Lollipop circuit on a 40A RCBO

[Octopus]

Tested a property, where Kev the kitchen fitter had fitted a new kitchen, rearranged the sockets and a new board - no paperwork provided.

The kitchen socket circuit is clearly a 6mm/2.5mm lollipop - no access to the JB's, but it tests OK.

If it was on a 32A - I wouldn't mark it down, but as its on a 40A my inclination is to give it a C2 on the basis we don't put a 40A MCB on a 2.5mm RFC.

Comments please

Thanks

 

[Andy78]

Yeah I suppose it has to be a C2. Not compliant with 433.1.1 and theoretically potentially dangerous, however unlikely.

 

[GB.Kayak]

Tested a property, where Kev the kitchen fitter had fitted a new kitchen, rearranged the sockets and a new board - no paperwork provided.

The kitchen socket circuit is clearly a 6mm/2.5mm lollipop - no access to the JB's, but it tests OK.

If it was on a 32A - I wouldn't mark it down, but as its on a 40A my inclination is to give it a C2 on the basis we don't put a 40A MCB on a 2.5mm RFC.

Comments please

Thanks

My reading of the Electrical Safety Council's guide on the matter is that if the OCPD rating is too high to provide protection to the cable then your coding is spot on.

 

[telectrix]

433.1.103 is quite specific in stipulating a 30A or a 32A OCPD for a socket RFC in 2.5mm, in your situation, as there is a possibility of overloading the 2.5mm, then a C2 is, IMO, spot on.

 

[7029 dave]

^^^^^ all above agreed.

 

[Octopus]

Thanks guys..... I was seeking assurance - just sometimes you begin to doubt yourself!

 

[Engineer54]

Personally i'd give the opportunity to the customer to change the OCPD for a 32A, if this is the only fail item you found on the installation.

 

[Octopus]

Personally i'd give the opportunity to the customer to change the OCPD for a 32A, if this is the only fail item you found on the installation.

Kev the kitchen fitter contributed to 2 other C2's, and there are 2 futher C2 issues on other circuits so its a "draft" unsatisfactory.

That said I have the go ahead to do "whats necessary" to bring the installation up to standard and make some light fitting changes so its a C2 for the RCBO!

Thanks for the constructive input all.

 

[D Skelton]

C3 for me.

Regulations aside, I can't quite see why a 32A OCPD is acceptable on a ring final circuit but not a 40A one???

 

[GB.Kayak]

C3 for me.

Regulations aside, I can't quite see why a 32A OCPD is acceptable on a ring final circuit but not a 40A one???

From first principles I would interpret that a 40 A OCPD might be acceptable on a 2.5 mm² RFC or lollipop circuit, if the CCC of the cable allows. I am open to correction but it seems to me that BS 7671, TABLE 4D5, would allow 2.5 mm² T+CPC cable and a 40 A OCPD to be used 'clipped direct' for example for the circuit in question, the CCC of the cable being 27 A for this installation method and therefore in a ring capable of carrying up to 54 A. However, if the cable were wrapped in insulation at the centre of an insulated stud wall then a 32 A OCPD would not give adequate protection as the CCC is reduced to 13.5 A requiring at most a 20 A OCPD in a RFC.

 

[HandySparks]

C3 for me.

Regulations aside, I can't quite see why a 32A OCPD is acceptable on a ring final circuit but not a 40A one???

Surely it's all down to the assumptions made about where on the ring the load is.

If all the load is at the middle of the ring, then the current will be split equally between the two routes between origin and load. If you have multiple loads spread evenly along the ring, then again you'll have an equal current split.

However, if you have all the load at one location near to the origin, then the current will split in the ratio of the resistance of the two routes which, assuming the same cable used throughout, will be in proportion to the two lengths involved.

For example, if you put a 32A load 15% along the ring, you'll get about 27A in the short leg and about 5A in the long one. A load of 40A at the same point on the ring will give 34A and 6A.

 

[GB.Kayak]

Surely it's all down to the assumptions made about where on the ring the load is.

If all the load is at the middle of the ring, then the current will be split equally between the two routes between origin and load. If you have multiple loads spread evenly along the ring, then again you'll have an equal current split.

However, if you have all the load at one location near to the origin, then the current will split in the ratio of the resistance of the two routes which, assuming the same cable used throughout, will be in proportion to the two lengths involved.

For example, if you put a 32A load 15% along the ring, you'll get about 27A in the short leg and about 5A in the long one. A load of 40A at the same point on the ring will give 34A and 6A.

Thanks for the practical reminder of the application of Ohm's law.

 

[HappyHippyDad]

From first principles I would interpret that a 40 A OCPD might be acceptable on a 2.5 mm² RFC or lollipop circuit, if the CCC of the cable allows. I am open to correction but it seems to me that BS 7671, TABLE 4D5, would allow 2.5 mm² T+CPC cable and a 40 A OCPD to be used 'clipped direct' for example for the circuit in question, the CCC of the cable being 27 A for this installation method and therefore in a ring capable of carrying up to 54 A. However, if the cable were wrapped in insulation at the centre of an insulated stud wall then a 32 A OCPD would not give adequate protection as the CCC is reduced to 13.5 A requiring at most a 20 A OCPD in a RFC.

I wouldn't have thought you could just double the CCC of a 2.5mm and say thats how much it can take. Thats basically saying 5mm cable can take 54A. Table 4D5 shows that 6mm cable has a CCC of 47A clipped direct, so in theory a ring has a lower CCC than this.

I've always found it odd that we have such specific tables for all the different CSA of cable but nothing for a ring, apart from appendix 15 which states a 30A or 32A OPD.

 

[davesparks]

I wouldn't have thought you could just double the CCC of a 2.5mm and say thats how much it can take. Thats basically saying 5mm cable can take 54A. Table 4D5 shows that 6mm cable has a CCC of 47A clipped direct, so in theory a ring has a lower CCC than this.

I've always found it odd that we have such specific tables for all the different CSA of cable but nothing for a ring, apart from appendix 15 which states a 30A or 32A OPD.

2x 2.5 in parallel is not the same as 1x 5mm cable, this is why parallel cables are used for larger supplies.

 

[GB.Kayak]

I wouldn't have thought you could just double the CCC of a 2.5mm and say thats how much it can take. Thats basically saying 5mm cable can take 54A. Table 4D5 shows that 6mm cable has a CCC of 47A clipped direct, so in theory a ring has a lower CCC than this.

I've always found it odd that we have such specific tables for all the different CSA of cable but nothing for a ring, apart from appendix 15 which states a 30A or 32A OPD.

The current carrying capacity of the 6.0 mm², 'stick' of the lollipop is well protected by the 40 A OCPD.

How then do you explain the 32 A OCPD of a 2.5 mm² RFC when a single piece of cable may be limited to a CCC of 13.5A? I suspect that in this case, the OCPD should be 'downrated' to 20 A.