The likely cause of this is the combination of an Instant Start ballast with tube guards. An instant start ballast should not be used with tube guards because there is no EOL protection in the circuit, and if the lamp begins to fail it can overheat and melt the tube guard. Robertson Worldwide recommends that our customers avoid using instant start ballasts with tube guards and offers an expansive line of ballasts with other start methods that will ensure the tube guards do not melt.