A fire at an electrical substation that shut down Heathrow Airport for most of a day was most likely caused by moisture which had been present in a transformer since 2018 but went ‘unaddressed’, a review by the National Energy System Operator (Neso) has found.
The report said moisture had been detected in oil samples at the North Hyde site in 2018 but action was not taken to replace electrical insulators known as bushings.
No flights operated at the west London airport until about 6pm on March 21 because of the blaze, which started late the previous night.
The final report by Neso said there was a ‘catastrophic failure’ in one of the transformers, ‘most likely caused by moisture entering the bushing causing a short circuit’, which ignited the oil. The report said: ‘An elevated moisture reading in one of SGT3’s (the transformer’s) bushings had been detected in oil samples taken in July 2018.
‘According to National Grid Electricity Transmission’s relevant guidance, such readings indicate ‘an imminent fault and that the bushing should be replaced’.
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‘While the reading was recorded in National Grid Electricity Transmission’s online system, the mitigations appropriate to its severity were not actioned.
‘The controls in place were not effective and failed to identify subsequently that action had not been taken in relation to the elevated moisture reading. This includes an opportunity in 2022 when a decision was taken to defer basic maintenance on SGT3. The issue therefore went unaddressed.’
More to follow.
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