Later National Grid's report revealed that the second fault, and the ultimate cause of the blackout, was the fitting of a wrongly rated part in a backup system - a Buchholz protection relay had the wrong setting; similar to fitting a 1 ampere fuse instead of a 5 ampere fuse. The first fault was a problem with a different transformer. The impression was given that this first fault was in a sense routine - about 13 such faults happen in a year. Normally it would be possible to switch the transformer that faulted first, out of the system and continue with power uninterrupted using spare capacity on the adjacent second transformer, also in Wimbledon, but in this case the switch over exposed the human error in using a part with the wrong setting on the second transformer.