Ring vs Radial : Identification.

Ray Pooley · Feb 11, 2022

Just a quick one

When you see two lines on one MCB that could be either a ring or two radials.
The only way to determine which is through testing. Is that true?

Are there any best practices regarding this?
For example, would you ever see three lines on an MCB? One ring, one radial? Or three radials?
Assuming the load calculations remained within the limits for the purposes of diversity.

Thanks

ImpededLoop · Feb 11, 2022

You're correct, the only way is to disconnect the cables and do and end to end continuity test.

It is permitted to have 2 ring cables and a spur cable in one circuit breaker, so long as the termination is secure. It's the same as spurring from the ring final circuit at any other point.

Ray Pooley · Feb 11, 2022

ImpededLoop said:
You're correct, the only way is to disconnect the cables and do and end to end continuity test.

It is permitted to have 2 ring cables and a spur cable in one circuit breaker, so long as the termination is secure. It's the same as spurring from the ring final circuit at any other point.

Thanks for that. Do you have any idea where that's covered in the regs or the on site guide?
What would it be under?
Cheers

timhoward · Feb 11, 2022

The MCB or RCBO size would be a clue of course but you can never assume.
I’ve also found a ring with both ends in different fuses before now!

There’s only one reg for ring final circuits - 433.1.204
Appendix 15 contains examples of how to comply, but is not a complete list of all compliant possibilities

ImpededLoop · Feb 11, 2022

OSG 7.2.2, page 76 in my old blue covered copy.

Ray Pooley · Feb 11, 2022

timhoward said:
The MCB or RCBO size would be a clue of course but you can never assume.
I’ve also found a ring with both ends in different fuses before now!

There’s only one reg for ring final circuits - 433.1.204

An interesting thought.
If you were planning this on a new installation you would do the diversity for each circuit and
then combine the results of those calculations in order to determine the MCB / RCBO rating.
Is that the case?

Thanks

timhoward · Feb 11, 2022

Ray Pooley said:
An interesting thought.
If you were planning this on a new installation you would do the diversity for each circuit and
then combine the results of those calculations in order to determine the MCB / RCBO rating.
Is that the case?

Thanks

Not really. In brief work backwards from required load, cable current carrying capacity, over current protective device.

Ray Pooley · Feb 11, 2022

ImpededLoop said:
OSG 7.2.2, page 76 in my old blue covered copy.

How old is your OSG. Mine's 2015.
7.2.2 is Socket Outlet Circuits Page 71.
I have had a look on page 76 and can't find anything relevant there either.

ImpededLoop · Feb 11, 2022

Paragraph 4 of 7.22 "A non fused spur..."

Ray Pooley · Feb 11, 2022

timhoward said:
Not really. In brief work backwards from required load, cable current carrying capacity, over current protective device.

Yes. Load (from diversity), cable size (from load and mounting) and finally overload protection device.
My thought process is that if each circuit had its own dedicated overload protection then the above process would determine
a particular overload protection device for that circuit.

If you plan to connect those circuits to a single protection device, rather than three separate devices, then surely you have to combine those loads in determining the appropriate overload protection device. It wouldn't affect the cable sizes. Just the shared protection device.

Ray Pooley · Feb 11, 2022

ImpededLoop said:
Paragraph 4 of 7.22 "A non fused spur..."

Ok. I can see paragraph 4. I was looking for a specific redference to "It is permitted to have 2 ring cables and a spur cable in one circuit breaker, so long as the termination is secure". That part. But it doesn't matter.

Thanks

timhoward · Feb 11, 2022

Ray Pooley said:
Yes. Load (from diversity), cable size (from load and mounting) and finally overload protection device.
My thought process is that if each circuit had its own dedicated overload protection then the above process would determine
a particular overload protection device for that circuit.

Ah ok, in abstract yes we are in agreement.

There has been debate on here before about whether a ring final circuit has any flexibility regarding the OCPD as the only reg regarding RFC's does state "protected by a 30A or 32A protective device". The consensus was that while it could be designed to be perfectly safe, the reg is rather prescriptive.

Here is the reg you want from BS7671....

433.1.204
Accessories to BS1363 may be supplied through a ring final circuit, with or without unfused spurs protected by a 30A or 32A protective device complying with BS88 / BS3036/ BS EN 60898 / BS EN 61009 etc.
The circuit shall be wired with copper conductors having line and neutral conductions with a minimum cross sectional area of 2.5mm sq. except for two-core mineral insulated cables complying with BS EN 6072-1 for which the minimum cross sectional area is 1.5 mm sq. Such circuits are deemed to meet the requirements of 433.1.1 if the current carrying capacity (Iz) of the cable is not less than 20A and if, under the intended conditions of use the load current in any part of the circuit is unlikely to exceed for long periods the current carrying capacity (Iz) of the cable.

Ray Pooley · Feb 11, 2022

timhoward said:
Ah ok, in abstract yes we are in agreement.

There has been debate on here before about whether a ring final circuit has any flexibility regarding the OCPD as the only reg regarding RFC's does state "protected by a 30A or 32A protective device". The consensus was that while it could be designed to be perfectly safe, the reg is rather prescriptive.

Here is the reg you want from BS7671....

433.1.204
Accessories to BS1363 may be supplied through a ring final circuit, with or without unfused spurs protected by a 30A or 32A protective device complying with BS88 / BS3036/ BS EN 60898 / BS EN 61009 etc.
The circuit shall be wired with copper conductors having line and neutral conductions with a minimum cross sectional area of 2.5mm sq. except for two-core mineral insulated cables complying with BS EN 6072-1 for which the minimum cross sectional area is 1.5 mm sq. Such circuits are deemed to meet the requirements of 433.1.1 if the current carrying capacity (Iz) of the cable is not less than 20A and if, under the intended conditions of use the load current in any part of the circuit is unlikely to exceed for long periods the current carrying capacity (Iz) of the cable.

Perfect. Thanks again.

Although, when you think about it, with 2 rings and 1 radial on a single device it only only takes 2 snips and a bit of trimming and you have 5 radials on a single protection device.

timhoward · Feb 11, 2022

Ray Pooley said:
Although, when you think about it, with 2 rings and 1 radial on a single device it only only takes 2 snips and a bit of trimming and you have 5 radials on a single protection device.

Yes indeed. And 5 radials with current carrying capacity considerably lower than the 32amp breaker. Which takes us full circle to where you started - testing RFC circuits is an essential part of any EICR.

loz2754 · Feb 11, 2022

Technically you'd have 1 radial circuit with five branches radiating from the protective device.
See definition of a circuit on p26 BBB.

Ring vs Radial : Identification.

Ray Pooley

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ImpededLoop

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Ray Pooley

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timhoward

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ImpededLoop

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Ray Pooley

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timhoward

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Ray Pooley

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