At 11.49pm, the adjacent transformer also tripped, which resulted in the simultaneous loss of connection to the remaining transformer, according to the document. The report said: "The consequence was the loss of all supplies from North Hyde 275kV substation, impacting thousands of customers including Heathrow airport." A Heathrow spokesperson said: "Heathrow welcomes the Neso review's initial report, which raises important questions for National Grid and SSEN (Scottish and Southern Electricity Networks) that we hope the final report will provide answers to, including the cause of the fire. "Further clarity on how the fire started and why two transformers were subsequently impacted can help ensure greater resilience for the UK's energy grid moving forward." National Grid owns the substation, while SSEN is responsible for power distribution in the area. The full report is expected to be published by the end of June.