Following discussions in a recent thread, I have been contemplating the extent to which the totality of my electrical installation is compliant/non-compliant with the current wiring regulations, and, in the case of things which are non-compliant, how crucial/important it may be to consider having them brought into alignment with current regs. This first couple of questions are fairly 'strategic' ones.....
I have (inherited) an installation with a number of sub-mains each served by a 60A switch-fuse (rewireable fuses), currently wired in 16mm² double-insulated singles (surface clipped, not grouped). Taking into account the 0.725 fusing factor correction due to protection being by a rewireable fuse, Iz of the cable is (just) greater than 60A, so ought to be OK in relation to Iz>In.
The potential problem relates to design current (i.e. a problem relating to In>Ib, rather than Iz>In). The circuits connected to some of the CUs supplied by the sub-mains theoretically have maximum current demands in excess of 60A, if one uses guideline diversity figures – so, although Iz >In, In would not be greater than Ib if one used that basis of calculation. However, the actual (rather than guideline) diversity for the situation concerned is such that the demand on each and every the sub-mains is, in practice, virtually always considerably less than 60A. However, it’s obviously difficult (impossible?) to ‘prove’ that assertion, and the fact that none of the 60A fuses have blown in many years of use merely means that there have not be sustained currents greater than about 120A. I suppose I’m therefore really asking whether (in order to satisfy In>Ib), it could be acceptable to work on the basis of a design current less than that which would result from application of diversity guidelines - although I recognise that such would not be an ideal situation, even if permissible. Changing to fuses of higher rating (in order to accommodate design current using guideline diversity) would obvioulsy not be an option with the current cable sizing. For completeness I should add that it’s a TT installation and that all of the sub-mains are themselves protected by 100mA S Type RCDs.
As a subsidiary question (which becomes much more important if the answer to the above is unfavourable), the switch-fuses are massive old MEM ones, albeit in good condition, and occupy half of a wall (which space I could make much better use of!). Would replacing these with, say, 63A double pole MCBs in suitable (small!) enclosures be a compliant option? More to the point, what about 80A Type B MCBs (assuming I could source them)? By virtue of the different fusing factor correction, the cable Iz should still just about be OK in relation to In, and we would then probably also have In>Ib using guideline diversity, thereby effectively eliminating the issue mentioned in the previous paragraph. If this were a viable approach, it would presumably be a good/safe idea (and easier to wire) to use double pole MCBs, whether or not that was required? - although the present switch-fuses are necessarily only single-pole in their overload disconnection, since neutral fuses are obviously positively dangerous.
A related question concerns the associated earthing. As`I said, it’s a TT system. The earthing conductor (replaced a while ago) from earth electrode to the MET and the one directly-connected CU (the one closest to meter) is 16mm². The inherited earth cables which accompany the sub-mains (which I presume also count as earthing conductors) are currently 10mm². As far as I can make out, this seems to be within the (surprising modest) requirements for TT systems. It might even do if our earthing system got upgraded to TN-C, although I don’t see that happening any time soon. The sub-mains (hence earthing conductors) are relatively short, since the CUs are all roughly in a vertical line, on different floors. Am I right in believing that what I have described is probably compliant, at least whilst the system remains TT?
Thanks for your assistance. Some more questions will undoubtedly follow.
Kind Regards, John
I have (inherited) an installation with a number of sub-mains each served by a 60A switch-fuse (rewireable fuses), currently wired in 16mm² double-insulated singles (surface clipped, not grouped). Taking into account the 0.725 fusing factor correction due to protection being by a rewireable fuse, Iz of the cable is (just) greater than 60A, so ought to be OK in relation to Iz>In.
The potential problem relates to design current (i.e. a problem relating to In>Ib, rather than Iz>In). The circuits connected to some of the CUs supplied by the sub-mains theoretically have maximum current demands in excess of 60A, if one uses guideline diversity figures – so, although Iz >In, In would not be greater than Ib if one used that basis of calculation. However, the actual (rather than guideline) diversity for the situation concerned is such that the demand on each and every the sub-mains is, in practice, virtually always considerably less than 60A. However, it’s obviously difficult (impossible?) to ‘prove’ that assertion, and the fact that none of the 60A fuses have blown in many years of use merely means that there have not be sustained currents greater than about 120A. I suppose I’m therefore really asking whether (in order to satisfy In>Ib), it could be acceptable to work on the basis of a design current less than that which would result from application of diversity guidelines - although I recognise that such would not be an ideal situation, even if permissible. Changing to fuses of higher rating (in order to accommodate design current using guideline diversity) would obvioulsy not be an option with the current cable sizing. For completeness I should add that it’s a TT installation and that all of the sub-mains are themselves protected by 100mA S Type RCDs.
As a subsidiary question (which becomes much more important if the answer to the above is unfavourable), the switch-fuses are massive old MEM ones, albeit in good condition, and occupy half of a wall (which space I could make much better use of!). Would replacing these with, say, 63A double pole MCBs in suitable (small!) enclosures be a compliant option? More to the point, what about 80A Type B MCBs (assuming I could source them)? By virtue of the different fusing factor correction, the cable Iz should still just about be OK in relation to In, and we would then probably also have In>Ib using guideline diversity, thereby effectively eliminating the issue mentioned in the previous paragraph. If this were a viable approach, it would presumably be a good/safe idea (and easier to wire) to use double pole MCBs, whether or not that was required? - although the present switch-fuses are necessarily only single-pole in their overload disconnection, since neutral fuses are obviously positively dangerous.
A related question concerns the associated earthing. As`I said, it’s a TT system. The earthing conductor (replaced a while ago) from earth electrode to the MET and the one directly-connected CU (the one closest to meter) is 16mm². The inherited earth cables which accompany the sub-mains (which I presume also count as earthing conductors) are currently 10mm². As far as I can make out, this seems to be within the (surprising modest) requirements for TT systems. It might even do if our earthing system got upgraded to TN-C, although I don’t see that happening any time soon. The sub-mains (hence earthing conductors) are relatively short, since the CUs are all roughly in a vertical line, on different floors. Am I right in believing that what I have described is probably compliant, at least whilst the system remains TT?
Thanks for your assistance. Some more questions will undoubtedly follow.
Kind Regards, John
