Fusible Plugs
Hello PofD
The fusible plugs let steam into the firebox when they melt - not to douse the fire but to provide warning to the crew. They were there to help protect the firebox from overheating and collapse if it became uncovered. So fusible plugs are for the protection of the firebox and safety valves are to protect the boiler.
A very good current source is How Steam Locomotives Really Work - Semmens and Goldfinch Oxford Press ISBN 019 860782 2. A good anorak's read!
To quote the story of the French Railways accident:
"The magnitude of a locomotive boiler explosion was well demonstrated in 1935, when the crew of a PLM 2-8-2 working a Geneva-Paris express up a 15 mile bank allowed the water level to fall and uncover the top of the inner firebox. This overheated, losing its ability to withstand pressure and the crown sheet burst downwards, discharging the contents of the boiler through a tear in the plates, which had an area of 17sqft. The whole boiler was ripped from the frames and the cab, killing the crew and causing the brake to be automatically applied as the air pipes were severed. This brought the train to a halt with the rest of the locomotive still on the track and attached to its coaches. Fortunately the train was going around a curve at the time and the boiler's trajectory sent it off unto the line side fields. It first hit the ground, firebox first, 272 ft from the point of explosion, after clearing a telegraph line and a row of trees in the process. It then cartwheeled three more times, finally finishing up 512 ft from where it had taken off"
The inquiry found that the fusible plugs had been blowing for 9 mins prior to the explosion after the water level dropped too low.