Basically, the dielectric constant of air is not infinite. If the electrical potential is high enough between two points, air can conduct electricity.
If you put a small bit of conductor near two legs of a high voltage system, and that conductor has a path to ground, you can get now get an arc between a voltage source and the conductor (e.g., someone with a screwdriver loosening a screw is too close to a high voltage leg). This initial arc ionizes the air, which makes a pretty great conductor, so the amount of current flowing through the air increases really quickly as if it's flowing through an open circuit. The arc is superheated, can burn through steel (or flesh) quickly, and expands.
One of the hazards of working on high voltage is gasping in shock after an electrical arc - you inhale superheated plasma and it cooks your lungs instantly, so you suffocate.
Anyway, to prevent this, you don't cut corners as an electrician - you have proper insulation on all your tools and body parts to prevent this sort thing. You isolate legs of high voltage circuits, or de-energize the circuit you're working on with proper lock-out tag-out procedures.
If you put a small bit of conductor near two legs of a high voltage system, and that conductor has a path to ground, you can get now get an arc between a voltage source and the conductor (e.g., someone with a screwdriver loosening a screw is too close to a high voltage leg). This initial arc ionizes the air, which makes a pretty great conductor, so the amount of current flowing through the air increases really quickly as if it's flowing through an open circuit. The arc is superheated, can burn through steel (or flesh) quickly, and expands.
One of the hazards of working on high voltage is gasping in shock after an electrical arc - you inhale superheated plasma and it cooks your lungs instantly, so you suffocate.
Anyway, to prevent this, you don't cut corners as an electrician - you have proper insulation on all your tools and body parts to prevent this sort thing. You isolate legs of high voltage circuits, or de-energize the circuit you're working on with proper lock-out tag-out procedures.