The main electrical source is connected to the cartridge panel which is then connected to the main breaker box. This has been set up for the past three years no problems. In the last two days the 60 amp cartridge fuse on the left side keeps blowing.

What could be causing the problem?

Gsmith,
You realize any answer would be at best a guess...
Call a professional, this is a hands-on type of answer.

What changes have been made to your electrical system.