Several factors contributed to this outage. The direct cause of the blackout was an incorrectly installed
protection relay at Wimbledon substation, which protected the circuit to New Cross. In 2001 a 1 ampere relay was accidentally installed instead of a 5 A model, resulting in the relay tripping at a current level five times lower than the correct rating. When the transformer at Hurst was isolated, the current in the circuit increased to 1,460 A — significantly below the 4,450 A operating capacity of the underground cable, but sufficient to cause the incorrectly sized
protection relay to trip. The Buchholz alarm which triggered the incident was found to be due to low oil levels in the
shunt reactor SR3 at Hurst, which was not in service at the time of the incident. However, the alarm message received at the National Grid control desk was ambiguous, and the operators interpreted it as referring to the transformer
SGT3, which was in use at the time. After the incident, an anonymous engineer highlighted the oil leak to the press, indicating that it was spotted some weeks before the blackout, and accusing National Grid of poor maintenance. The Ofgem report found that National Grid had been aware of the leak since March 2003, and had decided to manage it by periodically refilling the oil until scheduled maintenance. ==See also==