Is this a bolt-in or push in CB? 'sanding' the buss may not have been a good move. The contact tension on the push in type CB may be compromised, resulting in failure over a period of time.
If you are running 208 volt, 3 phase, with a 60 amp CB and 15KW of heat you also have an issue with the 'continuous load' (80%).
Conductor size?
Like Greg said, see if you can get your hands on an IR unit, you may have a heating issue due to the design.
