Heat is definitely the most common culprite for Ballast failure.
From the description you gave, it is obviously the Autotransformer section which has fried on these Ballasts - due to the breaker tripping from a nearly short circuit.
Running the Autotransformer section in high heat and at the lowest input voltage setting creates even more heat. Figuring 480 VA to drive a 400 watt lamp [apx. 86% P.F.] draws at least 4 amps @ 120 VAC.
Not capping the unused leads will cause trouble also, if any of the leads complete a circuit to the grounded box. It will draw current through all parts of the Autotransformer and make the coil heat up excessively.
Other than terminating to the wrong voltage input and the stuff mentioned above, a typical HID CWA Ballast should last a good 10 years minimum. Lamps, Capacitors and Ignitors fail sooner than frying a coil.
Scott S.E.T.
