What caused this spectacular fireball to zip across a power line?

Several days ago, a resident of Lachine, Quebec, captured harrowing video of a large electrical fireball that traversed across a power line, prompting everyone to ask: What the hell was that?

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The incident happened after a huge wind storm swept across the region, knocking out power lines in a Montreal suburb. At first the damaged device throbbed and hummed with electrical power, but then the fireball emerged, shooting down the street along the power lines. According to the utility, a lightning strike combined with safety switch failures was the likely cause.


Redditor BeesKnees21 says it was a fault with the function of a protective device:

This was a high impedance electrical arcing fault. Usually the upstream protective device (ie. fuse, relay/breaker, recloser, etc.) will interrupt these types of faults and prevent them from continuing. They can start because of tree branches or animals or lightning strikes or anything that can bridge two phases or a phase and ground. The air becomes ionized and no longer acts as an insulator but a conductor. This arc is interesting because it was able to be sustained for such a long period of time.

If the fault has a high enough impedance, the current is actually not very high in comparison to regular faults. This makes it very difficult for the protective device to clear the fault since it the current can even be lower than the normal "load" current. The utility should be able to analyze any event reports in the area from the relays to try and determine what happened and if it can be prevented in the future. Source: Electrical Engineer


Redditor SoTupps, an electrical line technician in training, put it this way:

The "fireball" is definitely an electrical arc probably formed by a fault current in the line i.e. a lightning strike in combo with faulty protective devices. Fault's current traveled up-stream (opposite of the usual flow of electricity) until they can be stopped by a protective device (which usually opens the circuit and allows the fault to clear). In this case I believe it was a fuse cutout/lightning arrester combo that finally snuffed out the arc. These are usually installed to protect transformers from dangerous faults such as these. Had that fault reached the transformer.... BOOM! An explosion would be likely.


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