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Disk drive fired 'Frisbees of death' across data centre after storage admin crossed his wires

Paul Hovnanian Silver badge

Many years ago, I worked for the local power company as a distribution engineer. At one point, we had to take a local substation out of service for maintenance. In order to feed local circuits, we ordered a truck-mounted temporary substation to be connected to the circuits. I decided to stop by and watch as the temporary station was switched on line.

The truck was delivered and the techs dutifully checked jumpers so that the transformer was properly set to step down 66 kV to 12.5 kV. Unfortunately, the input to the transformer was protected by stacks of three 25 kV surge arresters, connected in series. The previous use of this portable unit was to step 12.5 kV down to 4 kV. And part of that installation had been to bypass two of the three arresters on each phase. Leaving only one rated at 25 kV.

The lineman who was assigned the duty of closing the primary side switch opened a panel on the side of the truck, which swung upwards. He was a pretty tall guy and had to duck down under the opened panel to reach the switch handle. And he made certain that his displeasure with this setup was heard. Until he threw the switch.

A fireball erupted out the top of the temporary sub as the arresters failed. Surge aresters are designed to bypass relatively brief lightning strikes with lower levels of total energy. Not to bypass a continuous 66 kV feed. The guy who threw the switch was (fortunately) protected by that nuisance of a steel panel over the top of his head. The fault was cleared a few seconds later by a breaker in the transmission yard feeding that circuit some miles away.

After the power had evidently been cut, the crew reversed their hasty retreat and slowly moved toward the substation with the idea of putting out the fires. Until the foreman yelled, "Wait for the reclose!"

Transmission line circuit breakers are often programmed to attempt to re-energize a line in the event that the fault was a tree branch bumping the line. But in this case, we were treated to a second fireball and more porcelain fragments.

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