we simple have to adapt to what they want …

 

[EFLImpudence]

I was thinking more on the lines of them accepting or expecting their fuse should blow for faults on their 3 metre portion

Of course.

and if it also blows on a fault past that on towards the far end of the universe they are not concerned as they have done their bit.

I don't understand.

Why then stipulate another fuse to be fitted?

What happens to any protective device further down after the 3 metres they consider they are not responsible for.
If theirs blows or both blow they are not concerned really.

Are you saying that when an additional fuse is fitted because the tails are more than three metres that that fuse will be more than three metres from the DNO fuse (two metres I believe at least one DNO stipulates)?
Surely that would not met their criteria.

 

[JohnW2]

Quite and hence why it makes sense to understand the rules and approach of the DNO(s) in your area .... There is no one size fits all and as they are outside the scope of BS 7671 we simple have to adapt to what they want …

Yes, we would seem to have no choice but to accept whatever enforceable DNO rules/regulation actually exist.

However, I suspect that a lot of the variation we see results from personal views/interpretations on the part of individuals employed by DNOs which go beyond what the rules/regs actually say, in which case I think it would be a bit naive to simply accept what an individual DNO employee says or demands, without questioning the basis and validity of that demand. We have similar problems with indiidualBCOs.

 

[ebee]

Of course.


I don't understand.

Why then stipulate another fuse to be fitted?


Are you saying that when an additional fuse is fitted because the tails are more than three metres that that fuse will be more than three metres from the DNO fuse (two metres I believe at least one DNO stipulates)?
Surely that would not met their criteria.

No - sorry if I muddled it by making it unclear, what I meant was they accept that their fuse will be OK for 3 metres of tails (or 2 metres if they decide so) , if you then put your own fuse within that 3 metre point then yours is responsible from that point onwards not theirs.
Whether both fuses blow on some faults is not a concern but they have the first bit and you have the second bit , from that you can step down to further fuses etc as circuits dictate

 

[JohnW2]

No - sorry if I muddled it by making it unclear, what I meant was they accept that their fuse will be OK for 3 metres of tails (or 2 metres if they decide so) ,

... OK, but why will there fuse not also be "OK" for 20 metres of cable?

[ maybe I need to again wheel out an ancient story about an 'electrician' whose services I decided to do without - when he aggressively asserted that, although a shower circuit was protected by an RCD in the CU (a fair distance away), it was essential to install a second RCD much closer to the shower "because any residual current would have fallen to too low a level to trip and RCD by the time it got back to the CU" ]

if you then put your own fuse within that 3 metre point then yours is responsible from that point onwards not theirs.

I don't really understand what that means, given that the entire length of the cable (no matter how long) would be adequately protected by the cutout fuse. Hence, whist it may be literally true that the consumer would be "responsible for" everything downstream of the fuse they had installed, they would thereby be "being responsible" for something that was unnecessary/redundant

As I've said, the only 'point' (in having the additional fuse) I can see is that it somewhat reduces the probability of the DNO fuse blowing (and having to be replaced by the DNO) if a 'fuse-blowing event arises.

Kind Regards, John

 

[ebee]

John, in reality I think that is the only bit they are interested in. Nothing else. They do not want to do the sums calculating anything farther than 3m or so from their fuse. If you put a lower ampage fuse or even an identical fuse at the end of that 3m boundary then you accept responsibility for whatever cable of whatever length follows your fuse. Their fuse - the cut out, might give adequate protection for 6m or 10m or more but they will not bother to do the calculations on any other bit, just their own bit so long as it complies to their rules and they are happy.
You might well put an identical 100A fuse within 3m of their cut out and both might blow on some faults. That does not bother them, you might put a 20A fuse on your own bit, likewise that does not bother them either

 

[ebee]

... OK, but why will there fuse not also be "OK" for 20 metres of cable?

Well it could well be but they do not care, so long as their bit complies to their rules and a quick glance does not give them any great concerns ref your bit then they have covered themselves.

 

[JohnW2]

Well it could well be but they do not care, so long as their bit complies to their rules and a quick glance does not give them any great concerns ref your bit then they have covered themselves.

Maybe it's just age-related dimness on my part

In trying to understand your thinking, I wonder whether it is perhaps just loop impedance you're thinking about? - i.e. that they are 'confident' (perhaps wrongly - see below!) that they fuse will give adequate protection to a cable of up to 3 metres but that, without the hassle of 'inspection and calculations', they would not be confident for longer cables?

If so, then, as above, at least per BS7671 (albeit they work to different regs!), I think their 'confidence' may be misplaced - since I think that, even for tails od near-zero length, even a 60A fuse would be pretty marginal (and 80A or 100A fuses 'inadequate') in providing adequate (per BS7671) protection if it were a TN-S installation with the 'maximum permissible Ze' of a TN-S installation (0.8Ω), would it?

The implication of what I'm saying is that if the Ze is 'high'and/or the tails are long, the DNO fuse (and any additional fuse installed by consumer) would probably need to be lower-rated than it is if one wanted to provided 'BS7671 protection' to the cable - and that could introduce problems of the adequacy of the maxim,um 'supply current'. If the issue were the length (hence impedance) of the cable, that could bee addressed with a fatter cable, but if Ze itself were too high for a 'large fuse', then that would be insoluble (other, perhaps, than by conversion to TN-C-S).

Kind Regards, John

 

[EFLImpudence]

John, in reality I think that is the only bit they are interested in. Nothing else.

But they are showing an interest in the rest by insisting on another fuse.

They do not want to do the sums calculating anything farther than 3m or so from their fuse. If you put a lower ampage fuse or even an identical fuse at the end of that 3m boundary then you accept responsibility for whatever cable of whatever length follows your fuse.

Exactly - so why not leave it up to the installer to decide?

Their fuse - the cut out, might give adequate protection for 6m or 10m or more but they will not bother to do the calculations on any other bit, just their own bit so long as it complies to their rules and they are happy.

Ok, so why then insist on anything for the extra length.

You keep saying they are only interested in "their own bit" but what we are querying is their interest in "our bit".

You might well put an identical 100A fuse within 3m of their cut out and both might blow on some faults. That does not bother them, you might put a 20A fuse on your own bit, likewise that does not bother them either

So why do they bother about it?