...It looks as if the Plug was NOT Grounded during the Testing! That is Exactly HOW You Destroy a Coil!! I Did Warn You!!
With NO Ground, the High Voltage 'Seeks' a Ground with-in the Coil, and Finds it To the Primary Wiring! This 'Burns' a Permanent 'Path' through the Windings that will Forever be a Short in the Coil! The Coil becomes Junk! The HIGH VOLTAGE Has to 'DUMP' somewhere...and it's To the Primary Ground, INSIDE the Coil!! FWIW