From bigclivedotcom.

One of the most violent electrical failures is where an arc flash occurs. This is where an initial arc situation creates a conductive mix of ionised air and vaporized metal that can continue to sustain an arc, and also carry it across onto adjacent metalwork that could be grounded or a different phase.
That can result in an electrical explosion where it instantaneously bridges multiple phases in the vicinity, with the risk that the affected metalwork may be high current busbars that can literally supply megawatts or gigawatts of power instantly.

Arc flash poses a high burn risk – with the added complexity of the arc being metal laden, which makes the burns much worse. The plasma is also carrying current and when it engulfs a worker, some of that current can pass through them, posing an electrocution risk.

The carbon tracking issue can be very difficult to pin down if it is not to ground, and when it blows itself clear. If it was to ground then an insulation test would generally help pinpoint the area of the fault. But when it’s not to ground then you have loads across the conductors that defeat the insulation test and make troubleshooting more complex. Especially so when a cable fails in a long run and is underground or in inaccessible containment as happened here.

In some instances the time between the tracking causing flashover and blowing the fault clear can be so long that it becomes normal just to reset the breaker or change a fuse. Ultimately the fault may get more frequent until it does persist and becomes easier to locate.

Note that blowing faults clear shouldn’t be considered a normal thing. When it starts involving stupidity like uprating or bridging protective devices completely, then there is a serious risk of cable damage and fire when a fault turns into a solid short circuit.

Normally a circuit breaker like this will quench a high current arc by guiding it up conductive plates into a series of fins that effectively break the arc. But over time, if a circuit breaker is exposed to too many high current faults, it can create a progressive build up of carbon and metallisation inside that can lead to an issue like this, where the arc chose a different route to the normal quenching system and was sustained with explosive results.

It’s interesting that the mechanism interfacing ports between the trip mechanisms of adjacent breakers are close enough to live internal metalwork, that a high current arc flash in one looks like it did spread into the breaker either side.

If you’re in a situation where you believe you may be closing a breaker into a fault, I recommend at least wearing gloves that are suited to protecting you from ejected flame, and preferably eye protection too. With very high power equipment it really requires proper full body protection as used by substation workers. Because while it may be safe most of the time, that one in a million incident could be life changing.

An interesting comment by one of the channel’s Patreon supporters was that in some factories they keep a local log of breakers that have been closed into faults. And if a number of events is exceeded, they routinely change the breaker for a new one. That’s an excellent approach for reducing the risk of catastrophic failure in aggressive electrical environments.

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