Fused isolator

Discussion in 'Electricians' Talk' started by Bewlec, Jan 26, 2015.

  1. Bewlec

    Bewlec Member

    I need install new supply to consumer unit from meter. it is currently supplied by 10mm2 T&E, I will be installing 25mm 2 core SWA and seperate 16mm earcth cable. the run is around 22m. what is the situation with fitting a 80A fused isolator. do the electricty suppler require a fused isolator if the tails are or 3m in lentgh (i seem to think this is the case)
     
  2. Gman34

    Gman34 Member

    im not sure what your asking ?
     
  3. seneca

    seneca Screwfix Select

    Most do. An 80 amp fused switch fitted near the supply end will be fine.
     
    Last edited: Jan 26, 2015
    FatHands likes this.
  4. Bewlec

    Bewlec Member

    do electricty suppliers require a fused isolator fitted if run of cable to consumer unit is over 3m in length.
     
  5. nigel willson

    nigel willson Screwfix Select

    Answer is yes , but exact length dependant on suppliers, 3 mtrs is usual max, in practice switch fuse would be very near meter so tails would only be short.Hope that helps.
     
    FatHands likes this.
  6. Coloumb

    Coloumb Screwfix Select

    If you know the type of fuse and rating of the one in the cut out then and you can make sure your Zs is within limits at the end of the run you can just fit an isolator at the start of the run. If you put in a switch fuse with the same rating fuse as the cut out then all you end up with is two fuses of the same rating in series. Imo your just making things more complicated than they need to be. And yes I've done it (with runs longer than 3m) and no the Rec Police have never been round to bollix me.
     
  7. nigel willson

    nigel willson Screwfix Select

    W
    What,22mtrs, shame on you!
     
    Coloumb likes this.
  8. Coloumb

    Coloumb Screwfix Select

    What a well thought out, sciencetifically proven response. I bow to your superior knowledge.
     
  9. nigel willson

    nigel willson Screwfix Select

    science not , experience!lol
     
  10. Coloumb

    Coloumb Screwfix Select

    Your still not explaining WHY you would need to fit a second fuse in series with the cutout if the requirements of 7671 can be met, ie Max Zs, volt drop etc. That's nothing to do with experience, that's just hiding behind the fact you can't explain why. If anything two fuses of the same rating are deterimental to 7671 as it doesn't take account of discrimation.
     
  11. seneca

    seneca Screwfix Select

    It's because of the length of the "tails" (swa) Col.
    Apparently most DNO's require this for tails over a certain length.
     
    Last edited: Jan 27, 2015
  12. Coloumb

    Coloumb Screwfix Select

    Sen we have been over and over this. One one has yet managed to produce any proof that this is the case. Every time I've asked the rec about it they just say "what you do after the meter is up to you". Recently I had the rec come out to pull the cut out to fit an isolator and explained what I planned (ie SWA run / move cu with cut out fuse providing protection) and she (yup SHE) just shrugged her shoulders and said something like "it will be better than the ****** 1970's cu that's there now".
     
  13. FatHands

    FatHands Well-Known Member

    yes, i think this came up a few months ago and someone linked to ukpn/western powers guide on supply intakes
     
  14. Coloumb

    Coloumb Screwfix Select

    Right, which in the end said "it's ok to fit an isolator".
     
  15. Bewlec

    Bewlec Member

    thanks for the answers lads, my thoughts are the same as coloumb. why have two 80A fuses in series. I made a few phone calls today regarding this to northern power grid, meter plus etc etc did not get a definitive answer after speaking to about 5 different people, (most were just call handlers not technical dept.) I ran in the 25mm SWA and seperate 16mm earth today, I will terminate SWA into galv box and the wire into existing 100 DP switch
     
  16. FatHands

    FatHands Well-Known Member

    hi col,
    i seem to recall the attachments from the power distributors said something to the effect that past 3 metres isnt covered by their service fuse. i am on the dog at the moment but will try and find them when i get to the PC.
     
  17. Coloumb

    Coloumb Screwfix Select

    I found them and they don't. They say you can fit an isolator.

    Tbh I really don't understand why everyone is having such a baby over this. It's like the service fuse has some kind of sacred power that it won't behave just like any kind of normal fuse. I mean why not fit two consumber units? Or two fcu units? No one would. Anyway, what happens if the supplier changes? BG might say OK, but nPower might want less than 3m. So what happens? They come round and condemn the supply? Every time I've called the supplier all they say is everything beyond the meter is nowt to do with them. They are not interested and do not care how long the tails are.
     
  18. peter palmer

    peter palmer Screwfix Select

    The only issue I can see is the fault current would be traveling through their metering equipment, anyone know what the safe PFC is on a credit meter. If there was a fault I would prefer it to blow my fuse than the service fuse, I have always fitted an 80A switch fuse if I have ran in 25mm SWA, usually the cut out is 100A nowadays.
     
  19. Coloumb

    Coloumb Screwfix Select

    Peter, the fault current would run through the meter IF the fault happened on load side of the meter no mater how long the meter tails or if it fed a switch fuse. In fact, the fault current would be less the further away the fault happened. I would presume the cut out fuse would take care of this as all the equipment down stream from it would need to be rated high enough to take it, this would include the cut out, the meter as well as the consumer unit. You just need to make sure you don't exceed the Max Zs of the fuse in the cut out. Most are rated at 16ka I believe, with most mcbs being rated around 8ka. There would be no problem with fitting a 80 sw for a 100a cut out as discrimination is achieved, but if it was an 80a cu with an 80 switch fuse, my guess would be that, in a fault, both the fuses would blow at the same time, or it's 50/50 as to which one would go first, so I can't see any advantage to fitting two fuses in series.
     
  20. Coloumb

    Coloumb Screwfix Select

    Also looking at the pre-arc I2t time for bs88 and bs1361 fuses compared to the total I2t time the discrimation on a 100 to 80 amp fuse looks marginal, ie 2 or 3 1000'sth of a second.
     

Share This Page

  1. This site uses cookies to help personalise content, tailor your experience and to keep you logged in if you register.
    By continuing to use this site, you are consenting to our use of cookies.
    Dismiss Notice