Heathrow fire

Just my personal opinion regarding the fire at the sub-station that closed Heathrow Airport on Friday.
A transformer went on fire. That basically means it overloaded, the usual safety mechanism, a tripswitch, did not operate so it exploded. As it was bathed in 25,000 litres (6,000 gallons) of cooling oil, this caused that massive fire. 

Many moons ago, I worked in a lab that tested cooling oils. My Dutch readers will be familiar with KEMA. The test involved relative density and conductivity to electrical current. Those who lived closer to the lab than I did at the time (I was 6 miles away) will have been familiar with bangs, resulting from transformers being tested to and beyond their safety limits - to the point where they blew up. 
 
The ramifications at Heathrow were world-wide, and everybody is in a flap over resilience, back-up systems and all the rest. Just check your equipment on a regular basis. Maybe such a simple thing as a bird doing a poo on a switch - and it gets stuck.