Rotary Isolators on ring circuit

Is the proposed 3A load internally fused or otherwise incapable of overload ? If so, then you can connect it directly - the 32A fuse at the origin protects the existing wiring and isolator, and the protection local to the load protects it. If not, then a fused spur to protect the new work where the wiring steps down from the 2.5mm to the smaller cable is probably the easiest answer - or if nothing else is ever going to be added to the circuit, then change the 32A fuse for a smaller one. (after all, if each isolator fed a couple of  sockets, or a locally fused block of more sockets then you'd think nothing of a 13A fused spur fed from a ring final, and this is the same, but without the sockets.)


Certainly no need to create a new radial or remove anything unless there are other reasons that mean you want to do that anyway.  Is the wiring route obvious, because if concealed and not in earthed containment, it could need a 30mA RCD or RCBO at the origin.

Wiring route is obvious, however there is no RCD protection as this was installed back in early 00s. For obvious reasons I don't want a mechanical fan on an RCD circuit especially that it's fed off from the fuseboard rather than a standard DB. 


But from isolators point of perspective is it okay to have isolators on a ring rather than a twin sockets or a fused spur? Appendix 15 of BS7671 doesn't say anything about anything else than TSSO and spurs.

I am not aware of anything that prohibits use of the isolators as described, BUT I feel in all likely circumstances, that either a fused connection unit or a socket taking fused plugs would be required downstream of the existing isolators. The clear intention of a 32 amp ring final or a 32 amp general purpose small power radial circuit is that loads should be connected thereto by fused plugs or by fused connection units.

Most of us would presumably not connect say a toilet extract fan, or indeed the light in a toilet to a 32 amp circuit without an intervening fuse, this sounds rather similar.


Also, equipment should be installed according to the suppliers instructions, It seems probable that the fan unit instructions will specify a maximum fuse size of a lot less than 32 amps.

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.

okay that makes more sense now, but coming back to an RCD - that means I need to allow for an RCD protection then as I am using a ring circuit and the rating of the circuit is up to 32 Amp? As stated above it's not ideal in my situation as it will be a mechanical fan so I would rather avoid using RCD so it doesn't trip unwanted.

I would connect the new fan to the existing circuit via an RCD fused connection unit equipped with a suitable fuse. There is arguably no need to upgrade the whole existing circuit with RCD protection, but any additions should have RCD protection. I see no need to retain to retain the existing isolators, but also no harm in leaving them in place if this is simpler.

Thank you very much for your help! :)

that means I need to allow for an RCD protection then as I am using a ring circuit and the rating of the circuit is up to 32 Amp?

You'd only need 30mA RCD protection if your new work is supplying sockets rated 32A or less, outdoor portable equipment (again 32A or less), soft sheathed cables embedded in walls (<50mm deep in insulating walls, any depth in walls with metallic structural parts), passes through certain special locations such as a bathroom or where Zs is too high for the overcurrent protective device to provide ADS within the required time. The circuit rating or topology isn't a factor.


  - Andy.