Externally mounted Electric meter box - Consumer unit

Discussion in 'Electricians' Talk' started by DannyDoLittle, Sep 1, 2017.

  1. JP.

    JP. Screwfix Select

    Hmmmmm discrimination or lack of it seems - too wit

    Eeeeeny meeeeeny mini mo, I wonder what fuse will be the first to blow
    Last edited: Sep 17, 2017
  2. Bazza-spark

    Bazza-spark Screwfix Select

    The responsibility for certifying equipment and CE marking equipment from outside the EU lies with the importer into the relevant EU country. If, say Chint for example, wish to import Chinese circuit breakers and market them as theirs, they are responsible for ensuring the breakers comply with the relevant EU standards and they have to have them type tested to those standards. They then issue the DoC and are responsible for any failures.

    Any fake goods can be reported to the EU Commission via their RAPEX portal and appropriate action is taken.

    Kind regards
  3. c0d3r

    c0d3r Member

    But what business are going to shoot themselves in the foot but not fezzing up?

    When is a failure a design or manufacturer flaw? Its so subjective even without imports being actively policed and tested by say Trading Standards & Custom's & Excise.

    Kind regards too. :)
  4. unphased

    unphased Screwfix Select

    I rest my case. Someone else who just accepts everything at face value. That's exactly why so much rubbish is just repeated over and over because nobody seems prepared to challenge anything. If something doesn't look logical to me I check it out. Thats exactly what I did with the 3-metres 'rule' on tails because it just didn't seem logical. But, it won't ever be accepted by the vast majority of sparks and DNO installers because they have 'heard' it repeated so many times it has stuck.
  5. unphased

    unphased Screwfix Select

    An isolator switch in the tails is useful. It allows the supply to be isolated without having to interfere with the meter or the cut-out. Putting a fuse in the isolator switch is pointless. If, for example, you wanted to run 10m metres or so of tails in 16mm2 insulated and sheathed then you would need to install a fuse to comply with Regs. ie change in csa from 25mm2 down to 16mm2. Having chains of fuses along a conductor is achieving nothing, tbh. The 'reason' for the fuse seems to be lost by those who just fit them because they have always done it.
  6. Coloumb

    Coloumb Screwfix Select

    Tbh mate I can't really see a situation where you would want to do this. In GN7, the regs is explained in as much as you can "omit" protection, because your cut-out doesn't come under the auspices of 7671, technically you are omitting protection IF you want to reduce the CSA of what ever is carrying the leccy from the cut out to the CU. So where does this leave us? Well the only situation that would sensibly fit into this would be the bus chamber where the cut out feeds multiple supplies from a bus bar system, so the main bus bar is nice and chunky and each feed to a cu could be of a reduced CSA since it's unlikely that one CU will ever draw a current (overload) larger than the main cut out fuse. Each feed must also be less than 3m long and covered in type-tested material. Where would you ever come across this other than large scale industrial or commercial office premises? Never in a domestic. This is opposed to the other reg that I can't remember that allows "relaxation" of the csa but does not limit it's length, or we would know as it, a spur. And of course we can do this because we don't omit fault protection. I recall someone here was convinced for a long while that any spur could not be longer than 3m, I can't remember who it was though.
  7. Bazza-spark

    Bazza-spark Screwfix Select

    The following is a page from the current edition of the on-site guide.


    Kind regards
  8. Coloumb

    Coloumb Screwfix Select

    No three meter rule then...
  9. JP.

    JP. Screwfix Select

    Problem is is that nothing is cast in stone with ref to posted page..for example

    Many things can be read into the above examples

    I am going to see if I can find 7:3:2, I presume its in the OSG?
  10. JP.

    JP. Screwfix Select

    Nowt in 7:3:2 - just walls and partitions, so not really relevent within the parameters of the topic.
  11. Coloumb

    Coloumb Screwfix Select

    There are two issues here, one is - is there a three meter rule in 7671? Well, clearly there is not otherwise it would be written above.
    Is there a three meter rule for the distributor? Well yes there is, if they have such a "rule". Some of the pdf links posted have this rule. The problem is they have no legal framework to enforce the "rule". They might want it, they might desire, they might stay up all night worrying about it but can they enforce it? No they can not, well not easily. Ergo, there is no three meter rule anywhere at all ever. Full stop.

    So where does this leave us? Back where we started. Ie, you call up the distributor, you tell them what you want to do and they either agree or they do not. If they do not agree then on what basis? That it is somehow safer to fit a switch fuse as the same rating as the cut-out? How does that work then? If your Zs (or Zdb) is within limits of the fuse and the run is in swa etc there is no breach of any regs.

    My own personal experience of this is that they are 100% not interested in what you do. As far as they are concerned it's your electric, you paid for it. End off. I've had meter people come round fit isolators, smart meters, you name it. UK power networks have been out to change my cut out. Do they give a toss? Well I'm living proof that clearly they do not. And my meter tails are about 10m of 16mm SWA.

    They only people getting busted up about this are the sparks on this forum "who have always done it that way". What kind of argument is that mate? Have they not got any ability to think for themselves? If those pdf links said you had to wear pink frilly knickers to work on the meter tails would they being wereing them? As it's "always been done like that"?
    Bazza and Dr Bodgit like this.
  12. JP.

    JP. Screwfix Select

    I agree with you 100% Mr C..I'm just a little fish, but in the past when it came for the meter chap to hook up my completed rewires never once did he say that a switch fuse was required, and if I remember correctly some of the tails were pretty long, admittedly it was the REC then but I cant see that that makes much difference.

    My conclusion for what its worth - apply common sense in these scenarios, but just dont go fitting stuff because everybody else does because that will end up just a waste of money and clag up stuff with unnecessary connections, use your own judgement and act accordingly. From the dealings I have had with the DNO in the past, I find them excellent at there job, they dont mess about, and more then likely would have no concern over tail length/switch fuses or whatever as long as everything was done properly electrically.

    The only proviso for me would be if say I fitted a db remotely from intake position (say it was what I dunno 4 meters or more away and a TN) then I would use SWA for the run and fit a switch fuse at intake position appropriately discriminated against the cut out ( if discrimination is possible..;) you know the crack) thats the only thing for me.
  13. Coloumb

    Coloumb Screwfix Select

    I would think so yes. I think the three meter rule is more of a catch all situation that would be used as a "default" from which to build on. If you understand the principles at stake then why worry over it? Certainly not the DNO or what ever, otherwise half the bloody country would be disconnected.
  14. JP.

    JP. Screwfix Select

    Good topic though Mr C..conclusions? Have not really got any myself,certainly the OSG doesn't help towards anything definitive within the topic posts..its the same old thing, one person does it one way, and the other person does it another way. Its probably what makes the world go round.
  15. Bazza-spark

    Bazza-spark Screwfix Select

    There is a three metre rule in 7671 in 433.2.2(ii).

    433.2.1 Except where 433.2.2 or 433.3 applies, a device for protection against overload shall be installed at the point where a redcution occurs in the value of the current carrying capacity of the conductors of the installation.
    NOTE: A reduction in the current carrying capacity may be due to change in cross sectional area, method of installation, type of cable or conductor, or in environmental conditions.

    433.2.2 The device protecting a conductor against overload may be installed along the run of that conductor if the part of the run between the point where a change occurs (in cross sectional area, method of installation, type of cable or conductor, or in the environmental conditions) and the position of the protective device has neither branch circuits nor outlets for connection of current using equipment and fulfils at least one of the following conditions:

    (i) It is protected against fault current in accordance with the requirements stated in Section 434

    (ii) Its length does not exceed 3 m, it is installed in such a manner as to reduce the risk of a fault to a minimum, and it is installed in such a manner as to reduce to a minimum the risk of fire or dangerto persons (see also Regulation 434.2.1)

    If 433.2.1 or 433.2.2(i) are satisfied then the three meter rule does not apply.

    From the OSG, 2.2.4 an electricity isolator switch may be required, but if you read the text this should be supplied by the DNO. No requirement fo a fuse, because if you jump back to OSG 2.2.1 the DNO fuse will afford fault current protection.

    The only time an extra fuse is required is if the tails change size, environmental conditions change etc causing a reduction in current carrying capacity of the tails. The remaining requirements are set out in 2.2.3 and protection MAY be required, but if we were changing tail sizes and environmental conditions that would already be taken into account.

    Kind regards
    Coloumb likes this.

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