Late on Thursday night a fire broke out at the North Hyde substation in Hayes, west London causing a blackout at Heathrow airport as well as thousands of homes and businesses. Of three transformers on the site, one was completely destroyed by the fire, a second, adjacent transformer suffered heat damage, and a third had to be de-activated while the fire was brought under control.
The incident caused a local power outage which cut supplies to over 60,000 homes and businesses including nearby Heathrow Airport. The airport was without power for most of the day, disrupting thousands of flights and having a global impact as incoming flights were diverted and outgoing flights were cancelled. Thankfully there were no reports of serious injuries or fatalities.
What do we know happened?
According to the London Fire Brigade (“LFB”), at 23:23 on the evening of 20 March 2025, the LFB received the first of 212 calls to a fire in a high voltage substation near Heathrow Airport. The fire involved a transformer with 25,000 litres of its cooling oil fully alight.
This created a major hazard as the high voltage equipment was still live, and the difficult nature of oil fuelled fires. The first appliance was on scene within five minutes of mobilisation, and at 00:42 the Metropolitan Police declared a major incident.
In all, ten fire engines, two bulk foam units and one high volume pump were on scene at the peak of the incident, equating to approximately 70 LFB personnel.
29 people were evacuated from neighbouring properties and a precautionary 200-metre cordon was established. Around 150 people were evacuated to a rest centre.
Later updates noted that the oil was covered by a foam layer which was reducing the temperature of the fire:
“This is one of those types of fire that burns for a significant period of time, because what we have to do is cool that down, and it’s the nature of the cooling agent or the oil that was on fire. And that takes time to bring that under control,”
– Jonathan Smith, deputy commissioner, London Fire Brigade
16 hours after the fire brigade received the first call alerting them to the incident, the deputy commissioner said that approximately 5% remained alight in isolated hotspots.
Beyond that, we know very little about the incident. News channels provided wall-to-wall coverage containing a great deal of speculation but little concrete information. It is thought that this was an accident rather than a criminal or terrorist act, but this has yet to be confirmed. It took many hours to fully extinguish the fire – the exact time this happened has not been made public – and make the site safe for inspections to begin.
What are substations and why are they important?
Substations are integral to power grids and enable electricity to be transmitted at different voltages, securely and reliably. One of their primary roles is to convert electricity to different voltages. Voltage is stepped up or down through the use of transformers, which sit within a substation’s site.
Electricity is transmitted over long distances at high voltages (400 kV for the main backbone and 275 kV in older parts of the network) because this results in lower line losses, but the voltages used in homes and businesses are much lower for safety reasons (eg 230 V in the UK and 120 V in the US).
Transformers transfer electrical energy by means of a changing magnetic field. They consist of two or more coils of wire each wrapped around a metallic core – the difference in the number of times each coil wraps around its core determines the change in voltage.
There are two classes of substation: those that form part of the transmission network (which operates at 275 kV and above) and those that form part of the distribution network (which operates at 132 kV and below).
Transmission substations are found where electricity enters or leaves the transmission network and are the “junctions” where different circuits connect to each other. The North Hyde substation is a point where electricity leaves the transmission system ie is a “grid supply point” or “GSP”.
Where electricity leaves the transmission network a GSP substation steps the voltage down for safe onward distribution – often to an adjacent distribution substation.
Causes of transformer fires or explosions
The likelihood of a transformer exploding and then burning is fairly low, but it can and does happen. Often such fires are the result of a complex interplay of factors that compromise the integrity and functionality of the substation. The main causes of substation fires are:
Transformer failures
Mechanical failures and insulation breakdowns represent a significant cause of transformer fires. Over time the physical components of a transformer can deteriorate, and if they are not adequately maintained, create a fire risk. Bolts, wires, and connections can wear out or come loose, leading to internal damage. Insulation materials, such as paper and oil, play a crucial role in preventing electrical arcing and maintaining proper transformer function, but these can degrade due to factors like thermal stress, moisture ingress, or contamination. When insulation fails, it can result in short circuits or arcing, which may quickly escalate into a fire. Poorly maintained cooling systems can result in leaks of flammable oil.
Electrical faults
Electrical failures and overloads are among the leading causes of transformer fires. These issues can stem from a variety of factors, including aging equipment, inadequate maintenance, or sudden surges in demand. When a transformer is subjected to currents beyond its design capacity, it can overheat, which may in turn cause the insulating oil to break down and ignite.
Additionally, short circuits or internal arcing – where electricity jumps across a gap in the conductor – can create intense heat and spark a fire. Relay protection failures causing excessive current flow and faulty switchgear or circuit breakers can also lead to transformer fires.
External factors and human error
Environmental conditions such as extreme weather, lightning, or flooding can compromise transformer integrity and trigger fires. Lightning strikes can cause sudden voltage surges that overwhelm the transformer’s protective systems, potentially igniting the insulating oil. If flood water gets inside a transformer, it can cause electrical shorts leading to an explosion. Strong winds can cause trees to fall causing damage to transformers or their power lines. Very hot weather can cause overheating, while extreme cold can make insulation brittle and prone to cracking. Animal interference such as bird or rodent activity can create short circuits.
Mistakes during maintenance, improper handling of equipment, or accidental damage to transformer components can all lead to fires. As can deliberate acts of sabotage.
Transformers can explode due to internal failures, most often caused by short circuits. These faults generally occur when insulation breaks down – insulation is designed to keep electrical parts separate. If it wears out or is damaged, electricity can jump, creating a short circuit, and directing current in unintended ways. This can create a heat build-up inside the transformer, leading to a break-down of the cooling oil. When this oil becomes too hot, it gasifies. The gas can build up very quickly, raising the pressure inside the transformer tank above its operational tolerance, at which point it explodes. Most transformers have safety features to prevent explosions, but if the failure happens too quickly, there may not be enough time for these protections to act.
British electricity infrastructure is aging
Legacy electricity infrastructure in the UK as in most other developed nations, is reasonably old and in need of updating. This is a challenge that is coinciding with the need to build out extensive new infrastructure to connect renewable generation which is often built in areas without existing electricity infrastructure.
According to the National Infrastructure Commission, investment in electricity networks has not kept up with demand. Its recent assessment found that over 20% of the UK’s critical infrastructure is operating beyond its intended design life.
NGET reported in its RIIO-ET2 Business Plan Submission that it had obtained an extension of transmission transformer lives from 60 to 65 years. Typical distribution transformers in the UK are now over 60 years old, far exceeding their design lifespan.
Normally transformers that are in close proximity would have blast walls between them but this was not the case at North Hyde as can be seen from both this satellite image of the site and the images of the fire such as the one shown above.
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The absence of a separating wall means that since the central transformer was destroyed, the one nearest to it is also likely to have suffered damage eg due to the heat from the fire. A commentator on a Reddit forum who may or may not be reliable, said that the transformers at the substation were built in 1969, but that regulations for walls between transformers were introduced in 1970 and a wall was never built in retrospect.
Why did Heathrow airport have to close for so long?
Heathrow lost power when the fire broke out and closed. Some flights resumed at 7pm on 21 March but the airport did not resume normal operations until Saturday 22 March.
Thomas Woldbye, the CEO of Heathrow Airport told GB News (the segment begins with an interview with me at 2hr 24 min) that the backup systems and crisis management worked the way they should, but the backup supplies are not sized to run the entire airport. In fact he stressed that everything worked as it should. He described the incident as having “major severity” where the airport lost power equivalent to a mid-sized city. Each substation has a backup transformer and in this case that also failed.
The power supply had to be re-allocate from North Hyde to the other substations, which involved shutting down all systems, re-configuring them to run from the other substations, testing the new configuration and then re-start all the systems. As the airport has thousands of electrical systems, this takes a long time.
Woldbye described the incident was “as big as it gets” for the airport short of anyone getting hurt, and insisted that they cannot implement contingencies for everything.
There has been widespread criticism of the lack of resilience at the airport, and a lot of confusion about “backup” with Ed Miliband suggesting early on that backup generation had failed. There is no comprehensive public information about Heathrow’s electrical systems, but this is what I have been able to find…
Electrical supply
The distribution network around Heathrow is operated by Scottish & Southern Electricity Networks (”SSEN”). The airport is supplied by three substations. Clearly one of these is the North Hyde substation which experienced the transformer fire.
The North Hyde GSP network is made up of 66 kV, 22 kV, 11 kV, 6.6 kV, and low voltage circuits. It is an urban network located in west London where the land use is a mix of residential, commercial, and industrial with no agricultural land. There are two sites where the SSEN distribution network connects to Heathrow Airport within the GSP area. From the map this appears to be North Hyde directly and East Bedfont to the southeast of the airport.
“We have three of these substations, each of them has a back-up transformer. The back-up transformer in this case also went and then we had to restructure the supply. So we’re not out of power but we have to restructure our power supply. To do that we have to close down systems – that is safety procedure, we will not go around that…Two substations can run the airport but we need to re-engineer the structure of the power supply for all the terminals and that’s what we were doing during the day, and then we have to restart all the systems and that’s what we’ve done, and we now see operation coming back,”
– Thomas Woldbye, CEO, Heathrow Airport
It is unclear which the third substation is. Possibly Longford (132 kV) to the west or Staines (132 kV) and Laleham (275 kV) to the south although these are further away. However, if East Bedfont is supplied only by North Hyde there could be difficulties in maintaining supplies to the airport through the site. This means that either there is a fourth substation involved, which no-one has mentioned, or that East Bedfont is supplied by a second GSP, or that there was a belief from the outset that supplies would be restored to East Bedfont using the undamaged transformer at North Hyde once the substation could be re-energised after the fire. There is simply no information available about this.
What is clear is that when the airport talks about having “backup supplies” it means the ability to power the airport through substations other than North Hyde which is typically used, and that taking hours to activate this backup is within its business continuity plan. However some experts disagree and describe “several construction and engineering failures” which “likely contributed to the outage”:
- Power distribution pathways: modern critical infrastructure should have physically separated power supply routes, so if one cable trench is damaged, alternative pathways remain intact. The complete shutdown suggests Heathrow may have had insufficient separation of these critical pathways;
- Automatic transfer switches: these should seamlessly transition from the main power supply to backup systems. These are sophisticated components that require rigorous maintenance and testing – even a minor malfunction can prevent proper failover;
- Uninterruptible power supply systems: these systems should maintain power during the crucial seconds between main power loss and switching to alternative supplies. Battery-based systems are designed specifically to bridge this gap, preventing even momentary outages from affecting critical systems.
“What’s particularly troubling from an engineering standpoint is that these systems represent well-established technology. We’re not talking about experimental approaches or cutting-edge solutions that might reasonably experience teething problems. These are mature technologies that should function reliably,”
– Thomas Oldham, UK Construction Blog
In 2013, a consulting company prepared an assessment of Heathrow’s operational risks, identifying the electrical supply configuration as a key concern, and in particular “the transmission line connections to the airport”.
While the report noted that Heathrow’s backup systems “appear” to be compliant with relevant regulations and that its provision of “onsite generation and other measures” “appear” to be adequate to enable the expanded airport to withstand and recover from power supply interruptions, none of this means that the airport could maintain continuous operations in the event of a power supply failure.
All it really means is that it complies with regulations which are likely to relate to critical safety systems, rather than that it can maintain full site operation.
In addition, the distribution network in the area is becoming congested, which adds to the risks at the airport. SSEN has a major upgrade programme centred around the North Hyde substation.
A 2022 report for the London Assembly noted that the electricity grids in the Heathrow area have little or no available headroom to accommodate demand growth and that the North Hyde substation had recorded usage at 80.7 MVA versus a nameplate capacity of 76 MVA. Peak utilisation in 2021-22 was 106.2%. Other substations in the GSP operated at 94.7%, 89.1% and 86.2% of capacity.
Backup generation
Heathrow does have some on-site generation. There is a 10 MW (2 MWe) wood pellet biomass combined heat and power (“CHP”) plant and as of 2022 there was a small amount of diesel generation for powering runway lights and the control tower. There is no information to confirm either that these generators have been replaced although Heathrow would like to switch to a greener source of backup generation, or that diesel generators were removed when the biomass plant was opened.
There is some gossip that there may have been gas generation that was removed – a poster on Reddit said his father used to work for Heathrow as an auditor and once audited the disaster recovery procedures including those applicable after a power outage. The poster alleged that Heathrow replaced an entire gas-fired power station with reliance on the grid. He also said that managers had told his father that the airport relies on three different substations at different points of the compass, and that if one failed, they could get the necessary power from the other two. This is unsubstantiated gossip – hopefully the investigations will uncover whether any large fossil fuelled backup plant was removed at any time, and if so, whether this was because it was obsolete or because of sustainability and net zero concerns.
There has been widespread speculation that Heathrow’s lack of reliable onsite backup generation sufficient to power the entire site was a result of these net zero concerns. It is definitely true that Heathrow has repeatedly stated an intention of reducing its use of diesel and has for over a decade had a strong focus on sustainability. The installation of the biomass plant was part of the sustainability strategy (and possibly an attempt to secure subsidies).
The biomass plant was unable to operate during the power outage. It is only designed to supply heating and air conditioning to terminals 2 and 5 and needs to be connected to the grid in order to work.
The diesel generators are designed to maintain power to key safety systems in the event of an electricity supply interruption, so that aeroplanes in the final stages of landing are kept safe. Otherwise, they would lose ILS, runway lights and contact with the tower, which would impose a high risk burden on both cockpit crew and controllers at the National Air Traffic Control centre who would have to instantly take over contact with many flights at the same time, some of which could be in the final stages of their approach. These diesel generators power critical safety systems for long enough to see incoming fights on final approach safely on the ground, or safely diverted to other airports.
Could or should Heathrow have full onsite power resilience?
Heathrow’s peak electrical demand is probably in the 40-60 MW range. In 2023 the airport consumed approximately 271,080 MWh of grid electricity.
Certainly infrastructure with smaller loads has onsite generation able to provide backup. Various people noted that datacentres in west London did not go dark during the power cut. This is because they have backup generators, for example the London4 datacentre in Slough which has a load of 19 MW and two diesel generators that can run the site at full load for two days. Heathrow would need six similar generators to achieve similar redundancy, but this is something other large airports have installed.
A 2023 report to the United States Congress by the Government Accountability Office found that many (24) airports are able to power airport systems other than critical safety systems which in the US must operate for at least four hours after a power outage, such as operation centres and computer rooms, to maintain some level of terminal operations during an electrical power outage. For example, one medium hub airport reported that its generators can provide power to jet bridges to allow passengers to board or disembark from flights.
Seven US airports reported having enough generators to temporarily provide full power throughout a terminal in the event of an outage. For example, one large hub airport reported having 10 diesel fuel generators and enough fuel on site to power the entire airport for 3 weeks. Two airports reported investing in additional generators in response to an electrical power outage.
Another large hub airport has more than one substation and has installed a device that automatically switches power to an alternate substation in the event one of the substations loses power. One medium hub airport described working with its utility provider to obtain a dedicated power line that would only serve the airport, and that it planned to add additional power lines to provide an adequate power supply for future airport upgrades.
Several US airports including JFK in New York have installed CHP plant to improve electrical resilience and allow them to operate as micro-grids in the event of a power supply failure. The gas-fired CHP plant at JFK as a capacity of 110 MW and allows the airport to continue to operate in the event of a grid supply failure.
So the answer is yes, Heathrow could have enough onsite generation to power its operations if it chose to, and comparable airports elsewhere have installed such systems.
Will Heathrow be forced to think again?
The Heathrow airport shutdown is expected to cost the airline industry about £60 – £70 million which is probably not far away from what it would cost to install enough backup generation to allow Heathrow to operate off grid for a day or two. However to be a reliable backup, it would likely need to involve fossil fuel generation. Current emissions regulations limit the use of onsite diesel generation, but if the airport were to install such a large amount of backup, not only would it want to regularly test it, it would want to use it to optimise its operations and potentially provide electricity and other grid support services back to the grid.
As noted above, the west London area is running short of electricity capacity, so the ability to run Heathrow with onsite generation could be of value to both National Grid and SSEN as they manage local grid congestion and the challenges associated with demand growth, not least from datacentres. Heathrow is currently served by aviation fuel pipelines, and the local area is covered by the natural gas network, so providing fuel for onsite generation should not be difficult. In every respect apart from net zero, installation of a gas-fired power plant may be the most efficient choice.
Of course, this would be opposed by environmental groups, and while a 60 MW gas plant at Heathrow is not going to make a material difference to climate change, activists would likely prefer the airport to close than to increase its onsite fossil fuel generation. Ironically, replacing the biomass plant with a gas plant would be better for the environment (Heathrow can always plant trees regardless of the fuel it uses) as gas power stations emit less carbon dioxide from their stacks than biomass plants do.
It will be interesting to see what politicians conclude. There has been a lot of talk about how embarrassing the incident has been for the UK, and how bad it is that a single substation fire can close one of the world’s largest airports for many hours. The Government is currently struggling with balancing a desire for economic growth with a desire to promote net zero, with battles over the expansion of Heathrow airport at the centre of these debates.
At the very least Heathrow needs to upgrade its control systems so it can switch from one grid supply to another quickly, with batteries and/or small diesel generators providing power during the switch over period (which should be short). But it should follow the example of major US airports and install gas-fired CHP that is configured to run in full island mode and can also export power back to the grid. And the Government should consider whether critical infrastructure such as Heathrow should have exemptions to environmental regulations to enable this to be done efficiently.
Otherwise our creaky electricity infrastructure could see other days when tens of millions of pounds are lost to grid outages.
Thank you for an excellent article.
It’s certainly troubling how basic safeguards, in this case a firewall, was not in place simply because it wasn’t required until 1970.
Do the people in charge not know the importance of reviewing potential danger spots?
They remind me of those in charge of the the disastrous Railtrack Plc, who were so incompetent, that they didn’t even have a display of their infrastructure, showing all the maintenance hotspots.
Given Ed Miluband’s plans to throw billions at a (thoroughly debunked) carbon capture scheme, and the money used to subsidise Drax, it seems ludicrous that that critical infrastructure has just been left to rot.
Just picking up on a minor point given the reference to Railtrack Even Network Rail (and Railtrack and BR before them) design to allow the AC electrified rail network to operate in the event of one of their 400kV supply transformers going bang. (Albeit they may need to reduce the service if they loose a whole substation (pair of transformers)).
Sensible of them, just a pity about some of the past maintenance
Thank you for posting this excellent and detailed report. Myself and many others are interested in learning more about the Heathrow outage. I like many of your readers have flown through Heathrow and it is in fact well known world wide. Your in depth reporting is appreciated.
Great article.
Is electrical capacity in the area now lower given the damage to North Hyde?
There are a few new datacentres in the area scheduled to come online later this year, i.e. Stockley Park. They’ve already had connection approval from SSEN, with the damaged equipment at North Hyde I wonder if overall capacity in the area is now reduced and some new datacentres will be delayed?
Next to CHP, why not instal solar capacity? Panels are cheaper than ever and even boxed-in Heathrow should have sufficient unused space available on building tops and along taxiways, perimeters etc to generate a large proportion of its power needs. There are examples of other commercial airports in the world that have gone full on solar. The installation cost of solar may be double that of a gas plant but the savings on gas and CO2 emissions should make it an alternative worth considering.
Solar is tricky at airports because the panels can cause glare that dazzles pilots.
Airports in Singapore, Brazil and India have installed solar photovoltaics. Newer photovoltaics technology have reduced glare
Notwithstanding this is an NG equipment failure the loss of an infeed is a totally credible failure scenario albeit low probability but high impact. Thus in any basic risk analysis it should have come high up there to threats to airport operations and thus it ought to have had a response. One would expect any safety critical system to be supplied with at least two independently sourced firm supplies as well as being battery backed 2-4hr UPS and by all accounts there is no suggestions that aircraft movements were compromised. The main problem appears that Terminals 2 & 4 were without power and despite Heathrow having two other infeeds with interconnection around the airport reconfiguration isn’t automated. So Heathrow are saying it took took them time to reconfigure their HV system and bring all the loads back online presumably escalators, baggage handling, lifts etc. Anyhow despite Miliband and Heidi Alexander (DfT) doing the usual waffle and spaffle on the media on Friday and using the spin of praising the emergency services (it is what they are there for!) to divert attention Miliband has now tasked NESO to do an investigation into it
https://www.gov.uk/government/news/neso-to-investigate-heathrow-power-loss
however that is directed at the NG fault although full terms of reference are to follow. The problem lies with Heathrow Airports Ltd resilience so if NESO aren’t tasked to look at that DfT need to tee up their own investigation if Heathrow wont publicly declare what happened and I’m sure they will have the powers to do so.
I do not believe that Heathrow does not have sufficient stand by generation capacity (Diesel or gas turbine) totogether with UPS to ensure all critical systems such as nav aids, radar, radio and the tower do not experience a loss of power at all. The UPS (Uninterruptible Power Supply) keeps the power on between a loss of mains and the starting and switching on line of the stand by generators.
The provison of such stand buy, and its regular testing must be a minimum, surely as an air safety requirement by law?
In the US the FAA requires airports to run ILS, runway lights, the control tower etc for a minimum of 4 hours after a supply outage. I could not find an equivalent rule in the UK but expect there is one. There is no indication that any of those systems failed at LHR
I agree that the failure of Heathrow to cope with the loss of (part of) their grid connection reflects badly on them, the country, and any justification for expansion. But I don’t understand the justification for a leap to a continuously running Gas fired plant. Any local storage or generation should fit within the national plan unless it is intended for relatively short periods, say a week maximum.
That Heathrow is in the wrong place is clear and given the generally accepted views on climate change it should not be expanding. Instead an alternative hub or distributed but linked smaller hubs should be sought.
Plenty of jet engines at Heathrow. Why not flash up a few of those and feed energy into the system?
For me, there are two almost completely seperate issues:
1. It seems that Heathrow was not engaged in any way in managing its power supply. It looks like the general view was “we get our supplies from the DNO and if they fail then never mind, we’ve got safety covered and we can just close the airport”. A more engaged organisation would have put in place the necessary internal distribution and switching arrangements (and supporting commercial arrangements with the DNO) to rearrange their demand across all the substations they were connected to. [ There is a sub question here which is, if those arrangements were in place could the DNO have accepted the load transfer, or would the DNO’s network have been too disrupted by the fire to cope. ] An even more engaged organisation would have on site generation to cover a significant proportion of the total load – in Heathrow’s case, they usually have vast stocks of fuel specially designed for gas turbines so they could achieve significant autonomy.
Whether Heathrows’s management took the right approach is ultimately a political question driven by how reliable we want our airports to be.
2. The distribution network had flaws, was towards the limit of its capacity and hadn’t been kept up to date with modern practise. That is a natural consequence of the constraints on investment which exist in the industry. Whilst I would obviously like DNO supplies to be ultra reliable, I am less excited by this than I am about point 1. If North Hyde hadn’t been supplying Heathrow, this would probably have been a local news story at best.
I don’t think that any decision making process was that sophisticated. I am an IT engineer and was part of many datacenter designs. My first thought was how Heathrow wasn’t designed to the same standard as any data centre. They didn’t ask the right questions! They didn’t think about separate path for incoming electricity, didn’t think about single point of failure. Didn’t kept up with modern technology about automatic failover. Take into consideration DEI initiatives, instead of meritocracy. It’s a sad story
Great Article.
I have no doubt that the transformer failure was down to complacency. Load monitoring and heat monitoring would be in place but if you save money by having only basic monitoring this is what you get when the basic fails through lack of inspection & maintenance. To allow the transformer to reach the temperature for the oil to combust (140°c) is negligence plain and simple.
On top of that, when you hear that Heathrow had access to other substations you start to see a pattern of negligence.
Investment in what was required I have no doubt was rejected so protect executive bonuses and shareholders.
A transformer of this size and even of that age will have been equipped with a Buchholz gas and surge monitoring device and will also have temperature monitoring devices that are reported back to NG control centre via telemetry. Furthermore if it developed an internal fault (which is most likely outcome) thee are overcuurent protection devices that would have activated to trip it off without human intervention. Now from what I’ve said it is surprising that its got to the point that its caught fire so investigation will have to look carefully at our the system operated and what were, if any, the actions of the operators.
You can see that the layout design was wrong with transformers being too close together from the outset.
I really don’t understand why anyone would have put them so close together and in any modern risk assessment that would have been called out. Given these transformers are quite old contingency review should have increased monitoring while a plan to move or replace one of the transformers was drawn up.
Meantime, Heathrow’s desktop contingency review instead of actual planned testing, led to 200,000 passengers being let down and £m in law suites and compensation. Hopefully heads will roll.
Unfortunately, the UK will take years to shake off this reputational damage.☹️
Both Heathrow itself and the electricity distribution network are regulated asset bases. As such any significant investment program (and operating costs) is a negotiation between the relevant regulator (CAA and OFGEM respectively) and the owners of the facility.
As infrastructure assets the owners will generally WANT TO SPEND AS MUCH MONEY AS POSSIBLE on capital investment – such as adding redundant systems – while the regulator will want to minimise it to reduce tariffs to consumers. So executive bonuses and shareholder returns LOVE any excuse to invest more money because their extra returns are guaranteed. They would want to gold plate it as much as humanly possible (with the added benefit of reducing the risk of exactly this kind of fiasco blowing back on them).
Meanwhile for cost cutting around opex, mostly those costs are just pass through to the tariff payers. The regulator designs the incentive schemes for cost efficiency and has oversight of how they are implemented. Granted it may be possible to game those such that lower costs from reduced maintenance are kept by the owner as an “efficiency” signed off by the regulator. In which case fault might be distributed between the owners and regulators (assuming no outright fraud).
RAB assets are very much a “you get what you pay for” space and regulators themselves have been under enormous pressure from the political establishment to keep tariff costs down for consumers, which has been a big driver in underinvestment in critical infrastructure. “We have met the enemy and he is us”.
Vernon, aircraft produce 115vAC, but at FOUR HUNDRED Hz!
Each terminal is divided in two, with supply from North Hyde and East Bedfont HV switching centres. Each computer room is powered simultaneously from substations on North & East, with ELV DC supplies strapped together via diodes, or AC changeover units with half a cycle switchover. Critical systems also have online battery UPS.
It seems to me that National Grid somehowdisconnected the south supply connection, which forced all dual loads onto North Hyde, which overloaded and triggered the fire.
This will get absolutely worse. The UK under mad Ed wants to spend £290 billion on renewables infrastructure, whilst they don’t want to update the 65 years old substations as you pointed out. They are not only old but run constantly near or over full capacity. We said many times that the distribution network is not able to cater for the growing demand, but this government wants to mandate heat pumps, EV’s, etc. We will see more fires like this.
The other issues is with the UK utilities companies that for long years haven’t invested in the infrastructure, even though the authorities granted them cc 8% profit each year. They too out loans on low interest rates, gambled on the bond market, to enrich their shareholders. These all badly backfired, after the BOE started to hike rates. They are all under water, all for us to bail them out.
Heathrow is fed from three GSPs, but different parts are fed from different GSPs and neither at the SSEN BSP level nor at the Heathrow HV level are those normally interconnected.
This is partially due to NGET not usually allowing their transmission system to be paralleled at lower voltage because it can lead to system control issues. Therefore these connections are all normally open.
Had they run in parallel, then the failure of North Hyde would not have led to power interruption.
It then seems that it required manual intervention to isolate the NE quadrant of the airport which is fed from North Hyde and close the normal open points to feed from the other two GSPs (I don’t know if Heathrow is directly transmission connected or if they’re fed from the BSPs instead but it doesn’t really matter for this) in order to re-route power.
However what then took most of Friday was to restart all the equipment such as the luggage system. I would suggest that the most cost effective way of preventing something like this happening is:
1) Ensure that the switching sequences required to restore power using alternative GSPs are automated and do not require manual intervention.
2) Ensure that equipment is able to restart after a power failure without manual intervention and checking. Virtually all domestic and commercial equipment can do this (e.g. your Sky Box just starts back up automatically after a power failure and restoration) but lots of industrial equipment doesn’t do this and if a factory loses power it may not be trivial to restart it. At the very least, entirely electronic systems, internal lights, etc should be easy to do this. Mechanical systems like luggage handling are more complex to ensure that they can recover in this way if they weren’t designed for it.
This would increase resilience without building a substantial new gas power plant which may be used once every few decades.
Seems Heathrows CEO has to appear before Transport Select Committee next week to explain himself. Heathrow have also got Ruth Kelly ex Transport Secretary in to lead an independent investigation into what happened and they responded.
NESO have to report back within six weeks. Quick by parliamentary timings but NG will already know the sequence of events from the SCADA systems but of course leave it long enough and the news cycle will have moved on.
John Sullivan is a former SSE engineer who worked in the area in the past. Here’s his initial view, composed before more facts emerged:
https://johnsullivan.substack.com/p/disinformation-and-the-heathrow-shutdown
I guess through old contacts he established the proximate cause of the trip:
https://threadreaderapp.com/thread/1903715776634269836.html
Today we have the he said/she said from Pettigrew at National Grid
https://www.energylivenews.com/2025/03/24/grid-boss-says-heathrow-did-have-power/
As John pointed out what happens onsite is the responsibility of HAL: they should have had an internal grid capable of switching between substations with no more flicker than from a lightning strike. This they did not have, quite evidently. I suspect they have been far too much tied up with their Net Zero plan, on which they are spending £23m for design:
https://www.heathrow.com/company/about-heathrow/heathrow-2-0-sustainability-strategy/our-carbon-strategy
You have to suspect that the day job of making the airport systems robust plays second fiddle. The same is probably true of National Grid, where Alice Delahunty who is part of the Irish harem promoted by Fintan Slye (now CEO of NESO) who was Head of Networks prior to being elevated to President of Transmission after the NESO/NG split holds primary responsibility for overloaded 60 year old transformers being in close proximity to each other and being secondary to pursuing Net Zero projects such as East Anglia Green.
https://www.nationalgrid.com/about-us/our-leadership-team/the-executive-committee/alice-delahunty
Miliband’s appointment of NESO to investigate is thus like appointing a fox as coroner at a chicken inquest. Slye authorised the approach at NGESO.
P.S. I have seen a network diagram that shows Laleham, fed from Weybridge 400kV substation has a bypass that feeds East Bedfont and North Hyde, while Langford is supplied from the 400kV station at Iver. There is a 275kV link between Iver and Weybridge that is part of the outer London ring.
Very informative explanations and thanks for the links
Thanks Kathryn for an excellent write-up
I have a question regarding datacentres and AVR: Do you know if NGET was using Automatic Voltage Regulation at North Hyde?
https://www.reuters.com/technology/big-techs-data-center-boom-poses-new-risk-us-grid-operators-2025-03-19/
In the US, apparently datacentres are causing turbulence* in the grid, by disconnecting and switching to (Battery/Diesel/Gas) backup en masse whenever the grid voltage goes slightly out of tolerance. (they say, to protect the ‘sensitive chips’ and their cooling systems).
I don’t know what their tolerances are, but it’s likely that they have both a lower voltage limit and an upper one, and they may be set similarly enough between DCs that a lot of DCs trip off simultaneously, which would cause a rise in grid voltage and frequency. If they are tripping on overvoltage, there could be a positive-feedback effect where a DC tripping to UPS causes the grid voltage to rise further, causing another DC to trip. If enough trip simultaneously, it is too much for the grid to cope with, and could cause some generators to be shut down.
Why is this relevant to Heathrow: One way to avoid voltage peaks and dips is through Automatic Voltage Regulation (AVR) in which a control system automatically adjusts an On-Load Tap Changer (OLTC) at a substation to tweak the voltage output. But the drawback of AVR is that tap-changing causes small internal arcs in the transformer, which if done too frequently will contaminate the oil over time, and potentially lead to a catastrophe like we saw last Thursday.
If NESO were having to use AVR at North Hyde substation to mitigate the risk of datacentres tripping, then one could perhaps place some of the blame for the incident on the excessive concentration of datacentres in the West London area. (They should be being built in Scotland, where there is usually an excess of electricity.)
Regional Pricing might nudge them in the right direction, but it is a blunt implement that would lead to massive price hikes for ordinary Londoners, and could perversely disincentivise investment in transmission infrastructure.. (If they can charge more for electricity in constrained areas, why spend money to alleviate those constraints?)
* turbulence, heathrow, geddit?