Ignoramus19762 fired this volley in
:

Right, but why does the arc flash in the first place?

All electricity needs is a low-enough resistance path to start flowing,
and as it raises the temperature of its surrounds (say, by fusing the
metal grinding dust that started the connection), the air itself becomes
a conductor.

A "spark" is nothing more than the voltage overcoming the insulation
resistance of the air; when it begins to flow through the air, it heats
the gasses to plasma temps, and that's what you see as the "spark". If
something like a filament of lathe turning can assist in starting that
current flowing, all the easier for a big, fat arc to form.

Such an arc can form from having foreign material between the leads (like
your 3ph box), or it can form from having two high-voltage leads too
close together. Excess humidity shortens the required gap.

The "classic" man-killer short happens when an inattentive technician
creates a dead-short with a tool, then the tool AND the air around it
vaporize into plasma.

LLoyd