Hello, I am dealing with an office refurb and existing installation consisting of about 8 rotary isolators on a ring circuit wired with 2.5mm2 singles and fed by BS88 32Amp fuse. From as builts it appears isolators supposed to serve power poles which appears were never installed, but I would like to reuse the existing circuit to feed new HRU - about 3Amp 1ph supply. Do I need to change the circuit to radial/replace isolators with fused spurs or can I reuse one of the existing isolator? I feel I can't use the isolators but doubt arose in me when my boss said that he thinks using isolators is compliant with BS. Please let me know, thank you :)
But its not supplying sockets so the notes in App 15 is not really covering it.
So we cannot avoid thinking for ourselves and hide behind the ready calculated case, but must ask ourselves the exam question,
"is the choice of cable suitable for the load, and adequately protected against credible faults ? "
Yes, two lots of 2.5mm in parallel is way more than enough for this load, and a 32A fuse will blow long before the 2.5mmm if there is a short circuit anywhere on the ring.
But unless the load has a fuse or breaker inside, yes, you probably need to add a fused spur near the load to catch problems in that bit.
The only corner case to consider, (and for this, can be discounted) is how well the 2.5mm cable would fare if a load that did not quite blow the 32A fuse was connected at a point far round the the ring, such that one side took significanty more of the current than the other. In such a case, a lower rated fuse at the origin makes it unconditionally safe against cable overloads from single point loads.
More generally you can both find and design ring circuits that do not meet appendix 15, and are not for 13A sockets, but then someone needs to do a few design sums to be sure to avoid odd overload cases.
Its not unknown to see a 50A MCB protecting a ring of a few 32A outlets on a camp site for example, or ringed supplies to data cabinets, and I have seen the odd case of lights wired as a ring. In the same way I have found a 16A MCB in a box supplying a shed from part way round a ring of sockets. Much like a 13A fused spur, its load is limited, and as it was not especially near to one end, while it was non-standard, not an issue.Such a thing does not meet the advice of the appendix or the sketches in the OSG of course, but does not necessarily break the essential requirements of BS7671, though when found it really does need clear description or notes leaving at the board so as not to confuse the next man in as well.
But its not supplying sockets so the notes in App 15 is not really covering it.
So we cannot avoid thinking for ourselves and hide behind the ready calculated case, but must ask ourselves the exam question,
"is the choice of cable suitable for the load, and adequately protected against credible faults ? "
Yes, two lots of 2.5mm in parallel is way more than enough for this load, and a 32A fuse will blow long before the 2.5mmm if there is a short circuit anywhere on the ring.
But unless the load has a fuse or breaker inside, yes, you probably need to add a fused spur near the load to catch problems in that bit.
The only corner case to consider, (and for this, can be discounted) is how well the 2.5mm cable would fare if a load that did not quite blow the 32A fuse was connected at a point far round the the ring, such that one side took significanty more of the current than the other. In such a case, a lower rated fuse at the origin makes it unconditionally safe against cable overloads from single point loads.
More generally you can both find and design ring circuits that do not meet appendix 15, and are not for 13A sockets, but then someone needs to do a few design sums to be sure to avoid odd overload cases.
Its not unknown to see a 50A MCB protecting a ring of a few 32A outlets on a camp site for example, or ringed supplies to data cabinets, and I have seen the odd case of lights wired as a ring. In the same way I have found a 16A MCB in a box supplying a shed from part way round a ring of sockets. Much like a 13A fused spur, its load is limited, and as it was not especially near to one end, while it was non-standard, not an issue.Such a thing does not meet the advice of the appendix or the sketches in the OSG of course, but does not necessarily break the essential requirements of BS7671, though when found it really does need clear description or notes leaving at the board so as not to confuse the next man in as well.
