Pat wrote:
When the coil creates a high voltage, there will be an arc wherever
the gap is the smallest. ... it could arc inside the coil and thereby
destroying some high voltage insulation. ...

You're right, but there are limitations. First, the voltage isn't
unlimited, it has some maximum, whether there's a gap or not. I don't
know the mechanisms limiting it - parasitic capacitance, leakage
current, I dunno. Hopefully the coil designer knew and insulated
against it, maybe.

Then there's the path of an internal arc. There's certainly not enough
voltage for an arc between adjacent turns. Also very unlikely between
adjacent layers. The highest voltage is between the last turn and the
core. But that's the most heavily insulated path.

I'd be convinced by an actual case of an internal arc - anybody ever see
one?

Bob