The Heathrow Wake Up Call - Why Oil Test Sets Matter

The Heathrow substation fire made global news in back in March, with the blaze that broke out at the North Hyde substation sparking a complete loss of power to part of the airport’s electrical distribution network.

An estimated 300,000 passengers were affected by the airport’s subsequent closure, with airlines reportedly losing up to £100m because of care for stranded passengers. While the full investigation of the incident pointed to a failure in the substation’s components, questions have been raised about whether this was an unavoidable disaster, or a result of missed early warning signs – particularly those that a routine oil test is designed to detect. This article will explore what oil test sets are, what they measure, and why they play such a critical role in preventing catastrophic failures like the one seen at Heathrow.

Oil test sets are critical to maintaining the health and operation of critical infrastructure, and ensure that issues are not present within the insulating oil found in electrical equipment. This oil is crucial to this equipment but, over time, it can degrade due to a number of factors, whether that be oxidation, electrical stress, or the intake of moisture, which the latter believed to have caused the issue at Heathrow substation. Oil test sets are able to perform several diagnostic tests, such as dielectric breakdown voltage (BDV) tests, which is able to test for degradation within asset.

When operating large-scale infrastructure, oil testing is absolutely imperative in order to prevent issues from arising. Megger’s range of Oil Test Sets applies a high voltage to a sample of oil and measures the voltage at which it breaks down, indicating its ability to resist electrical stress. This would have highlighted issues in the oil on the transformer. According to the National Energy System Operator (NESO), who released their findings on July 2, 2025, the National Grid were aware of the fault which caused the issue for seven years before it occurred. This means that an oil test set was most likely used, which revealed elevated moisture levels in the oil that was insulating the machinery. A Megger Oil Test Set, such as the Megger OTS100AF would have been able to detect the elevated moisture levels, and with action taken on the back of this reading, could have prevented the disaster from happening in the first place.

The blaze was likely caused by moisture in the bushings, which are cylindrical components that are designed to reduce friction between moving parts. Oil testing conducted by the National Grid Electricity Transmission in 2018 found that moisture levels indicated a risk of “imminent failure”, and an order to replace these components was left unanswered. Had frequent oil testing been conducted and repairs actioned, this disaster could have been avoided, and millions of pounds could have been saved in the process.

Megger Oil Test Sets aim to reduce the impact of unplanned downtime and failures by identifying these issues before they have progressed. These oil testers can perform breakdown detection in terms of both voltage and current, which even ensures that they can keep up in terms of further developments in testing standards. Overall, the importance of oil testing cannot be overstated. Simply put, if an issue is located during this process, it must be corrected before it progresses, just like the Heathrow substation incident. At Test Meter, we stock a range of Megger Oil Test Sets, which are available on our website along with a range of equipment suitable for High Voltage Testing inside a substation.

If you have any questions about these instruments, or a general enquiry, do not hesitate to get in touch, either by emailing our sales team at Sales@test-meter.co.uk, or by phone on 0113 248 9966.