Heathrow Airport shut down by electrical substation fire: FAQs

Originally published 21 March 2025

Heathrow Airport in London closed all day on 21 March 2025 as a major power outage disrupted 270,000 journeys.

A fire broke out at the nearby North Hyde electrical substation in the early hours of the morning, damaging equipment and leading to the power cut. About 70 firefighters worked to put out a transformer that had gone up in flames.

It's believed this was the first time in 15 years that Heathrow Airport has been completely shut.

The following provides accessible answers to frequently asked questions about the incident.

What is an electrical substation?

A substation is a ‘stopping point’ for electricity on its journey from where it’s generated to where it will be used. They help manage this flow safely and efficiently.

To travel long distances, electricity has to be very strong, so its power doesn't get lost. Think of it like a really powerful horse charging around at a gallop. You can't ride that!

So, it needs to 'calm down’ before it can be used safely in our buildings - this is essentially what substations are for.

Why can incidents such as a fire cause significant outages?

Simply because of the amount of power we're dealing with.

Using the horse analogy again, if a galloping horse hits something, it's going to break probably itself and whatever it hits.

Meanwhile, a horse travelling at a more controlled pace is much less likely to do damage.

Do fires often break out at substations?

Fires at substations are very rare, because they are usually very well managed because electricity is so dangerous. That's why the public aren't allowed anywhere near them.

If a substation breaks down, then the strong and powerful electricity could cause a lot of damage.

Usually, high-voltage (really powerful) electricity can go up in flames when it short-circuits, or meets an obstacle.

This will cause further damage to the substation and prevent it from delivering electricity to users.

What are the causes of fires at substations?

Anything that stops the flow of electricity unexpectedly can cause an electrical fire.

It could be as simple as a tree branch, or a bird or animal getting into the substation and causing a short-circuit which can start a fire.

It could also be equipment failure, though monitoring equipment is used to mitigate these risks.

What substation equipment can be damaged by a fire?

The fire damaged transformers, devices that reduce or ‘calm’ the power voltage from very high to usable levels.

The cables that were short-circuited were likely damaged as well.

Is it normal for a substation fire to affect the running of an airport?

There are usually safeguards and spare power supplies for critical infrastructure like airports.

This can be in the form of back-up generators onsite, or a separate power supply that comes from a different place.

This creates resilience, so that if one supply breaks down, the lights stay on.

That Heathrow Airport could not continue operating was highly unusual and unprecedented.