I have a customer that we have performed electrical work for many years. They have 30' poles with 1000w metal halide large,old "cobra head" fixtures to illuminate the parking lots. We are having a problem with the fixtures failing within 2-3 months of repair. The last repair date was early April 2010, 6 fixtures had complete lamp and ballast kits replace, now five of the six are no longer working. The source power feeding the lights is 480v. This problem has been continual for at least 18 months to 2 years. I am at a loss to find the cause. Voltage at the pole base/head is 475-485v, wire size is # 8 from the breaker to the base of the poles, # 10 up the poles. The only thing I can think of is the poles are not grounded properly and that is causing capacitor/ballast failure. Customer is becoming rightfully frustrated, as well as I am at the end of my patience for warranty issue. Any suggestrions are appreciated. What am I missing?