The contactor went to shut off and only 1 leg shut off, leaving the high leg energized and killing the ballast.? Idk.
You don't understand high-leg 240/120V 3-phase 4-wire systems.
When you have 2-wire loads connected to 240/120V, the troubleshooting, and potential problems, are no different than any other 240V system. If you routinely hold hold one meter probe to ground, or neutral, the 'low leg' will be about 120V and the high-leg about 208V. It is straight math, if the high leg is not 208V, then the low leg is not 120V.
So, if one contact stayed closed, then one ballast wire would still be energized. At worst, the ballast would see 208V to ground, which is exactly the same voltage it sees when it is operating. How is this a problem?
The biggest issue is when the equipment uses a neutral connection. Unless you know the internal wiring of the equipment, do not wire it to a high-leg.
Bad contactor contacts can lead to abnormal currents. Abnormal currents can lead to device failure.
