Faulty fuse brings rush-hour chaos to London in a blackout that brought the famous underground tube network grinding to a halt.
The 28 August outage, which affected parts of southern London and north-west Kent, was the biggest blackout in the region since the Great Storm of 1987. More than 410,000 homes and businesses lost their electricity supplies.
Power dropped at roughly 6:30 pm. It was restored in most places after half an hour, although it took a couple of hours to restore supplies in some areas.
Around 60% of the London Underground was affected, with all main rail services in south London and the south-east of England at a standstill, leaving 250,000 passengers stranded.
Nearly 2,000 mainline trains were hit and it took several hours for normal service to resume.
A hundred people were rescued from stuck lifts by the emergency services.
Digging Deeper Into The Power Cut Causes
National Grid published its 43-page investigation on 10 September. Energy regulator Ofgem and the London Assembly also produced reports into the incident.
Initially, National Grid analysis went no further than “there was a fault in the 275 kV system affecting a ring around London”.
But the full review revealed a failed transformer at the Hurst substation near Bexley as the initial cause of the outage.
Although power was re-routed through other circuits, a second fault seven seconds later resulted in in the flow of electricity stopping on the underground cable between New Cross and Wimbledon substations.
This second fault happened because automatic protection equipment thought there was a fault with the load. In turn, this disconnected supplies to three power stations.
The technical report uncovered this protection mechanism only kicked in because “an incorrect protection relay was installed when old equipment was replaced in 2001”.
The Buchholz protection relay on the circuit had the wrong rating. It was the equivalent of fitting a 1 amp fuse instead of a 5 amp one. So when the fault occurred, the relay’s reaction was too sensitive and shut down the system when it shouldn’t have.
When the report was published, National Grid had already checked around 9,000 of 45,000 similar pieces of equipment across the network.
All were working as normal. The operator stated all remaining relays would be checked within four weeks.
By mid-October, it was discovered that the first transformer fault was due to an oil leak that had been spotted some weeks before the blackout.
While the oil was topped up, the leak wasn’t fixed at the time, which chimed with wider criticisms of National Grid’s approach to maintenance and investment in the network’s upkeep.
National Grid’s Director of Transmission admitted there was a “small backlog” of maintenance checks.