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

Former Community Member over 4 years ago

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!

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0 AJJewsbury over 4 years ago

Would it be true to say that for a radial circuit one equation (more or less) can be used to determine the limits to how that circuit may be used?

Sort of. There'll be a number of different requirements, each of which can usually be reduced to a simple equation. Each calculation will have a slightly different basis - loop impedances will just depend on the cable size & length, so relatively straight-forward, but voltage drop also depends on the load current - which means making some different assumptions especially for circuits like socket circuits that have many different points where loads may or may not be connected. You get quite different results if you assume that the loads are concentrated at the far point, or spread evenly, or spread unevenly in some way. If calculating in full manually you'd often have to do several calculations and then pick the most limiting (shortest) result. The OSG used to note against some circuits what the limiting factor was - e.g. zs for earth fault loop impedance, sc for L-N loop impedance or ad for adiabatic (usually due to the smaller c.p.c. size), otherwise it was voltage drop.

In many cases you can treat as ring as being similar to a radial with half the cable of twice the c.s.a.. - which would suggest to me that a spur from the MCB could be up to a quarter of the overall length of the ring for the same result (I'm still puzzling where 1/8th has come from, unless someone thought that a 2x safety margin would be a good idea)

Applying the rule of thumb given in 7.2.2, I can install an unfused spur at the consumer unit, not longer than 1/8 the cable length from the spur to the furthest point of the ring, which just happens to be the cable length of the ring i.e. 106m.

I would have said that the furthest point of the ring was half way around (after that you start getting closer to the start again) - so even with the spur taken from the MCB you've only 1/8th of 53m rather than 1/8th of 106m to play with.

Don't discount the fact that these things are written by humans, and to err is human... Even IET texts aren't necessarily word perfect.

- Andy.
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0 AJJewsbury over 4 years ago

Would it be true to say that for a radial circuit one equation (more or less) can be used to determine the limits to how that circuit may be used?

Sort of. There'll be a number of different requirements, each of which can usually be reduced to a simple equation. Each calculation will have a slightly different basis - loop impedances will just depend on the cable size & length, so relatively straight-forward, but voltage drop also depends on the load current - which means making some different assumptions especially for circuits like socket circuits that have many different points where loads may or may not be connected. You get quite different results if you assume that the loads are concentrated at the far point, or spread evenly, or spread unevenly in some way. If calculating in full manually you'd often have to do several calculations and then pick the most limiting (shortest) result. The OSG used to note against some circuits what the limiting factor was - e.g. zs for earth fault loop impedance, sc for L-N loop impedance or ad for adiabatic (usually due to the smaller c.p.c. size), otherwise it was voltage drop.

In many cases you can treat as ring as being similar to a radial with half the cable of twice the c.s.a.. - which would suggest to me that a spur from the MCB could be up to a quarter of the overall length of the ring for the same result (I'm still puzzling where 1/8th has come from, unless someone thought that a 2x safety margin would be a good idea)

Applying the rule of thumb given in 7.2.2, I can install an unfused spur at the consumer unit, not longer than 1/8 the cable length from the spur to the furthest point of the ring, which just happens to be the cable length of the ring i.e. 106m.

I would have said that the furthest point of the ring was half way around (after that you start getting closer to the start again) - so even with the spur taken from the MCB you've only 1/8th of 53m rather than 1/8th of 106m to play with.

Don't discount the fact that these things are written by humans, and to err is human... Even IET texts aren't necessarily word perfect.

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

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