Applying the 3 meter rule properly

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

  1. Alan sherriff

    Alan sherriff Member

    I thought the 3 metre rule was if you wanted to take say 6 mill for a shower from a Henley block to switched fused isolator as to get 25 mill tails int a 60 amp switched isolator would very difficult as long as the outgoing supply from the switched isolator is fused correctly to protect for overload this would be o/k as lo as the supply from the Henley to the switched fuse was no further than 3 metres . Not sure how the 3 metre is derived but it could be to avoid a volt drop if there was no Length of cable given People may install switched fue isolator in the loft and excesive volt drop could occour
  2. Coloumb

    Coloumb Well-Known Member

    You would have to cover the 6mm with some sort of type tested non-combustible material in order to achieve compliance.
  3. Lectrician

    Lectrician Screwfix Select

    You wouldn’t. You would if you omitted fault protection, but not if you move over current protection. This 3m rule appears twice. Once for over current and once for fault protection. People confuse the two and argue.

    In a domestic (for example) with a 100amp service fuse, you can still fit a 30amp switch fuse and feed it with 6mm DI tails if you wish. It’s rarely done these days, but compliant.

    A cable can have fault protection provided by a device which is larger (in amps) than the CCC of the cable.
    Magicspark likes this.
  4. Coloumb

    Coloumb Well-Known Member

    So long as your reasonably confident of what the max. load on the cable will be.
  5. Lectrician

    Lectrician Screwfix Select

    ? You have fault protection and over current protection. Two separate requirements, usually met by a single protective device. With tails in my example, two separate devices provide protection, the switch fuse downstream of the tails protect the tails against overcurrent. The device upstream of the tails may be larger than CCC of the tails, but can still provide fault protection to them.
  6. Coloumb

    Coloumb Well-Known Member

    This statement, literally interpreted, would suggest it's possible to design a ctt. so as to provide ONLY fault protection. Yes it is PROVIDING you know what the load is going to be as overload protection is discarded. ie, spurs from rings can only feed one socket. The three meter rule gives even more relaxation of this rule by allowing the designer to omit both fault and overload protection, within certain limits (3m), for example, the reduced CSA of the cable is not to be regarded as expendable which is a bit daft given that you would have to protected the cable for the length of its run with typed tested material designed to contain the fault.
  7. Risteard

    Risteard Well-Known Member

    LOL. So you were completely wrong then...
  8. Bob Rathbone

    Bob Rathbone Well-Known Member

    The 3 metre rule is just a quick solution to a common issue usually found with meter tails and connections to bus bars. Since the introduction of the 16th Edition we have been given a more powerful tool to use in all situations, the adiabatic formula. All you have to prove, is that under fault conditions, the protective device at the origin will operate before the conductors overheat and insulation or conductors are damaged. It does assume that conductor overcurrent is provided by other means or it cannot occur, i.e. a fixed known load.
  9. retiredsparks

    retiredsparks Well-Known Member

    I think I can help you with this problem and not get too technical.
    You are retired and now have too much time on your hands....and you have started 'pondering'.
    Dont do it.
    You need a hobby which does not involve any skills or plumbing.;)
    Try to spend more time in the pubs/tapas bars and things become clearer...or in fact blurred.
    WillyEckerslike likes this.
  10. Coloumb

    Coloumb Well-Known Member

    Wrong about what?
  11. Risteard

    Risteard Well-Known Member

    Nearly every claim you have ever made.
    Magicspark likes this.
  12. Coloumb

    Coloumb Well-Known Member

    Nearly? ooooowah. So have been right about some things? How exciting!
  13. Coloumb

    Coloumb Well-Known Member

    434.2.1 provides relaxation from using the adiabatic equation when you need to design a system where fault protection can not be provided, typically a bus bar chambers, where an adiabatic equation is of no use. One of the conditions is limiting the length of conductors where the csa changes to 3m. This is where the myth of meter tail length comes from.

    I expect most DNO's would prefer you to stick to 3m, and in 99.9% of all households this is a given. But it's certainly not exclusive and has nothing to do with the 3m rule in 7671.
  14. Magicspark

    Magicspark Active Member

    Boring....I bet you love the sound of your own voice. You can keep repeating it time and time again...Anyone with half a clue has almost given up replying as your lack of comprehsion is quite astonishing. You select bits of peoples posts and seem to disregard the most valid points that’s been written in them. I just hope new members that join, looking for advice, steer clear of you as you are a classic case of a little bit of knowledge being dangerous. Reason being this little bit of knowledge gives you the confidence to post “advice” on here that is most often, complete and utter tosh.

    You are going on my Ignore list with immediate effect, so I won’t see your drivel of a reply!
    Risteard likes this.
  15. Coloumb

    Coloumb Well-Known Member

    Your the boring one. You need to re-read the regs, particularly 434.2.1. Only you won't because you simply can't get your head around it. You tell me, how does this relate to the 3m rule in tails? Only you won't tell me because you don't actually know. Also you got totally called out when it was pointed out to you that it's totally stupid it was in a TT install and why there's no mention of it in a TT in regs there.

    Neither you or ristard have yet come back with a convincing argument. Just the same rubbish, oh your wrong, your wrong your wrong.

    Change the record Lee, this one's stuck in a scratch.
  16. spinlondon

    spinlondon Well-Known Member

    Flaming heck, thought this had all been dealt with in that other thread?
    If this 3metre rule only applies to busbars, why is there no mention of this in the Regulations, and why is there a prohibition against installing branch circuits or socket-outlets between the reduction in CCC and the protective device?

    This 3m rule applies to all conductors.

    A spur can be longer than 3m on RFCs, if the Zs is low enough for the 32A protective device to operate.
  17. Coloumb

    Coloumb Well-Known Member

    Not really, it just got a pointless.

    It doesn't only apply to bus-bars, it can apply to any such system where the csa of the conductor is reduced so the upstream conductor doesn't provide sufficient protection. The most common situation would be bus bar chambers.

    Yes, but only in certain situations, ie reduced csa etc.

    And adiabatic is satisfied.
  18. spinlondon

    spinlondon Well-Known Member

    Well I’m glad that you accept that it doesn’t only apply to busbars now.
    Though I don’t understand why you brought them up in this thread?
    Yes reduced CSA is one circumstance, another would be a change in installation method.
    I wouldn’t bother with the adiabatic equation if the Zs is ok.
  19. Coloumb

    Coloumb Well-Known Member

    Not really my point, the only "other" system I can think of where this would apply would be underfloor bus-bar systems, much the same as bus-bars.

    Tell me, do you still think this reg applies to meter tails? So they are limited to three meters?
  20. spinlondon

    spinlondon Well-Known Member

    No, I don’t think it at all.
    I know it.
    There’s lots of sites that can be found through Google about this, I’m surprised you haven’t looked them up.
    Here’s an excerpt from the Association of Meter Operators Stakeholder information pack:

    “The meter tails should be as short as possible. Different metering companies have different policies but the length should always be less than 3 metres of cable from the cut-out through the metering equipment to the consumer unit. A example may be 1m of cable from the cut-out to the meter, then 1.5m from the meter to the consumer unit. The meter tail cable size must be consistent. BS7671 Reg. 434.2.1 gives some guidance. If the consumer unit is required to be further away, then a switch-fuse unit should be installed close to the meter and a sub-main compliant with BS7671.”

    This advice is not in fact in line with the requirements of BS7671, as (as already explained in the other thread) the installation begins at the consumer connection point at the meter, so the length of the tails between the cut-out and the meter should not be taken into account.

    This is an excerpt from a Northern Power guide:

    “The meter tails between the meter position and your consumer unit should not be longer than 3 metres. If the length is more than 3 metres, you should install an additional protective device at the nearest point to the supply inside the customer’s premises, as specified in the current IEE Wiring Regulations.”

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