…for a week.
For those that don’t know, a big generator spins in hydrogen as there is less friction.
To do certain generator maintenance that hydrogen must be removed. This is done by pumping out the hydrogen and replacing it with nitrogen.
This is because nitrogen and hydrogen are a safe mix, and air and nitrogen mix are also safe.
However, air ( oxygen-rich) and hydrogen are not to be mixed within the generator casing.
If there is a spark there will be a loud popping noise.
In this case, it woke people living 20 km away.
The end caps of the generator were emancipated, and the power station roof for that unit was blown off.
This happened at night and thankfully no one was physically hurt, barring, of course, the nightshift staff that were treated for shock.