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Scientists comment on a power outage caused by an electrical substation fire that has caused Heathrow airport to close.
Stuart Mortimore, a Fellow of the Institution of Engineering and Technology and a fire and forensic investigations expert, said:
“The initial press reports suggest that the fire is due to an electrical fault that developed within an oil filled transformer at the substation. Such a failure would have caused severe and widespread electrical arcing activity – “sparking” – which would have ignited the oil as it was ejected forcefully from the transformer. This would have resulted in a large fire that was difficult to extinguish.
“The damage caused makes it unlikely that much useful physical evidence will remain. The investigation is therefore likely to concentrate on the operation of the transformer and its maintenance. Specifically, if it had an on-load tap changer – a complicated switch that can automatically change the output voltage while supplying electricity – that was operating at the time of the fire, this could point to a fault with the switch.
“Maintenance checks will be undertaken to see if periodic oil tests had shown any signs of leakage from the transformer that might have caused electrical components within the transformer to become uncovered, or whether the oil was degrading with age that meant it failed to insulate components adequately. Either of these problems could have caused a loss of insulation, which precipitated electrical sparking and thus the incident.”
Dr David Laverty, Reader, School of Electronics, Electrical Engineering and Computer Science, Queen’s University Belfast, said:
What is an electrical substation?
“An electrical substation is a crucial part of the power grid, acting as a hub that transforms high-voltage electricity into lower, usable voltages for homes, businesses, and critical infrastructure like airports. It also plays a key role in controlling and protecting power flows to prevent overloads and blackouts.”
How/why would one substation have such a big impact on an airport? Is that normal / what we would expect from our energy infrastructure?
“Heathrow relies on a steady and high-capacity power supply to run everything from air traffic control to baggage systems. If a key substation goes offline and backup systems aren’t enough to bridge the gap, operations grind to a halt. While major infrastructure is designed with some redundancy, a single point of failure—especially in a high-demand environment—can still have a dramatic impact.”
Are fires at substations common? Is there anything we can tell from this fire?
“Substation fires are rare but not unheard of, often triggered by equipment failure or external damage like weather events. The scale of disruption at Heathrow suggests this was a serious incident, but until investigations are complete, it’s unclear whether it was due to a technical failure or an external factor.”
What sort of things can cause fires at substations – do we know yet what might have caused this one?
“Electrical faults, insulation failures, lightning strikes, or even deliberate tampering can all spark a substation fire. Right now, it’s too early to say what caused this one, but investigators will be looking at everything.”
Is it too early to know what has happened here?
“Yes—engineers will need time to assess the damage, trace the cause, and determine whether the fire was due to a technical issue, an external incident, or something more unusual.”
Some of the information suggests the fire damaged equipment at the substation – what would that equipment be?
“The most vulnerable components would be transformers, circuit breakers, and switchgear—vital for controlling and distributing electricity. If these are badly damaged, it can take days or even weeks to repair or replace them.”
Why do incidents like these cause such significant losses of power?
“Substations act as central hubs in the power network—when one fails, electricity can’t be safely routed to where it’s needed. Heathrow’s demand is enormous, and if alternative power sources aren’t able to take over immediately, the impact is felt within seconds.”
What makes this incident ‘unprecedented’, if it is?
“If this fire caused an extended or widespread outage despite existing safeguards, it could highlight previously unknown weaknesses in the power infrastructure. A failure of this scale, affecting one of the world’s busiest airports, would certainly be a rare event.”
What will the authorities/companies etc. be doing at the moment to try to restore power?
“Engineers will be working around the clock to assess the damage, restore power via alternative routes, and bring in emergency generators where possible. In parallel, investigations will be underway to understand what went wrong and prevent it from happening again.”
What sorts of investigations will be going on to try to work out what happened?
“Specialists will be analysing damaged equipment, reviewing maintenance records and operational data recorded by instruments in the substation. They will also considering external factors like weather conditions. They may also use forensic techniques to pinpoint where the fire started and how it spread.”
Is this an issue specific to aviation and airports or does it just happen that Heathrow was affected because it is supplied by this substation?
“This isn’t just an airport problem—any large facility relying on a single substation could suffer a similar fate. Heathrow was particularly unlucky this time, but the incident raises questions about the resilience of power infrastructure supporting other critical sites.”
Mark Aizlewood, member of the IET’s Aerospace Technical Network Committee and Head of Aerospace Integration Research Centre and Programmes at Cranfield University, said:
“The aviation industry is at its best during a crisis. It really comes together, and companies that day-to-day compete to be the best airline will work together with the airport and businesses to resolve from the situation as quick as possible. Airports are like communities, where everyone works together as quick as we can.
“At this stage we don’t really know what the impact will be. If they will be back on at midnight we should have flights back tomorrow, then it’s a question of how long it will take to bring all services back online to function fully again. At the moment the airport is unable to operate and it will come back online as safely and swiftly as possible. The industry really prides itself on safety so there will not be a rush to do so in an unsafe manner.
“It’s too hard to say how long it will take to clear the backlog; some flights will be cancelled, some rearranged, and so it’s difficult to know the impact at this time.
“There was a similar level of disruption with the Gatwick drone sighting a few years ago; at that point aircraft were unable to land for an extended period, this time aircraft already airborne had to divert to other airports or to return. Does the UK need to have lots of spare capacity to deal with these major events? That would not seem sensible, as these events do occur, but they are not regular, so we do not need to have more runways to cater for them.
“Everyone will be working as a team to get Heathrow back online; this is a strength of aviation and something working in the industry I feel proud to be a part of.”
Grant Spence, Chair of the Power Industries Division, Institution of Mechanical Engineers, said:
“This is a highly unusual event, which goes beyond the designed resilience of the electricity network, and due to the close proximity of the substation to Heathrow Airport has resulted in a very high impact. This is far beyond what might have been experienced if the incident had occurred at a similar substation located elsewhere in the UK.
“Disconnection of potentially healthy supply circuits in the substation will have been necessary to enable the firefighters to work to put out the fire. This will have contributed to the extent of the impact experienced.”
Prof Martin Kuball, Professor of Physics at the University of Bristol, and a Royal Academy of Engineering Chair in Emerging Technologies, said:
“The event near Heathrow highlights how dependent we are on the reliable provision of power to businesses and homes, and the scale of catastrophic effects when a single power sub-station fails. I was also, like so many others, due to be catching a flight tonight, which has been cancelled.
“Unfortunately there is no resilience built into the National Grid. In part, this is because we still rely on old technology in sub-stations that use copper windings to distribute power rather than new technology, so-called solid state transformers. Due to this, there is currently still a rather sparse distribution of these substations rather than a more resilient network. We need to develop our national grid, with substations that are more densely located and smaller so if instances like this occur, the impact is much less. This infrastructure upgrade is only possible using semiconductor power device technology for solid state transformers to distribute power – substations of suitcase size, which could create a much stronger national power distribution grid.
“The challenges are recognised, so we are at the forefront of developing these devices and working with industry for their commercialisation but it will be some years away, between five to ten, before we can fully move away from the existing power provision.”
Graham Kenyon, an electrical engineering expert, the Institution of Engineering and Technology, said:
“An electrical substation like this takes very high voltage power from the national grid, and switches it to various transformers, distribution protection equipment, and other substations that eventually supplies end users such as homes, businesses and airports with power.
“This particular substation is a major distribution centre for the national grid, in a very populated area of the city. In addition to Heathrow Airport and local residences and businesses, there is a significant amount of commercial and industrial power use in that area.
“It’s too early to provide a definite answer on what has happened here. Heathrow airport has multiple supplies from the national grid, and means of diverting power if it is safe to do so. Power in an airport is required for safety equipment for flight operations and safety and security of the public in and around airport terminals.
“The Civil Aviation Authority has rules that govern the impact of power loss on airport operations, which includes consideration as to how long the loss of some, or all, power at an airport is expected to last. Airport operations can also be affected by the consequences of a major incident in the vicinity of the airfield or flight paths, for example smoke affecting flights.”
Dr Conor Murphy, Vice President of Engineering, NovoGrid, said:
What is an electrical substation?
“An electrical substation is a critical component of the power distribution network, responsible for regulating voltage levels between high-voltage transmission systems and lower-voltage distribution networks. These facilities transform electricity to safer, usable levels for homes, businesses, and critical infrastructure like airports. Substations house essential equipment such as transformers, circuit breakers, busbars, and capacitors, which collectively manage power flow, maintain grid stability, and reduce energy loss during distribution. Think of it like the fuse board in your home, this routes power to different rooms on different circuits, a substation is a point in the network where power is dispersed to circuits many km long and serves thousands of customers.
How/why would one substation have such a big impact on an airport? Is that normal / what we would expect from our energy infrastructure?
“Redundancy is typically built into grids, such widespread impact from a single substation is unusual, but it is doubly newsworthy because it has knocked out an airport. This incident suggests a great deal of damage was caused at the substation, I would speculate that the site was a primary and standby source of power for Heathrow Airport.
Are fires at substations common? Is there anything we can tell from this fire?
“While transformer failures do occur 2.4–4% over a 40-year lifespan, fires causing mass outages are rare. However, the oil-cooled equipment in substations poses inherent fire risks, particularly from aging infrastructure or overloaded systems. While not common, they are planned for, there is typically a concrete blast wall surrounding transformers at this voltage level.
What sort of things can cause fires at substations – do we know yet what might have caused this one?
“Common causes of substation fires include transformer overloads, insulation failures, arc flashes from faulty wiring or maintenance errors, and voltage surges or equipment degradation. The exact cause of the Heathrow fire remains under investigation, with no confirmed details yet available. I’d speculate it was an aged asset in this case, but have limited knowledge of the equipment specifics.
Is it too early to know what has happened here?
“Yes. Full root-cause analysis (e.g., equipment forensics, grid resilience reviews) will take weeks.
Some of the information suggests the fire damaged equipment at the substation – what would that equipment be?
“I’ve heard the fire involved a transformer, prudent grid operators carry surplus equipment in their stock for an event like this. If these assets are to be replaced with new equipment, there is a lead-time of over a year for a new transformer of this size. From footage I have seen it looks like the air insulated switchgear may have sustained significant damage from being in the vicinity of the fire. Damage to these components would disrupt power routing and safety mechanisms so will need to be made good again prior to energising the site.
Why do incidents like these cause such significant losses of power?
“Substations are central hubs; disabling one can sever downstream connections. In an urban setting like this there are always standby routes available to ensure continuity of supply.
What makes this incident ‘unprecedented’, if it is?
“The scale, that is the impact from a single point of failure is extremely high because of its impact on Heathrow Airport. To me the event is unprecedented owing to its impact on critical infrastructure impact. Complete airport shutdowns due to power failure is extremely rare. This suggests a simultaneous loss of primary, standby and local backup power sources, which is unusual.
What will the authorities/companies etc. be doing at the moment to try to restore power?
“I can see already that Scottish and Southern Electricity Network and likely UK Power Networks, are reconfiguring the distribution network to bring power via alternate routes to customers. Customers on this network will be more exposed to future outage events until the damaged equipment is restored at North Hyde Substation.
What sorts of investigations will be going on to try to work out what happened?
“The source of the fire will need to be determined to prevent similar events in the future. Assessing whether the cause was electrical fault, human error, or external factors.
Is this an issue specific to aviation and airports or does it just happen that Heathrow was affected because it is supplied by this substation?
“No. Heathrow was affected solely due to its dependency on this substation. Any critical facility reliant on a single substation (hospitals, data centres) would face similar risks. The incident underscores broader grid vulnerabilities, in particular the design of Hyde North Substation, not aviation-specific flaws.”
Prof Chenghong Gu, Professor in Smart Energy Systems, the University of Bath’s Department of Electronic and Electrical Engineering, said:
“Electricity substations are used in electrical power systems to change voltage levels and distribute power flows. They are critical components in power systems. There are many devices in substations to make it work, including transformers, switch gears, circuit breakers, insulators, lightning protection devices, measurement device, data acquisition and control systems.
“This is a very rare event. Substations are built and operated according to very strict standards, and they are monitored 24/7. There are also many automatic devices in substations like this one to deal with faults.
“It is very unusual for big substations like this to catch fire, but it is possible due to many causes. It is too early to tell the causes of the fire at North Hyde Electricity Substation without further information.
“It is too early to tell what has happened, many things can cause fires to substations. One major cause is extreme weather, such as lightning strikes, which could cause extreme high voltage on equipment. Extreme hot weather together with high demand can cause transformers to be overheated, thus leading to faults. Another factor is aging of transformers, as the insulation gas can degrade, and this could cause explosion inside transformers together with high temperature. There could be malfunction of other auxiliary devices, such as insulator, switch gears, circuit breakers inside the substation. There could be other causes. One is malicious attacks on the substation, for example someone set fires deliberately. Cyberattacks on the IT systems can also cause the malfunction devices in the substation, leading to fire.
“In a substation, there are many components, include transformers, circuit breakers, isolator, busbars, and other measuring equipment. Transformers are the most vulnerable to fires. There is insulation oil in them and under high temperature high pressure situations, transformers can explode, and the insulation oil is leaked and can catch fire.
“Incidents like these can cause such significant losses of power because Hayes Primary Substation is a high voltage, big capacity substation. According to SSEN’s North Hyde Grid Supply Point: Strategic Development Plan, the Hayes Primary Substation has a capacity of 31.44 MW and serves 17,300 customers (source: https://www.ssen.co.uk/globalassets/about-us/dso/current-consultations/north-hyde-grid-supply-point—strategic-development-plan—for-consultation.pdf)
“It is very rare for a substation to catch fire, although it is not impossible. However, it is even rarer for a critical customer such as Heathrow to completely lose power.
“Normally, when we design power systems for critical customers, redundancy is put in place to make sure power supply is not interrupted during power system failures, i.e., they normally have a very high reliability level such as 99.9%. This is done automatically when the power detects any faults. However, in this event, it seems that it did not happen.
“On the other hand, some critical customers normally have standby generators (such as oil-driven generators) on site to provide power for a short time in events like this. But again, we did not know whether any standby generators are used by Heathrow.
“There are a few steps to restore the power supply:
“The repair of the substation could take much longer time depending on the damage of assets, ranging from months to years.
“There are a couple things to be done to understand what happened:
“In this case, Heathrow was affected because it is supplied by this substation. But, these types of interruptions can happen to any critical customers as long as they use electricity, because substations or other power system components can fail during particular circumstances.
“The immediate action for critical customers, I think, is to review their power supply schemes, conduct contingency test, find out potential causes of failures/interruptions, migrate them to enhance the supply reliability. In longer them, they might increase redundancy to enhance reliability, and install onsite standby generators.”
Prof David Flynn, Head of Research Division, Autonomous Systems and Connectivity (ASC), James Watt School of Engineering, University of Glasgow, said:
“Today we are witnessing the significant and disruptive impact of the fire at the electricity substation of Heathrow airport. Regarded as an unprecedented event, given that the risk of fire in substations has been historically low, it provides a lens towards the clear and emergent threat we face whilst the UK accelerates the coupling of transport and energy networks – we are building more direct links between all transport modes and energy (consider when their is a power cut, cars which ran on petrol/diesel will be potentially brought to gridlock with the loss of electricity services; in August 2019 when there was a power cut in London the tube and train services, as well as traffic management systems failed).
“Regarding the surprise of this event – substations do have international standards, namely IEEE 979-19141, which provide substation designers with guidance on the mitigation measures relating to fire risk hazards. These practices are also underpinned by a generation of advanced monitoring technologies, which can deliver near to real time critical health monitoring across the breadth of diverse components and assets within a substation2,3.
“In terms of probable cause, it depends on the type of assets within the substation but historical evidence would suggest that for an AC substation, circuit breakers and transformers are some of the most likely component failures resulting in fire in a substation.
“What this event demonstrates, is a lack of readiness and resilience to “events”. On this occasion the event is a fire – cause to be determined. In terms of fires, we know they are uncommon in substations thanks to the standards and technologies we use to monitor the substation assets. In terms of probabilities, its most likely a failure of a component in the substation, but what we cannot rule out until the investigation is completed is an arson or terrorism event (around 8-9% of substation fires are attributed to those two things). And that is why counter terrorism personnel will be present – it’s unlikely but needs ruled out.
“This event, has evidenced both a front line failure in monitoring and mitigation of fires within a critical asset, but it also highlights the significant failure to understand the risk and resilience of this interdependent energy-transport hub interdependence.
“Lessons will be learned, but what is immediately clear is that important research of Hubs, such as DARe and TransiT, are integral to how we advance practices, understanding and enabling technologies towards improving the resilience of our increasingly interconnected transport and energy infrastructure and services.
“Beyond airports, future mobility and freight, across rail and road e.g. EVs, Electric HGVs etc along with maritime ports are all becoming increasingly interdependent on the integrity of energy networks. In August 2019 the power outage in London brought huge disruption to parts of England and Wales, which were left without electricity following a major power cut that had a serious impact on rail and road services, including city traffic lights6. It’s vital we develop with key government agencies, regulators and stakeholders, an improved understanding of how to model and enhance the resilience of the transport-energy nexus. This critical national infrastructure (CNI), most be resilient to both unforeseen events e.g. asset failures, as well as other threats such as climate events and cyber-attacks. We are leading the UKs research in cyber physical infrastructure and digital twinning, as to deliver an advanced ability to predict and mitigate such threats. This research is vital towards underpinning UK national security and economic growth.”
1 979-1994 – IEEE Guide for Substation Fire Protection | IEEE Standard | IEEE Xplore https://ieeexplore.ieee.org/document/8684713
2 Huynh, N., Robu, V., Flynn, D., Rowland, S., & Coapes, G. (2017). Design and demonstration of a wireless sensor network platform for substation asset management. Paper presented at CIRED 2017, Glasgow, United Kingdom.
3 https://www.mdpi.com/1424-8220/14/10/18748
4 David Flynn – DARe https://dare.ac.uk/about-us/our-team/professor-david-flynn/
5 Meet the Team – TransiT https://transit.ac.uk/meet-the-team/transit-team/
6 e.g. https://www.theguardian.com/business/2019/aug/09/power-cut-hits-london-and-south-east-england
Dr Malte Jansen, an energy lecturer and researcher, University of Sussex, said:
What is a substation?
“An electrical substation is a site in the electricity grid where power is handed over between different parts of the grid. For example, where one overhead transmission line ends and is being connected to another line. It can also be a point where high voltage electricity in the transmission (220/400 kV) grid is stepped down to lower voltage electricity (110 kV or less), and then connected to the distribution grid.
How/why would one substation have such a big impact on an airport? Is that normal / what we would expect from our energy infrastructure?
“The connection point in question is in a 300-330 kV line which is part of NationalGrid’s transmission network (see the network map (https://www.entsoe.eu/data/map/). Electrical infrastructure is usually redundant (called n-1), which allows for the failure of one critical component. In the substation, there would have been two separate switch gears to allow for n-1, with the lower voltage being n-1 as well. As both switching points are within the same site, going into Heathrow, they may have both been damaged at the same time, resulting in the loss of two critical pieces of equipment.
“The map shows that the substation in North Hyde is the only one near Heathrow, and likely the only substation supplying the airport with power. Heathrow is most likely connected to the High Voltage NationalGrid, considering its power draw.
Are fires at substations common? Is there anything we can tell from this fire?
“Substation fires are not very common, especially at high voltage levels. It’s hard to pinpoint what happened at this stage, but high voltage infrastructure is very heavy gear, and not much like usual light switches. There are multiple causes of failures. For example, sometimes the air between two switch ends can become ionised and cause an electrical arc, which could lead to a fire. However this fire could also have resulted from a transformer stepping down the voltage. These are often oil filled for cooling, and can catch fire if malfunctioning, or overheating. Another alternative is interference from local vegetation causing a short-circuit to start a fire.
Some of the information suggests the fire damaged equipment at the substation – what would that equipment be?
“Not known, but likely the high voltage part of the electrical switch gear, or the transformer stepping down the voltage.
Why do incidents like these cause such significant losses of power?
“This is probably the only site delivering the large volumes of power needed to supply Heathrow. While the equipment is redundant, it is not geographically redundant.
What makes this incident ‘unprecedented’, if it is?
“The highly unlikely failure of several components at once.
What will the authorities/companies etc. be doing at the moment to try to restore power?
“NationalGrid will likely try to replace the broken components a quickly as possible, after diagnosing the error. Large components often have lengthy procurement timelines, but either spare parts in stock, or salvaged from other substation would expedite the process. It is possible that Heathrow will be reconnected reasonably quickly, but backup redundancy will be restored later.
Is this an issue specific to aviation and airports or does it just happen that Heathrow was affected because it is supplied by this substation?
“Not specific to aviation, but to all consumers which have a large power draw on the grid.”
Prof A. Manu Haddad, Director, Advanced High Voltage Engineering Research Centre, Cardiff University’s School of Engineering, said:
“From the videos I saw, it may be a fire at the transformer. Such fires are very rare but there were cases of such fires in the past (see some online videos).
“Transformers are devices that handle the power coming in and going out of the substation. Usually, the supply power arrives at very high voltage and is reduced to lower voltages for distribution to customers (businesses and domestic).
“With customers like Heathrow airport and the surrounding area, the expected power supply is significant and losing such a substation is not easy to compensate for easily unless there are already other alternative supply routes which are in place. For big load centres, there is always alternative routes for power flow in case one feeding substation has an outage.
“(The transformer is similar to a big water reservoir fed through a river/s, and then the water is distributed to local users/businesses through smaller pipes. Even at local levels, there is another lower capacity distribution system to individual houses and other taps. Transformers work in both directions, unlike the water tank example.)
“As such, the supply substation handles significant amounts of energy and hence the transformers can get hot which is mitigated by its cooling system. In most cases, they have a big margin of operation under normal service conditions, and the risk of overheating is negligible. Although, yesterday was the hottest day of the year so far this year, an ambient temperature of 20°C is more than acceptable for such devices.
“Most transformers are equipped with monitoring systems that warns the user about the state of the transformer so that action can be taken well before a catastrophic failure. The transformers are insulated with insulating oil which, if ignited, can cause a significant fire and can last a long time due to the amount of oil in the transformer tank.
“However, under fault conditions (e.g. a current or voltage surge on the electrical system), damage could occur, and this needs to be investigated.
“The company will have records of current flows and voltages at the substation and measurements of any faults on the system. Therefore, it is difficult to speculate on the causes without access to the network circuit configuration, power flows, condition monitoring data, and any related fault data in the area. The owner company has the data.
“The high voltage substations usually have a main and a reserve busbar to reroute power and more than one transformer in the substation to allow switch power flow to the healthy transformer. Depending on the damage caused by the fire, companies usually have spare and strategies to deploy solutions quickly (if no spares, transformers have a long delay for delivery at the moment given the expansion of electricity networks worldwide1). If the airport/area have other alternative routes for power supply, they will be used in the meantime to provide the necessary power to restore services/power. This question will be better answered by the company who owns the affected substation.”
1 Andrew Moseman, “Engineers transform transformers to save the power grid”, IEEE spectrum magazine, pp. 20 – 26, February 2025.
Dr Barry Hayes, Associate Professor in Electrical Power Systems, University College Cork, said:
What is an electrical substation?
“An electrical substation is a key part of the electricity grid. Substations contain important electrical equipment that enable the voltage of the electricity to be converted to different levels for transmission and distribution using “transformers”. They also contain electrical switches, circuit breakers, and monitoring equipment that enable the safe and reliable transport and delivery of electricity.
How/why would one substation have such a big impact on an airport? Is that normal / what we would expect from our energy infrastructure?
“It appears that a transformer fire in the North Hyde 275 kV substation caused the power outage (videos from the scene clearly show one of the large power transformers ablaze). The North Hyde substation is a large electrical substation that supplies the area to the Northeast of Heathrow airport and the Heathrow airport site. Scottish and Southern Electricity Networks, the local electricity distributor, said 67,000 homes and businesses in the area were cut off overnight as a result if this issue.
“While the North Hyde substation is a very important part of the West London electricity grid, it is generally not expected that this would cause such a big impact at Heathrow airport. There are also dedicated supplies to other parts of the Heathrow Airport site [Source: https://www.ssen.co.uk/globalassets/about-us/dso/current-consultations/north-hyde-grid-supply-point—strategic-development-plan—for-consultation.pdf]. Typically a critical electricity load such as Heathrow would be served from multiple supply points in the electricity grid and therefore there would be an option to feed the loads at Heathrow from an alternative supply point. There are some reports that parts of the airport (e.g., Terminal 5) have power this morning.
“The exact reasons for such a big impact are unclear at this point, but we do know that the North Hyde substation is in a highly-constrained area of the UK electricity grid, in an area where there has been “been a steep increase in the number of new electricity connection requests across west London, driven by new housing developments, commercial investment and datacentres” [Source: https://www.ssen.co.uk/globalassets/about-us/dso/current-consultations/north-hyde-grid-supply-point—strategic-development-plan—for-consultation.pdf]. The UK power grid (like in many developed countries) is generally old/outdated, with many of its components at the end of their anticipated service lifetime and in urgent need of upgrading/modernisation. These issues may be a factor in the power outage affecting Heathrow. However, it will take some time before the exact causes of this incident are established.
Are fires at substations common? Is there anything we can tell from this fire?
“Fires at electrical substations can be caused by the breakdown in insulation between high voltage electrical equipment in the substation and/or overheating, usually due to an equipment failure. This outage appears to have been the result a large transformer fire in the North Hyde substation. Most power transformers use oil as a cooling and insulation medium. Electrical faults or excessive heat can cause this oil to ignite.”
Mark Coles, Head of Technical Regulations, the Institution of Engineering and Technology, said:
“Electrical substations are a critical part of the electrical distribution system. Power stations generate electricity which is then distributed across the country at high voltage. Substations connect the power station to the distribution network and then, at the other end, substations connect the distribution network to where power is needed.
“Substations are electro-mechanical installations. Transformers are used in substations to raise the voltage for distribution across long distances and then to lower the voltage to a more usable level for use in electrical installations in, for example, commercial buildings and homes.
“The law, that is the Electricity at Work Regulations 1989, requires that all electrical installations are maintained in order to be safe for continued use. This applies to electrical substations which should be regularly checked. As part of this, records will have been kept of testing and inspection and, for example, thermal imaging cameras will have been used to make sure connections aren’t overheating. Maintenance work will look to diagnose possible failures of equipment and then any required repair work can be carried out. For this situation, the investigation will look at the history of the equipment, such as the manufacturer’s data from when it was manufactured. The investigation will be far reaching, and every aspect will be looked at to ascertain what has happened. It is very unusual for one incident to cause the entire shut down of an entire site like Heathrow airport.”
Prof Tim Green FREng, Head of Department of Electrical and Electronic Engineering, Imperial College London, said:
My Flight
“I was on a flight that left Charlotte, North Carolina at 19:10 on Thursday, due into Heathrow at 07:10 Friday. About 3 hours into the flight the pilot announced that he had to turn back to Charlotte because Heathrow was not accepting flights because they had lost their electrical supply. He said they’d looked to divert to Gatwick or Manchester but they were quickly filling up and he didn’t think we’d get in. So we landed back at Charlotte about 3am local time. The airport was very quiet but the few staff there did work hard to deal with a long queue of people needing help. Because lots of flights were turned back, there weren’t many options for rebooking. I’m not departing Saturday and changing planes in Baltimore to arrive Sunday.
Why was I in Charlotte?
“I was meeting with the Electric Power Research Institute (https://www.epri.com) which is a global collaboration between electric utility companies. I’m on their advisory council. We’d visited their labs and look at the diagnostic tests they do on failed bits of electrical power equipment. We also saw some of the monitoring devices that check the condition of the oil that helps cool and also insulate transformers and those sensors are intended to spot incipient problems before they become a real problem.
Why did Heathrow lose power?
“Large sites like Heathrow have multiple connections through the distribution network (run by SSE Networks in that area) to the big National Grid Substations. That protects against faults in the local network. Here the problem was with a large substation run by National Grid. It too has multiple routes in and it has three large transformers to step down the voltage. It can run on two transformers and normally if one develops a fault, circuit breakers open to remove it from the circuit and customers don’t notice a thing. This was more dramatic, it looks rather like a large fire in one of the three transformers that may have also damaged other equipment and so the substation exhausted the redundancy it had and customers were disconnected. Reports are of around 60,000 customers. The local distribution network can be reconfigured to take supplies from other locations on the National Grid but it takes some minutes to do the switching operations. It looks like not all customers could be reconnected this way but most were.
Why did the fire happen?
“Too early to say. It is very well known that the oil in the transformer tank (that is circulated via radiators to cool the transformer) is a fire risk. That is why the condition of the oil is check regularly for signs of degradation. They are also analysers that look for signs of gas being produced by the breakdown of the oil so they can be switch off before there is any danger. Such transformers also have pressure switches the detection build up of pressure in the transformer tank and automatically open circuit breakers. So there are multiple layers of precaution but still something went wrong. It is possible that the fire did not start in the transformer but in adjacent equipment that might also be oil cooled.”
Prof Richard Dawson FREng, School of Engineering, Newcastle University, said:
“This event has highlighted the vulnerability of our infrastructure to cascading failure. Damage to one asset in our energy network has exposed the vulnerability of our critical national transport infrastructure to major disruption. It is surprising that there is no resiliency in the system – backup power source and redundancy in the number of network connections. Government has been repeatedly warned of the risks of cascading failures by a number of agencies. For example, the Climate Change Committee has highlighted how extreme weather events such as flooding, heatwaves and storms, have caused similar cascading failures. As our society and economy are increasingly dependent on continued electrical and digital systems, there is an urgent need for a more joined-up Government approach to manage cascading failures such as this.”
Prof Rory Hadden, Chair of Fire Science, University of Edinburgh, said:
“The fire appears to have been in a transformer at the substation. Transformers are electrical equipment in the power distribution network and are often filled with oil as a coolant. Although oils are flammable their use is common in these installations because they do not conduct electricity so can be used effectively to cool the electrical components. Although there are strict precautions to minimise the risk of a fire – the use of low flammability (high flashpoint) oils, containment in the event that there are leaks, and potentially even automatic fire suppression systems – the risk of fire in these installations remains real as there are large amounts of electrical energy around which could, usually when more than one thing foes wrong, cause a fire. Once ignited these oils usually burn and produce large quantities of smoke which can make firefighting challenging and pose air quality issues to residents. Clearly it is too early to say what the cause may have been but investigations will be commencing to understand what happened and to learn from that.”
Dr Paul Cuffe, Assistant Professor, UCD School of Electrical & Electronic Engineering, UCD (University College Dublin), said:
What is an electrical substation?
“A substation is where different power lines link together; like a junction on a motorway network. Substations house large electrical transformers that interlink power lines that operate at distinct voltage different voltages: most of the UK’s transmission lines operate at either 400,000 Volts or 275,000 Volts. These high voltage power lines function to transport bulk electrical power around the country. In most substations, there will also be transformers that step down to yet lower voltages; these withdraw power from the national grid for distribution out to the surrounding region at lower voltage levels; typically at 66,000 Volts or 33,000 Volts. Substations also house a lot of sophisticated switching equipment, to allow lines and transformers to be turned on and off to allow maintainance or to reconfigure the network in emergencies.
How/why would one substation have such a big impact on an airport?
“An airport like Heathrow requires a lot of electricity to operate; equivalent to a large town. As such, it would be typical for an airport like Heathrow to be given a dedicated connection from the substation at Hayes; there is likely a dedicated power line and transformer there that connects the airport to the wider grid. When a major fire severs that link, it will no longer be possible to bring bulk electricity to the airport.
“I would anticipate that a major airport like Heathrow would have some on-site emergency capability to ride through a grid disturbance; I would hope that the traffic control tower and runway lights weren’t totally plunged into darkness! However, processing planeloads of passengers requires Heathrow in its totality to consume a town’s worth of electricity, and the inability to meet this requirement is likely why flights had to be cancelled.
Is that normal / what we would expect from our energy infrastructure?
“It is not overtly abnormal; we can anticipate that, from time to time, substation equipment will fail and downstream power outages will result. One could argue that a critical piece of national infrastructure like Heathrow deserves special grid connection arrangements to secure its supply of electricity further. For instance, sometimes critical loads like this are fed from two separate substations to provide redundancy when outages happen. I am not familiar with the exact connection arrangements for Heathrow airport but neither setup would surprise me.
“It is ultimately a political and economic question to determine the right level of capital investment into grid infrastructure to avoid the problems that outages like this cause. Redundant power supplies for an airport the size of Heathrow do not come free!
What sort of things can cause fires at substations – do we know yet what might have caused this one?
“The power flowing through a large substation is immense: they are by no means an inert environment! Sometimes malfunctioning switching equipment, called circuit breakers, can cause explosions. Transformers are typically cooled using oil, which can be flammable. It is too early to speculate on what exactly happened in this case, but there are several reasonable guess you could make.”
Prof John Loughhead CB OBE CEng, Fellow and electrical engineering expert, the Institution of Engineering and Technology, said:
“An electrical substation is where high voltage electricity from the national grid is transformed to lower voltages and distributed to users, and contains various switches and safety devices. It is therefore the supply point for users. It is surprising that, as a part of our national critical infrastructure, Heathrow does not have an alternative supply point in case of accidents like this.
“Fires do occasionally, but not frequently, occur at substations and can have various causes – from electrical short circuits to failures in transformers. It’s not stated what caused this one and it may take a while for what happened to be determined. A fire within a substation area can damage switchgear, transformers, measurement and control equipment, and cables. Fires are not unprecedented but also not frequent. If they damage equipment any user ‘downstream’ of the substation will lose all power. In this case Heathrow just happens to be the key user, although it seems other users are also affected.
“To restore power, supply companies may be able to route power to Heathrow through an alternative circuit, although it seems this is not easily done or may not be possible, or any damaged equipment will need to be replaced to get the substation back into service. To determine the original cause will mean examination of the damaged equipment and operating records to judge the sequence of events leading to the fire which may be obvious or require some time to work out.”
Dr Robin Preece, Reader in Future Power Systems, University of Manchester, said:
“An electrical substation takes electricity from the transmission grid (the big pylons) at very high voltage and transforms this to lower voltages so that it can be distributed through cables to homes and businesses. This is done using transformers which are filled with oil. The oil is a very good electrical insulator and is needed to stop the electricity sparking to places it’s not meant to go inside the transformer while it is operating during normal conditions.
“If there is a catastrophic failure of a component, this can cause a big electrical spark or flashover which is extremely hot and which can cause nearby things to catch fire. Unfortunately for a transformer, this can lead to the oil catching fire – which is why the fire can be so large and difficult to control. The whole substation would probably need to be switched off and de-energised in order to let firefighters safely deal with the blaze. You cannot fight a fire in an electrical substation that is energised. Certainly all of the equipment adjacent to the transformer on fire would need to be de-energised.
“Fires like this are not common at all, as equipment is monitored to check its health and the equipment gets replaced once it starts showing signs of age that mean it could be dangerous (for obvious safety reasons). Until the reports are completed by the equipment owners, it’s not possible to know the cause of the failure and the fire.
“The electricity grid will have sufficient alternative supplies to get pretty much everyone back on supply very quickly. There is a lot of redundancy built into the grid. The fact that the transformer was on fire would have led to a larger amount of disruption (due to the need to shut down the whole substation to fight the fire) compared to if it just broke down without catching fire – in which case you might never have noticed anything as we have sufficient alternative routes for the electricity to follow.
“This substation is one of many that supplies Heathrow, but it is not the only substation that provides power to Heathrow. I do not know, but perhaps it supplies some of the more critical services at the airport.
“To restore power, there will be some switching happening across the network to allow the electricity to find alternative paths to get to affected areas. We don’t run the system with all of these areas connected at once, as it helps to isolate problems to specific areas when issues like this happen. So the engineers will be reconfiguring the network so that the faulty bit is isolated, and to get the rest of the network reconnected to healthy parts again. A lot of this will be done using remote switching from a control room.”
Dr Guy Gratton, Associate Professor of Aviation and the Environment, Cranfield University, said:
“All airline flights must declare their destination and one or two “alternates”, which are airports they will go to in the event of problems at their destination. The UK has many large airports and flights have been diverting from Heathrow to airports at least as far away as Manchester. Alternates don’t have to be in the UK so for example Frankfurt or Schiphol may be used. With such a major airport closing however, there is significant risk of running out of capacity at those places.
“The absolute priority will of course be safety, with convenience to the passengers inevitably secondary to that. This is inevitably going to lead to significant disruption and frustrations.”
Dr Komal Raj Aryal, Lecturer in Crisis and Disaster Management, Aston University, said:
“This morning’s unprecedented incident at Heathrow Airport has forced us to urgently reconsider how we teach and govern risk management for operational continuity.
“The incident highlights how a localised disruption to one of the UK’s critical infrastructures can trigger cascading impacts across the globe.”
Declared interests
Prof Martin Kuball: “No conflicts of interest.”
Dr Conor Murphy: “I, Conor Murphy, declare the following interests which may be relevant to this commentary:
Current Position: Vice President of Engineering at NovoGrid, a company specialising in grid optimisation technology.
Academic Background: PhD Electrical Engineering from University College Dublin (UCD).
Professional Affiliation: Chartered Engineer with Engineers Ireland.
Previous Employment: Experienced network planner with ESB Networks, the distribution system operator in Ireland.
These roles and experiences provide me with relevant expertise in electrical grid systems and infrastructure. However, they may also be perceived as potential sources of bias, particularly my current position at NovoGrid and past employment with ESB Networks.
While I have no direct involvement with the Heathrow Airport power infrastructure or the specific substation in question, my professional background in the energy sector could influence my perspective on this incident.
I have not received any industry funding for research related to this specific event, nor do I have any current advisory roles or committee memberships directly related to Heathrow Airport or its power supply.
My comments are based on publicly available information and my professional expertise.”
Prof Chenghong Gu: “No interests to declare.”
Prof David Flynn: “I am member of the Transit and DARe hub leadership teams;
I am an independent advisor to SP Energy Networks, and;
I am an independent advisor to Cadent Ltd.”
Dr Malte Jansen: “Malte also works at UCL as Research Director of the Centre for Net Zero Market Design. NationalGrid is one of their philanthropic donors to the Centre for Net Zero Market Design, although he doesn’t benefit personally in any way.”
Graham Kenyon: “To the best of my knowledge, the only potential conflict of interest is that I worked for a company, Ultra Electronics Airport Systems, that provided specialist systems to Heathrow Airport, including for Terminal 5. Some of those systems involved control systems to link Heathrow’s central control room to electrical substations and other utilities in and around the airport.
This employment, including information that I worked on systems at Heathrow, along with the national standards committees I have been involved with, is openly and publicly available through my LinkedIn profile: https://www.linkedin.com/in/ghkenyon/.”
Prof A. Manu Haddad: “Most of our funded research in the AHIVE group at Cardiff University is supported by National Grid through strategic partnership since 2005.”
Dr Barry Hayes: “I am independent academic and I am not funded by the electrical power industry. I have an active research collaboration with ESB Networks, and I am an academic member of ESB Networks’ Innovation Stakeholder Panel, but this does not cause any conflict of interest in relation to the comments above.”
Mark Coles: “None.”
Prof Richard Dawson: “I am on the Climate Change Committee. The recommendation for more joined up govt approach is also in the Joint Committee on the National Security Strategy review which I gave evidence to:
https://publications.parliament.uk/pa/jt5803/jtselect/jtnatsec/132/report.html#heading-3”
Prof Rory Hadden: “No conflicts on this issue.”
Dr Paul Cuffe: “I have no direct links with the electricity industry in the UK. As an Irish academic, I have had occasional collaborations with Eirgrid, our transmission system operator, and ESB Networks, our distribution network operator. I have received funding as part the ESIPP and NexSys projects; these were co-funded by stakeholders in the Irish energy sector.
Paul Cuffe received B.E. and Ph.D. degrees in Electrical Engineering from University College Dublin in 2009 and 2013, respectively. He is currently an assistant professor within the School of Electrical & Electronic Engineering, and is a member of the UCD Energy Institute. His core research interests are in the optimization and analysis of electrical energy systems.”
Prof John Loughhead: “No conflicts of interest.”
Dr Robin Preece: “I have industry funding from National Grid Electricity Transmission, and NESO. Numerous industrial partners also support the Supergen Energy Networks Hub (which I am deputy Director of). I previously acted as a technical panel member for Northern Powergrid. I was sponsored by EDF Energy Networks during my undergraduate degree (2005-2009) and National Grid Electricity Transmission during my PhD (2009-2013).”
Dr Guy Gratton: “No conflicts to declare.”
Mark Aizlewood: No conflicts to declare.
Stuart Mortimore: No conflicts to declare.
For all other experts, no reply to our request for DOIs was received.