Donna Ohl writes:

Hi Dave,

Is this what probably happened in my "typical" oven fire?

0. Something (grease perhaps) was on the broil element in one spot
1. The grounded jacket thinned in that one spot, ever so slightly
2. Over time, that one spot cracked, ever so slightly
3. Over time, stress increased on the resistor core encased in concrete
4. At some point, the cantilever stress broke the inside resistive core

I don't think we know why the element failed. Oven elements routinely
get organic stuff (food) splattered or dripped on them, and the usual
result is a bit of smoke as the organics stuff gets burned to carbon or
ash, while the element is completely unhurt.

So, either your element was defective somehow (e.g. a gap in the
insulation somewhere, or it was subjected to some unusual stress at some
point. The initial failure could have been an open resistor core, but
it could equally well have been the core touching the grounded metal
jacket.

By the way, the insulating fill is not concrete - concrete contains lots
of water and doesn't like being red hot. It's some kind of
high-temperature insulator (e.g. a ceramic).

5. I heard a "sound" as the air gap ionized to a conductive plasma
6. The core melted, causing the plasma gap to expand (more sound)
7. The super-heated plasma conductivity extended to the metal jacket
8. The core-to-jacket arc melted the jacket "backward" in a spiral pattern

All of that's possible, though we don't know for certain.

9. I turned off the broiler but that only reduced to half the pressure
10. The other 120v pressure still allowed the plasma to remain heated
11. The plasma arc spiralled backward toward the remaining 120v pressure
12. Opening the house circuit eliminated the pressure & the "fire" went out

Substitute "voltage" for "pressure". Electrical voltage is in many ways
like fluid pressure, but they're not the same thing. This assumes you
turned off the switch, but the switch only interrupts one side of the
240 VAC line.

Yes, turning off the house breaker removed the voltage, and the arc
ceased. Turning off the circuit breaker for the stove alone would have
accomplished the same thing.

Is this what happened?
Did I miss any steps?

We don't know for sure what happened (at least I don't). But the
sequence is plausible.

Dave