Back up generator power is toasting ballasts on some, not all, branch 277v. night light circuits. Under normal purchase power, all these circuits operated ok. This problem only occurs when the gen set is cycled on. No back fed circuits have been found at panels with these trouble circuits. The problem seems to occur when purchase power comes back on line. Could both power sources on at the same time be it ?, why only some and not all circuits toasting ballasts ? Any thoughts ?