Applying the 3 meter rule properly

Discussion in 'Electricians' Talk' started by PeterMouse, Sep 7, 2018.

  1. PeterMouse

    PeterMouse New Member

    Hi. I am an recently retired industrial electrician, I am worried about aninstallation in my own home I didabout 30 years ago. I supplied myburglar alarm control box. By Spuring off my ring main (unfused) about 4feet to the alarm box. I may also have down sized the cable to 1.5mm T&E. I did not fit a fused Spur on the supply as the Alarm box is fused internally on the supply as it enters the box. At the time I thought this was O.K. as I understood the 3 meter rule to allowDown sizing and omiting of a fused spur at the point of connection to the ring main. As long as the run was less than 3 meters and the load to be suitably limited by protection at the end or along the supply cable. Since retiring I have had time to ponderthese things. Any clarification would be a big help.
     
  2. unphased

    unphased Screwfix Select

    Peter you are worrying far too much. Strictly you should have fitted an fcu at the point you spurred off because you downsized the cable. If you had kept it at 2.5mm2 then you would be fine. In reality it is going to be safe because an alarm panel draws very little current. Its not dangerous it just doesn't strictly comply with the Regs.
     
  3. Coloumb

    Coloumb Screwfix Select

    The three meter rule only applies in situations where bus bar chambers are used.
     
  4. unphased

    unphased Screwfix Select

    Or three metre rule. :)
     
    Coloumb likes this.
  5. PeterMouse

    PeterMouse New Member

    Thank you.
     
  6. chippie244

    chippie244 Super Member

    You've had no problem in 30 years so it's probably OK.
     
  7. PeterMouse

    PeterMouse New Member

    Thanks
     
  8. Magicspark

    Magicspark Active Member

    Where does it say bus bar chamber in reg 434.2.1?
     
  9. Coloumb

    Coloumb Screwfix Select

    Lee, you so want to be part of this forum don't you?
     
  10. chippie244

    chippie244 Super Member

    Is Magic from Essex by any chance?
     
    Coloumb likes this.
  11. mhar

    mhar Member

    Isn't the point of 434.2 to allow the fusing down within 3 metres of where the reduction in csa occurred rather than saying it must happen at the point of reduction.

    Even reading the reg with the book upside down, I cannot see any reference to bus bar chambers as referenced by another poster
     
    Magicspark likes this.
  12. seneca

    seneca Screwfix Select

    I've never seen that either. As far as I've always understood it the 3 metre rule is for cases such as running a spur in 1.5mm t/e from a 32 amp ring circuit, it's ok as long as the cable isn't likely to suffer mechanical damage and that it's only supplying a fused spur or socket so there's no chance of overload.
     
  13. Magicspark

    Magicspark Active Member

    Another example would be coming off a ryefield/bemco unit to a board with a MCCB incomer. (Iz) could be calculated using (In) of the MCCB as long as the other requirements of 434.2.1 were met. I only asked for clarification with regards to the reference of the bus bar chamber as I can’t see any reference to the regulation ONLY applying to a bus bar chamber? Maybe @Coloumb is confused or wrong?
     
    Risteard likes this.
  14. Magicspark

    Magicspark Active Member

    Hi @Coloumb you’ve been online and read the thread....Any chance of you giving us a clue where it says the 3 metre regulation applies only if it’s taken from a bus bar chamber? I’ve had another flick through the big blue book and still can’t see anything stated as such. Is your silence an omission of what you’ve stated is not correct? Kind Regards :)
     
  15. MGW

    MGW Screwfix Select

    I raised this on another forum, I was told by many that the 3 metre rule did not apply to spurs taken off a ring, I think they all realised they had exceeded the 3 meter rule many times, as have I, I remember fitting a ring main in a warehouse refrigerated and due to size a 6 mm SWA was run above the ceiling conduit box to conduit box and a cable was then run down from each conduit box to a socket on the wall, that drop was over 3 metres with 2.5 mm SWA so it did not comply with 3 metre rule, however I still consider it was safe and complied with loop impedance requirements where running down and up to each socket would not have, so would have needed 10 mm SWA and you can't get two 10 mm wires into a socket terminal.

    I am sure even with 1.5 mm cable as long as the loop impedance and/or RCD protection are within limits, then a short circuit will trip the B32 or RCD before the cable melts. However from a bus bar with a 600A supply then cable would melt before supply fuse, so yes it does seem sensible for bus bars and not so sensible for a ring final so I would not worry about over 3 meters of a ring final, but would also agree it does not comply.

    However there were other items which also seems to have missed out exemptions to rule, there is nothing to say RCD protection does not have to be provided for SELV cables buried in the wall or the earth wire is not required, however since it is a separated supply and extra low voltage I would not expect to see a RCD on the output of a door bell transformer. Some times you need to forget regulations and use common sense.
     
  16. Coloumb

    Coloumb Screwfix Select

    Usually reg 434.2 applies and requires a device for protection against fault current where a reduction in cross sectional area or reduction in current carrying capacity occurs. But we can apply regulation 434.2.2 and 434.5.2 in BS 7671. (I think I don't have my reg book to hand)

    You simply do not need to invoke the 3m rule regs if you can prove the above to be true. The 3m rule regs are simply there to "design out" the adiabatic equation if you follow the rules, ie do this, this and this and it will all be fine.
     
  17. Magicspark

    Magicspark Active Member

    Is that only from a bus bar chamber or does it sometimes apply to other situations as well?
     
  18. Coloumb

    Coloumb Screwfix Select

    Now the answer to THAT would be about as allusive as the one about the "rules" for the BNO...;)
     
  19. Magicspark

    Magicspark Active Member

    oh you were wrong then...glad that’s been cleared up :) Enjoy the rest of the weekend.
     
    Risteard likes this.
  20. Coloumb

    Coloumb Screwfix Select

    Well, to be fair I was on the bus when I posted that and had to get off before I completed it. I meant to edit it but I thought, nah, leave it. Lets see if some pedant picks up on it...

    And you too, re the weekend. :)
     

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