The customer called because half the downstairs sockets kept losing power, and there was a faint burning smell near the lounge. Another electrician had reset the RCD and left.
Finding the fault
Most UK homes wire their sockets in a ring, with both ends of the cable going back to the fuse board. When a connection in the ring works loose, the sockets often keep working, which is why the fault was so hard to spot. Meanwhile the remaining cable carries more load than it should, and the loose joint heats up.
We tested the ring end to end and narrowed the break down to one double socket behind the sofa. The terminals were loose and scorched.
The fix
We cut back the damaged cable, fitted a new socket, re-made every connection on the ring and tested the whole circuit again. The customer got a Minor Works certificate, and the burning smell is gone.