Wrong relay blamed for London blackout

An undersized relay protection unit was at the heart of August's power cut that brought London Underground to its knees and paralysed the busy rush hour traffic.

An investigation into the incident by National Grid Transco has revealed that an incorrect protection relay was installed when old equipment was replaced in 2001. A 1 A-rated relay had been installed rather than the 5 A unit specified. The unit disconnected supplies, including those to London Underground and Network Rail.

The incident was the last link in a chain of related events. Normally four power lines serve the south of London, but two were out due to maintenance.

Problems on the remaining two began earlier when an alarm revealed that gas has built up in the oil used for insulation at a substation near Bexley, Kent, requiring a transformer to be shut down. This left supply dependent on a single transmission circuit from Wimbledon. The blackout was caused by protection equipment "interpreting the change of power flows, due to the switching, as a fault."

The protection equipment that operated was an Inverse Definite Minimum Time (IDTM) relay. The relay had been correctly specified at 5A but a 1A unit installed instead. the mistake was not picked up during commissioning. The report concludes that "the rating of the automatic protection equipment that is included on the documentation used for commissioning could have been more clearly visible to the commissioning engineers."

An extensive survey of similar equipment has been initiated. To date, 9000 items (20%) on the National Grid system have been surveyed with no similar cases found.