What happened to the overload device?

That must have taken some explaining, earth faults and phase to phase faults really should have operated a breaker somewhere.

I presume there was an un-detected fault or serious configuration error with the fault detection.


What this film very clearly show is how the energy to  maintain an arc is more important than the voltage. The insulators are clearly just fine holding off  that voltage, and the air does not break down. However, once an arc is established, either by brief over-voltage or some conducting object falling through the high field region, the cloud of ionised gas shorts together regions that would have otherwise been many kV apart, and in effect makes the full voltage available across any remaining gap, which soon also breaks down - and the total volume of ionised gas rises until it is really limited by how fast you can get the power into the gas that is already conducting - the main mechanism being that the electrons are knocked from the outer orbits of the nitrogen and oxygen atoms in the air, by other already charged atoms bumping into them . There is a race between generation of new ions by impact, and natural recombination of electrons with their denuded atoms, and any effect like a high wind or convection currents that might take the conducting plasma away from the direct path.

This is the same physics that makes any arc from the substation side of the company fuse much more dangerous than the same from a fault on a 5A lighting circuit, even though the voltage is the same in both cases, the effect of the slipped screwdriver would be far more serious.

Mike.