As a little kid, I used to think electrical substations would make really awesome jungle gyms. This video helpfully demonstrates why 5-year-old Maggie was an idiot.
This is the Eldorado Substation near Boulder City, Nevada. What you're seeing: A substation like this one is connected to long-distance transmission lines and electricity has to be very high voltage to travel on those. The substation "steps up" the voltage so the electricity can travel. Everything at a substation is hot, in that shock the bejeezus out of you sense. So that maintenance can be done, substations are built with switching functions that allow you to disconnect and reconnect various parts of the system in modular sort of way. The big, crazy spark in this video happened when some of the switching mechanisms failed. The Arcs 'n Sparks page at Stoneridge Engineering explains what happened next...
There's some technical jargon involved in this, but I thought you'd find it interesting and it gets the point across.
During normal operation, the switcher will first open the SF6 interrupters which disconnects the HV circuit so that the air break switches can open with no current flowing. Once the air break switches completely rotate to the "open" position, the SF6 interrupters then reclose. Normally, this sequence insures that the air break switches operate de-energized and arc free.
Instead, here, the air break switches opened while the current was still coming through.
The arc stretches upward, driven by rising hot gases and writhing from small air currents and magnetic forces, until it easily exceeds 100 feet in length. As impressive as this huge arc may be, the air break switch was really NOT disconnecting a real load. This arc was "only" carrying the relatively low (about 100 amps) magnetizing current associated with the line reactor. The 94 mile long transmission line associated with the above circuit normally carries over 1,000 megawatts (MW) of power between Boulder City, Nevada (from the massive generators at Hoover Dam) to the Lugo substation near Los Angeles, California. A break under regular load conditions (~2,000 amps) would have created a MUCH hotter and extremely destructive arc. Imagine a fat, blindingly blue-white, 100 foot long welding arc that vaporizes the contacts on the air break switch and then works its way back along the feeders, melting and vaporizing them along the way. Still, you've got to admit that this "little" 33 MVAR arc is certainly an awesome sight!