Switchfuse for Long Tails

Discussion in 'Electricians' Talk' started by Moses, Jun 27, 2012.

  1. Moses

    Moses Member

    Is it a requirement of 7671, as opposed to that of the DNO, that a switchfuse be installed, instead of a 100A isolator, if tails are going to be longer than 3m?

    If a 100A DNO fuse is supplying a 100A customer switch fuse, then how is discrimination achieved, to ensure that only the customer fuse ruptures in case of fault conditions.

    If there is no discrimination, then why can we not just install an isolator only?
     
  2. removed 7

    removed 7 New Member

    Moses dear, it isn't a requirement of BS7671 at all.  Neither is it a Suppliers requirement to have a switched fuse for tails exceeding 3 Metres.

    But some suppliers insist on a 100A DP isolator for domestic services.  In fact, the current trend for suppliers now, is to energise new-builds with a live meter and DP isolator even before the property has been first-fixed by the electrician.

    It never has been a requirement of any Regulatory Body or any non-statutory code, such as BS7671 for such a ridiculous situation as two 100 fuses in close proximity.  There are too many old wives' tales in this industry..........


    Lucia
     
  3. Moses

    Moses Member

    Thankyou lucy dear.

    Can I quote you on this?
     
  4. removed 7

    removed 7 New Member

    Yes Moses, of course you can - otherwise I wouldn't have written it.

    I spent more than 20 years working for the EEB and LEB dealing with such myths.  You've answered your own question in the O/P haven't you?  It's simply illogical to have two 100A fuses side-by-side!  The only thing that we used to stipulate (and suppliers still do) is that the extended tails should be protected from mechanical damage.........


    Lucia
     
  5. sinewave

    sinewave Screwfix Select

    DNO's are a bit like Bouncers on Club Doors!

    What they say 'on-the-night' goes and that's final, no point in arguing.

    Quite a few like a fused down switch fuse if going over 3-5m others don't give a toss.

    It can vary within the same DNO's area depending on the guidance of the local Area Engineer etc.


    Bottom line is ASK and Obey and all will be well.
     
  6. removed 7

    removed 7 New Member

    Sinedoll, I happen to know that Scottish Power (for whom you worked) have never insisted on a second fuse for extended tails.  This business is built on hearsay and myth...........

    Bought any designer-work trousers lately, dear?




    Lucia
     
  7. sinewave

    sinewave Screwfix Select

    All you 'happen to know' Looche is what one man may or may not have told you.

    Whether that came with any sequential authority or not is irrelevant as there are too many 'Cooks' in local DNO's and hence why what is instructed at 'local level' goes so there is no use in ringing a Call centre 300 mile away and avin a winge cause it's going to cost you another £40.

    :)
     
  8. removed 7

    removed 7 New Member

    So you haven't bought any more floor-layers pants today?


    Pffftttttt
     
  9. Lokkars Daisy

    Lokkars Daisy New Member

    Sinedoll has a new preferred type , the knee pads are twice normal size and are situated in the ar se pockets
     
  10. sinewave

    sinewave Screwfix Select

    Might do tomorrow, split me crotch whilst mounting me bike for a few Jars early doors tonight.

    Nover £60 then!  :(
     
  11. removed 7

    removed 7 New Member

    Lardy boy ^


    Lux
     
  12. J.P.

    J.P. New Member

    100 cutout and 100 switch-fuse = 100 cutout and main-switch in enclosure

    100 cutout and 80 switch-fuse = flip the coin

    ;)
     
  13. Lectrician

    Lectrician Screwfix Select

    WPD don't give a monkeys on the length of the tails at all.  When we asked them a good few years ago they simply replied "as long as they are installed to BS7671" then they will connect.  They don't install isolators generally, but will if for some reason, no tails are presented to them at the time the metering is fitted.  Metering is actually done by EDF in the area and not WPD which can get confusing, and is annoying when relocating or upgrading services.
     
  14. unphased

    unphased Screwfix Select

    There is a regulation in BS7671 that may be the root of the 3m nonsense spouted about tails.

    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 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 in Section 434.
    ii Its length does not exceed 3m, it is installed in such a manner as to reduce the risk of fault to a minimum.....

    I believe this could be the origin of the misinformed requirement. If you had tails at 25mm2 from the origin, and wanted to serve a fuseboard more than 3m away, provided you kept the tails at 25mm2 all the way and met the other requirements then there is no need to install another protective device (switched fuse) along the run. However, if you wanted to save some dosh and, say, have the first 3m in 25mm2 and install the remaining in 16mm2 if it was an exceptionally long run, then it could be done using that reg. This is an example of how hearsay and misinformation gets entrenched in this industry.

     
  15. seneca

    seneca Screwfix Select

    That's the same reg. that allows a 1.5mm t/e spur from a ring to an fcu UP.
     
  16. unphased

    unphased Screwfix Select

    Indeed so, sen. But I still tend to keep the spur to 2.5mm2. :)
     
  17. seneca

    seneca Screwfix Select

    Same here UP, the only exception I make is for something like under cupboard kitchen lights where the fcu is right next to or very close to a socket.
     
  18. We're not forgetting about the requirement for RCD protection if your long tails are buried etc are we?
     
  19. removed 7

    removed 7 New Member

    Irrelevant^

    This thread is about discrimination between fuses.  Read the original post for goodness sake.........


    Lucia
     
  20. unphased

    unphased Screwfix Select

    There are laws against discrimination If one fuse discriminates against another woe betide the fuse.
     

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