Indeed, which is why there are so many specialist techniques (4 point contacts and such).
Anyway, I said I'd say how I narrowed it down - without any extra gear.
Linked L&N in the CU and went round the ring checking R1+RN at each socket. Since the rogue resistance I was looking for was about twice the resistance of the ring, I figured I might see the difference. As it turns out, when I got to the last socket on the ring, the measurements were still getting higher (or at least, not going down as they should) - so I figured the problem was either at the end socket or the end in the RCBO. It wasn't the socket (by chance, that was the one I did the initial test at), so that leaves the end in the RCBO.
Terminal screw was tight, so eased the ends out*, measured rN (which was now normal), put them back*, and rechecked R1+RN at a socket (which was now normal) to check that I'd got both ends in properly.
* Which reminded me why I was trying to avoid having to do it. A tad fiddly, not a lot of space, with cables that reach a long way in to the terminal** - ie a recipe for problems. It didn't help that the CU is high up, in a cupboard, and I was having to be careful not to mess up the door that I'd only varnished a couple of hours earlier
** In fact, the neutral ends are further into the RCBO than the gap between the end of the RCBO and the neutral bar. Ie, you can't just withdraw the ends like you can with a short MCB and a large space.
Anyway, I said I'd say how I narrowed it down - without any extra gear.
Linked L&N in the CU and went round the ring checking R1+RN at each socket. Since the rogue resistance I was looking for was about twice the resistance of the ring, I figured I might see the difference. As it turns out, when I got to the last socket on the ring, the measurements were still getting higher (or at least, not going down as they should) - so I figured the problem was either at the end socket or the end in the RCBO. It wasn't the socket (by chance, that was the one I did the initial test at), so that leaves the end in the RCBO.
Terminal screw was tight, so eased the ends out*, measured rN (which was now normal), put them back*, and rechecked R1+RN at a socket (which was now normal) to check that I'd got both ends in properly.
* Which reminded me why I was trying to avoid having to do it. A tad fiddly, not a lot of space, with cables that reach a long way in to the terminal** - ie a recipe for problems. It didn't help that the CU is high up, in a cupboard, and I was having to be careful not to mess up the door that I'd only varnished a couple of hours earlier
** In fact, the neutral ends are further into the RCBO than the gap between the end of the RCBO and the neutral bar. Ie, you can't just withdraw the ends like you can with a short MCB and a large space.
