The investigation into the fire at the North Hyde substation near Heathrow Airport in March 2025 has pointed to a moisture-related failure in a high-voltage bushing as the most likely cause of the transformer fire that disrupted power to Heathrow Airport and grounded over 1,300 flights, says Versaperm…

The official report concluded that “a catastrophic failure on one of SGT3’s high voltage bushings caused a fire to ignite on the supergrid transformer.” While the precise cause remains under investigation, moisture ingress, either through permeation or leakage, is believed to have played a key role in the bushing’s electrical failure.

Elevated moisture levels were reportedly detected as early as July 2018, yet the component was neither replaced nor fully maintained in the years since, including a documented deferral of maintenance in 2022. The event is widely considered to have been preventable.

“This incident serves as a stark reminder of the importance of effective moisture management in high-voltage equipment,” said Chris Roberts, Technical Director at Versaperm, a UK-based leader in moisture vapour permeability testing. “Whether the ingress results from leakage, permeation, or inadequate sealing, the consequences can be severe if left undetected or unaddressed.”

Versaperm’s Testing Solutions Help Mitigate Moisture Risks

Versaperm designs and manufactures specialist equipment to measure and monitor moisture vapour transmission through materials, seals and assemblies used in power, electronics, aerospace and other critical sectors.

Among its key technologies is the Versaperm Mass Spectrometry Meter, which offers:

  • Up to 100x faster testing than traditional gravimetric methods
  • Multi-gas analysis for a complete understanding of permeation characteristics
  • Suitability for electrical insulation, bushings, enclosures and coatings
  • High-throughput capability with minimal operator training

“Moisture ingress doesn’t always lead to failure—but when it does, the cost can be catastrophic,” Roberts added. “By integrating accurate, repeatable testing into the design and maintenance process, organisations can dramatically reduce risk.”

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