Replaced a wall light with a brand new 400w metal halide wall pack on an old hotel for a customer.
Upon energizing to test it, the lamp would not light. While trouble shooting, it turns out that when only the ground wire was disconnected, the lamp would light. Of course, when the ground was connected, the bulb would go out.
Is the capacitor in the fixture metallic by chance?? I would start looking at something between the ballast output lead (if not the ballast itself) and the socket accidentally touching ground or faulted to ground, and my first bet would be a capacitor with a metallic case and an internal problem ..