Greg,
I had one customer who had a 40 circuit 200 amp service panel. The main was fuses (Not breakers) but the branch circuits were all circuit breakers. He had trouble once where all the breakers tripped, but the fuses didn't blow. I think I even posted this question a dozen years ago. I never found out the problem, I always thought it was the POCO problem. I think the transformer outside was sending in a higher shot of voltage that would trip breakers but not blow fuses. After 2 or 3 service calls from me, and 1 or 2 from the POCO and a new transformer, the problem just disappeared.
