by ericmark »
Fri Nov 25, 2022 7:44 am
Always taught not to have loops in a ring final, makes testing near impossible, but as a spur you would need to fuse down, I found the Screwfix grid system was handy as could turn a double socket into a single with a fuse, or two singles allowing the ring final to be split to add more sockets.
The big problem for DIY is to test the ring final, for a MCB/RCBO to trip on overload on the magnetic part of the trip, with a B32 you need 160 amp (5 times thermal rating) to flow, plus 5% margin for safety, so around 1.37 ohms, the cheap plug in testers test down to 1.9 ohms, so no good for a ring final, which means using a proper loop impedance meter, or use existing readings with a low ohm meter.
But the 1.9 ohms is OK for a fused spur, the point is you don't know where the centre of the ring is, so will take the R1 + R2 reading. It is not the doing of the work which is the problem, it is testing the work to ensure it is safe. i.e. will it still trip the MCB/RCBO with a short circuit fault?
An electrician who tests every day gets a feeling for when on the edge, and when well within limits, one looks at the size of house etc, so test when completed when unlikely to exceed limits, for the DIY harder as you have no idea if getting near the 106 meter limit for cable with a ring final, so it needs testing before you start, unless you have a record of the last tests done.
