London Heathrow Airport Shutdown

[toucana]

London Heathrow airport has been completely shut down today by a major fire in a nearby electricity distribution substation which also took out power to 62,000 local residents. Authorities say that it may take a number of days to restore power to the airport, and air travellers are being advised to avoid Heathrow completely until further notice.

https://www.bbc.co.uk/news/live/cly24zvvwxlt

The fire at the North Hyde electricity substation in Hayes began last night at 23:33 GMT with an explosion in a large transformer which set some 25,000 litres of cooling oil on fire, destroying both the transformer and a nearby backup generator. This is a nightmare scenario for firefighters, because the cooling oil is electrically conductive. You cannot fight a fire like this without first de-energising the entire substation.

One point that has already attracted the attention of power engineers is that a power distribution substation of this type has extensive monitoring equipment which is designed to detect dielectric stress or thermal overload. They are also equipped with a safety cutout known as Bucholz relay which detects the presence of gas in oil-filled transformers caused by dielectric failure. These relays quickly trigger alarms and disconnect the power feeds, and have been in use since 1940.

https://en.wikipedia.org/wiki/Buchholz_relay

In the case of a major civilian airport like Heathrow, critical systems like the ground approach radars used by the ATC tower are run on a ‘Reverse Standby’ system - meaning that those systems are normally powered at all times by special generator sets, and they only lapse back onto national grid power if the local generator sets fail. The reason why Heathrow was completely shut down was because of the large number of other systems such as check-in, ticketing, digital signage and passport control computer systems which aren’t protected by reverse standby arrangements - Terminals 2 and 4 still have no power according to latest reports.

[exchemist]

On 3/21/2025 at 12:00 PM, toucana said:

London Heathrow airport has been completely shut down today by a major fire in a nearby electricity distribution substation which also took out power to 62,000 local residents. Authorities say that it may take a number of days to restore power to the airport, and air travellers are being advised to avoid Heathrow completely until further notice.

https://www.bbc.co.uk/news/live/cly24zvvwxlt

The fire at the North Hyde electricity substation in Hayes began last night at 23:33 GMT with an explosion in a large transformer which set some 25,000 litres of cooling oil on fire, destroying both the transformer and a nearby backup generator. This is a nightmare scenario for firefighters, because the cooling oil is electrically conductive. You cannot fight a fire like this without first de-energising the entire substation.

One point that has already attracted the attention of power engineers is that a power distribution substation of this type has extensive monitoring equipment which is designed to detect dielectric stress or thermal overload. They are also equipped with a safety cutout known as Bucholz relay which detects the presence of gas in oil-filled transformers caused by dielectric failure. These relays quickly trigger alarms and disconnect the power feeds, and have been in use since 1940.

https://en.wikipedia.org/wiki/Buchholz_relay

In the case of a major civilian airport like Heathrow, critical systems like the ground approach radars used by the ATC tower are run on a ‘Reverse Standby’ system - meaning that those systems are normally powered at all times by special generator sets, and they only lapse back onto national grid power if the local generator sets fail. The reason why Heathrow was completely shut down was because of the large number of other systems such as check-in, ticketing, digital signage and passport control computer systems which aren’t protected by reverse standby arrangements - Terminals 2 and 4 still have no power according to latest reports.

The cooling oil is not electrically conductive. In fact its dielectric properties are a key part of the specification. But it is a hydrocarbon, fairly similar to gas oil (diesel fuel) in nature, so it will burn if ignited. It's just an oil fire, but obviously one needs to isolate the high voltage feed into the substation before anybody can get near it. 

There seem to be a lot grandstanding by politicians over this incident. Whether it is worth the considerable cost to arrange duplicate supply into the airport will need careful study. Something like this is a very rare occurrence. The airlines that are huffing and puffing about lost business and disruption will have to pay for any new supply system, in the form of higher landing fees. Let's see what they think when confronted with the numbers.  

 

 

[toucana]

36 minutes ago, exchemist said:

The cooling oil is not electrically conductive. In fact its dielectric properties are a key part of the specification. But it is a hydrocarbon, fairly similar to gas oil (diesel fuel) in nature, so it will burn if ignited. It's just an oil fire, but obviously one needs to isolate the high voltage feed into the substation before anybody can get near it. 

The mineral cooling oil in itself is an excellent dielectric medium as you say, but unfortunately that situation changes rather rapidly when it escapes, catches fire (it has a low flash-point) and is then mixed with with water and firefighting foams. Aqueous Film-Forming Foam (AFFF) which is designed for fighting Class B fires (flammable liquids) is positively lethal if applied to a Class C electrical fire, because the chemical foam concentrate is even more conductive than water.

Quote

Foam is Not Effective on Class C Electrical Fires
Class C fires involve energized electrical equipment; water conducts electricity. Since foam
contains 94-97% water, it is not safe for use on this type of fire. In some cases, foam concentrate
is even more conductive than water. Class C fires can be extinguished using nonconductive
extinguishing agents such as a dry chemical, carbon dioxide (CO2), or halon. The safest
procedure for this type of situation is to de-energize the equipment if possible and treat it as a
Class A (ordinary combustible material) or Class B (flammable/ combustible liquids) fire.

https://www.transcaer.com/sites/default/files/documents/Instructor-Manual-Module-6-FINAL.pdf

That is what I meant to convey.

[exchemist]

1 hour ago, toucana said:

The mineral cooling oil in itself is an excellent dielectric medium as you say, but unfortunately that situation changes rather rapidly when it escapes, catches fire (it has a low flash-point) and is then mixed with with water and firefighting foams. Aqueous Film-Forming Foam (AFFF) which is designed for fighting Class B fires (flammable liquids) is positively lethal if applied to a Class C electrical fire, because the chemical foam concentrate is even more conductive than water.

That is what I meant to convey.

OK but that’s the firefighters’ foam, which would ordinarily be the method of choice for putting oil an oil fire. (I’ve done that myself on a safety course). But, to be clear, electrical cooling oil itself is an  insulator, not a conductor.