The coil works as a dynamic resistance or impedance if you like. The secondary DC resistance of 7.5K has virtually no effect on the primary current draw under steady state (DC) operation.

The basics are: Run current through the primary until the core is "saturated" then abruptly stop the current flow in the primary. The field induced by the saturated core collapses and causes a high voltage spike to occur on the secondary lead to the distributor. This spike >20KV is timed to correspond to the juxtaposition of the rotor and one (or more) of the distributor contacts causing a >20KV "spark" or arc at the spark plug gap. Mr. Lenz said it well with the voltage across a "coil" being proportional to the change in current. With an "infinite" change in current over time one would expect a large voltage change across the output terminals.

From Kettering..."the only parts with an infinite service life are those which are never installed" Kettering invented this type of ignition system and it bears his name.

So a primary fire cound be a DC issue with a short to ground causing the points when closed to fuse and direct battery current through the primary of the coil. Measure the DC primary resistance. Divide 12VDC by this resistance and you will calculate the current through the primary. It has been my experience that you can short the primary to ground with 12VDC applied without inflammatory results. But hey, I'm just a dwEEb.

A better answer might be a primary feed wire grounding without going through the primary coil. This would seriously smoke the insulation on said wire and maybe with enough inflammable grunge on the wiring cause a "fire".

Donning my nomex and awaiting the flames ....

pensioner