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On-Site Guide BS 7671:2018 - 7.2.2 Socket-outlet circuits

Former Community Member
Former Community Member
Hello, I am studying the On-Site Guide and have reached Section 7 Final Circuits.


7.2.2 Socket-outlet circuits

...

As a rule of thumb for rings, unfused spur lengths should not exceed 1/8 the cable length from the spur to the furthest point of the ring.

...


I should like to ask, what does this mean?


The length of the cable used for the non-fused spur is the length of the cable from where it connects to ring final circuit at the terminals of a socket-outlet (or junction box...), to the terminals at the other end of the cable where it connects to a single or twin socket-outlet.


What is the length ‘from the spur to the furthest point of the ring’? Does ‘from the spur’ mean from the the point where the spur connects to the circuit? Does ‘to the furthest point of the ring’ mean to the furthest point away from where the spur connects to the circuit i.e. half the length of the ring final circuit? If so, the maximum lengths would be the same for all spurs on that circuit.


Or, does it mean from where the spur connects to the circuit to the point which is the furthest away from the protection device in the consumer unit i.e. a point half way around the circuit beginning from the consumer unit? If so, the maximum length of the spur cable would be longest nearer to the consumer unit and shorter as the furthest point away is reached... Surely this cannot be correct, because at the furthest point the maximum length would be zero - !


So, in conclusion: 

1/ establish the full length of the ring final circuit;

2/ find the furthest point of the ring by dividing the full length by two;

3/ divide that length by 8 to find the maximum length of a spur cable for that particular ring final circuit.


Please would someone confirm my understanding or explain what the sentence within 7.2.2 actually means?


Many thanks!


Parents
  • 0
    My guess would be your second option:

     
    Or, does it mean from where the spur connects to the circuit to the point which is the furthest away from the protection device in the consumer unit i.e. a point half way around the circuit beginning from the consumer unit? If so, the maximum length of the spur cable would be longest nearer to the consumer unit and shorter as the furthest point away is reached... Surely this cannot be correct, because at the furthest point the maximum length would be zero - !

    And zero length at the furthest point does make sense in a way. A spur will always add some resistance - so increasing both voltage drop and Zs above that at the point where it's connected to the ring. If the ring is already as long as allowed - then yes the maximum spur you could add at the furthest point would indeed be zero - otherwise you'd be exceeding permitted loop impedances or voltage drop.


    I agree the wording you've quoted isn't that clear. Really the limit should be based on the maximum permitted ring length rather than the length of the ring you've actually got - and indeed they could be clearer about meaning the point on the ring where the spur is connected.


    (I'm still puzzling where they've got 1/8th from - I'll have to think about that).


       - Andy.


Reply
  • 0
    My guess would be your second option:

     
    Or, does it mean from where the spur connects to the circuit to the point which is the furthest away from the protection device in the consumer unit i.e. a point half way around the circuit beginning from the consumer unit? If so, the maximum length of the spur cable would be longest nearer to the consumer unit and shorter as the furthest point away is reached... Surely this cannot be correct, because at the furthest point the maximum length would be zero - !

    And zero length at the furthest point does make sense in a way. A spur will always add some resistance - so increasing both voltage drop and Zs above that at the point where it's connected to the ring. If the ring is already as long as allowed - then yes the maximum spur you could add at the furthest point would indeed be zero - otherwise you'd be exceeding permitted loop impedances or voltage drop.


    I agree the wording you've quoted isn't that clear. Really the limit should be based on the maximum permitted ring length rather than the length of the ring you've actually got - and indeed they could be clearer about meaning the point on the ring where the spur is connected.


    (I'm still puzzling where they've got 1/8th from - I'll have to think about that).


       - Andy.


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