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TECHNICAL MEMBER QUESTIONS


>>> Continued from page 37


protection. If you’ve been on SELECT’s 218 training course, then these risk assessments will have been covered. Indeed, some Members may remember our own in-house example, where Technical Adviser Darren Sweeney carried out the calculation. Given that he has overhead lines in his back garden


which are 20m from the house, you can probably guess the outcome too! Within this


regulation there


is also an exception for single dwelling units, which


mentions the total


value of the installation and the equipment therein


not justifying the protection. On this, we always advise caution to those attending if the job to be done is


signifi cant, and it may well be an ideal time to install the SPD, e.g. consumer unit upgrade or rewire. Discussion with the client and advising of what an SPD is, does and protects would certainly be advisable.


As the competent and knowledgeable electrician, having some knowledge in this will help. If you attended the SELECT Toolbox Talk in 2017 then you will have had the pleasure of our SPD presentation and may have gained some valuable understanding. If not, then I recommend the BEAMA Guide which can be accessed at bit.ly/BEAMA_SPDs


NOTE: T ere are proposals to change the wording to these regulations in the draft for public comment (DPC) of Amendment 2:2022 to BS 7671:2018 which includes plans for a discussion between the designer and the installation owner to determine if SPD protection is required or not in dwellings. T e outcome following review of DPC comments by JPEL/64 will no doubt be of interest to SELECT Members.


38 CABLEtalk APRIL/MAY 2021 03


circuit using 2.5mm² cable and a 20A protective device?


Appendix 4 Table


4D5 (page 409) of BS 7671: 2018. Thermoplastic insulated and sheathed fl at cable with protective conductor totally


surrounded ref method 101


current carrying capacity is 17A. Then let’s look at


Regulation 523.9 (page


143) which also references to Appendix 4 that we have just considered. A cable should preferably be installed in a location where it is unlikely to be covered in thermal insulation. Where a cable is to be run in a space to which thermal insulation is likely to be applied, it shall, where practicable, be fi xed in a position such that it will not be adversely aff ected by the amount of thermal insulation. To meet the requirements of Regulation 523.9 requires “where a single cable likely to be totally surrounded by thermally insulating material over a length of 0.5m or more, the current- carrying capacity


shall be taken, in the absence of more


precise information, as 0.5 times the current


carrying capacity for that cable clipped direct to a


surface and open (Reference Method C)”, also discusses the derating factor form table 52.2 regarding the cable surrounded by thermal insulation. Where applicable, the nature of the load (e.g. cyclitic) and reference to chapter 31,Maximum demand and diversity, may be taken into account.


To overcome thermal insulation issues, is it permissible to protect a ring fi nal


Firstly, let’s look at reference methods and follow on to


For economic and reliable design of an installation within thermal limits and admissible voltage thereof, diversity may be taken into account reference to On-Site Guidance Appendix A and in particular Appendix H standard circuit arrangements for household and similar installations. The installed current carrying capacity (Iz) calculated must not be less than 20A for a ring circuit using 2.5mm2


. Regulation 433.1.204 also


states that the load current in any part of the circuit is unlikely to exceed for long periods the current carrying capacity (Iz) of the cable.


A sensible approach when it comes to ring fi nal circuits reference can also be made to Appendix 15 (page 505) which has useful information and guidance. For background information,


ring fi nal circuits were developed in Britain in 1927, in the 9th


Edition of


the wiring regulations. Use was intended for light loads only, e.g. vacuuming etc. Regulation 433.2.1 (page 89), Position of devices for protection against overload footnote, states: “A reduction in current- carrying capacity may be due to a change in cross-sectional area, method of installation, type of cable or conductor, or in environmental conditions.” So, in answer to the initial


question, it WOULD be acceptable to protect a ring fi nal circuit using 2.5mm2cable protected by a 20A overcurrent protective device. However, the


designer of the installation must take into account the load connected to the socket outlets, the method of cable installation and switch-on surges when using reduced current overcurrent protective devices.


A similar answer to the above is given below, and is what electrical industry bodies including SELECT agreed under the auspices of Electrical Safety First’s Wiring Regulations Advisory Group (WRAG):


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