Heathrow substation fire raises important issues about electricity infrastructure

The recent fire which broke out at the North Hyde substation (SST) in Hayes, west London received significant publicity over the resulting blackout at Heathrow airport and ‘global’ pandemonium caused to flight schedules. However, the significant power outage also cut supplies to over 60,000 homes and businesses. A considerable number of people had to be evacuated from their premises although there were no reports of serious injuries or fatalities from the fire.

Of three transformers (TFs) on the site, one was completely destroyed by the fire, a second (adjacent) TF suffered heat damage, and a third had to be de-activated while the fire was brought under control. Police counter-terrorism units and other industry investigators have to date said that the speculation about criminal or terrorist intentions behind the fire appears to have been unfounded. Instead, it has been suggested that a failure within a high-voltage oil-filled TF likely resulted in electrical arcing (where electricity passes through the air-space between two conducting points), which ignited the TFs cooling oil, leading to a large fire. 

What are substations and why are they important?

It is more efficient for electricity to be transmitted over long distances within the UK at high voltages, but the domestic/commercial voltages distributed to homes and businesses are set much lower for safety reasons. SSTs enable the electricity to be transmitted at different voltages, securely and reliably. SSTs convert electricity to different voltages, stepping it up or down through the use of transformers located within a substation’s site. Electrical energy is ‘transformed’ by means of a magnetic field, alternating between coil units within the transformer. 

There are usually two classes of SST: those within the transmission network (which operate at high voltage) and those within the distribution network (which operate at low voltage). The SST which affected Heathrow airport was a transmission SST powered directly by National Grid, of which there are approximately 300 across the country. Most housing developments will be powered by distribution SSTs which tend to be smaller and hold less oil on site. 

Causes of SST/TF fires or explosions 

The likelihood of a TF exploding and/or burning is fairly low, but it can and does happen due to one or more of the following:  

• Electrical failures and overloads are among the leading causes of TF fires, stemming from a variety of factors, including aging equipment, inadequate maintenance, or sudden demand surges. If a TF is subjected to currents beyond its design capacity, it can overheat, which may in turn cause the insulating oil to break down and ignite. Arcing is also an issue in this instance. Relay protection failures causing excessive current flow and faulty switchgear or circuit breakers can also lead to TF fires.
• Mechanical failures and/or insulation breakdowns due to deterioration in physical components over time and/or due to inadequate maintenance/inspection. Bolts, wires, and connections can wear out or become loose, leading to internal damage. Insulation materials, such as paper and oil, can degrade due to thermal stress, moisture ingress, or contamination. When insulation fails, it can result in short circuits or arcing, which may quickly escalate into a fire. 
• Mistakes during maintenance, improper handling of equipment, or accidental damage to TF components can also all lead to fires, as can deliberate acts of sabotage.
• Environmental conditions such as extreme weather, lightning, or flooding can compromise TF integrity and trigger fires. Lightning strikes can cause sudden voltage surges that overwhelm the TF’s protective systems, potentially igniting the insulating oil. Flood water ingress to a TF can cause electrical shorts leading to an explosion. Strong winds can cause trees to fall causing damage to TFs 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.

Was the Heathrow fire an isolated incident?

At 12:18am on September 15, 2021, an electrical fire broke out at the Sellindge Interconnexion France-Angleterre Interconnector site in Kent, a joint venture between National Grid and its French counterpart, RTE providing a sub-sea electricity link between France and the UK. 

Examples of other SST fires include, in February 2018, a fire at a substation in Upton Warren, Worcestershire, which reportedly left a man in hospital and cut power to some 5,000 homes. In March 2018, firefighters attended a fire at a SST in Saltburn, North Yorkshire, leaving around 22,000 homes without power and in April 2018 a fire at a SST in Leicester’s city centre caused a power failure affecting 45,000 homes and businesses.  

Mitigating the risks

All SSTs and their owners/operators are subject to strict safety standards and regulations, including the Electricity Safety, Quality and Continuity Regulations 2002. These include ensuring that buildings/live equipment are properly/suitably enclosed; provision of visible safety signage; and that all reasonable precautions are taken to minimise the risk of fire associated with the infrastructure. As with other fire and building safety regimes, in the context of SSTs: 

• Utility companies will employ their own expert teams and/or engage the services of professional fire safety consultants, who will monitor risks and undertake regular fire safety reports;
• Passive fire protection such as fire alarms, extinguishers, blast protection and automated doors/other compartmentalisation protect key structural parts of a building from failure and stop/delay fire (from) reaching critical assets or vessels within a wider area;
• Performance testing and maintaining information from the supplier or manufacturer of the passive fire protection products and systems installed is vital, and also enables an operator to demonstrate that it is meeting the required safety thresholds. 

Is the British electricity infrastructure aging?

As in most other developed nations, ‘legacy’ electricity infrastructure in the UK can be in need of updating. Also, newer, renewable energy generation infrastructure is often built in areas without existing electricity infrastructure which requires costly new grid connections - so there is competition for prioritising the resources for these aspects. 

The National Infrastructure Commission has stated that 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. Typical distribution TFs in the UK are now over 60 years old, which is outside of their intended design lifespan.

Normally TFs that are in close proximity would have blast walls between them, but this does not appear to have been the case at North Hyde (the ‘Heathrow’ TF site), likely because when it was constructed, rules requiring blast walls were not in place. We understand that older SST sites are not currently subject to retrospective requirements or remediation programmes to ensure that there is consistency in this regard.

Housing context

New developments will often require SST upgrades/new installations to facilitate the electricity supply to the residential (and in some cases) commercial premises. This will usually involve granting easements to the electricity company (and district network operator, or DNO) and leasing or transferring the SST site to them in some instances.  

The electricity company and DNO must comply with their statutory obligations relating to minimising fire risk and ensuring the safety of the SST site. However we would always recommend to clients to include express covenants relating to the maintenance and repair of the SST (and any ancillary equipment) in the appropriate easement deed, transfer or lease, and the forms of documentation often used by the utility companies ought to provide for this.

Closing thoughts

Since the fire in Sellindge, upgrades at this strategically important facility have meant that the site now benefits from a comprehensive fire protection system aimed at ensuring protection from both the spread of fire and the risk it could pose to the building’s structural integrity.

It is reported that 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 enable Heathrow to operate off grid for a day or two. 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 (Scottish and Southern Electricity Networks) 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 on-site energy generation should not be difficult.  Installation of a gas-fired power plant may be a logical choice, but this would unlikely be a net-zero solution.

The incident was seen by some as embarrassing for the UK and that the Heathrow blackout after the substation fire highlights our infrastructure vulnerability across the country. It also points towards a tension between economic growth objectives and the promotion of net-zero. It will be interesting to follow what approach is taken by politicians and the energy industry, both in the context of one of the world’s busiest airports, but also across the wider network.