During WW2, guards in the New York City subway system had a standing order to shoot on site anyone who came down a certain elevator holding a bucket of sand.
The subway is powered by DC rather than AC electricity. Today solid-state converters are used. But back then, the only option was direct mechanical conversion. Literally a giant AC motor connected to a DC generator. An AC motor was powered by the grid, and its drive shaft was joined to the shaft of a DC generator, which energized the lines of the subway.
But it represented a single point of failure. This one crucial machine was powering the whole subway system. And all it would take is one person throwing a single bucket of sand into the contraption to cause the whole NYC subway system to grind to a halt. Oh, and NYC was the primary port city for American troops and supplies going back and forth to Europe.
During WW2, guards in the New York City subway system had a standing order to shoot on site anyone who came down a certain elevator holding a bucket of sand.
The subway is powered by DC rather than AC electricity. Today solid-state converters are used. But back then, the only option was direct mechanical conversion. Literally a giant AC motor connected to a DC generator. An AC motor was powered by the grid, and its drive shaft was joined to the shaft of a DC generator, which energized the lines of the subway.
But it represented a single point of failure. This one crucial machine was powering the whole subway system. And all it would take is one person throwing a single bucket of sand into the contraption to cause the whole NYC subway system to grind to a halt. Oh, and NYC was the primary port city for American troops and supplies going back and forth to Europe.