Discuss Ring vs Radial : Identification. in the Periodic Inspection Reporting & Certification area at ElectriciansForums.net

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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
 
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
 
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
 
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
 
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.
 
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.
 
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.
 
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.
 
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.
 
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.
That would be valid so long as the OCPD is suitable for the use.

So with 32A and 2.5mm it should be a RFC only, but with 4mm it could be a radial on 32A (in many cases, but subject to method for CCC)

If you have a dubious mix of radials and spurs that are not OK then simplest safe option is to drop OCPD to 20A as then it protects the 2.5mm however used.

Sometimes you would see RFC on 20A fuse wire if the supply Ze was too high tor meeting 0.4s disconnection, etc. While not as useful in terms of diversity as 32A (i.e. number of mixed loads up to 13A supported) it works just fine if that limit is respected.
 
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?
If the MCB is 32A and the cable is 2.5mm2, then it should only be a RFC if there are more than two sockets.
If there are three lines, there should only be one socket on the spur, and the rest on a RFC.
Testing for a ring, at its most basic, is disconnect one leg, turn the power back on, and check that all the sockets still work.
 
If the MCB is 32A and the cable is 2.5mm2, then it should only be a RFC if there are more than two sockets.
If there are three lines, there should only be one socket on the spur, and the rest on a RFC.
Testing for a ring, at its most basic, is disconnect one leg, turn the power back on, and check that all the sockets still work.
Thanks for your input.
 
So three radials (or more?) can come off one OCPD 's terminal in the CU. That has been determined on this thread.

My Question. A radial can be spurred off a radial, but can a radial be spurred off radial spur?
 
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