I see this is your first post so welcome to the board. There are several things that will cause the fuse to blow. A fuse protects against two things, overloads and short circuiting. Before you even think about replacing the fuse, you have to identify why it did what it did. There are too many things that it can be and since your bio states that your are a future apprentice, you are better off and safer to call in an electrician. For whatever reason the main blew before anything else typically indicates there is a other, possibly serious issue with your system. Even if it is just a problem with your heat pump, your over current protection for you heat pump did not kick out before the main therfore you have other issues as well especially sense you service is almost 40 years old.
Are you certain the fuse blew? The is the possibility that the you lost a leg from your service. This requires working around energized wires. Another reason to call an electrician.
This may not be the answer you are looking for, but without additional info and putting you at risk for getting additional info, again you are better off calling an electrician. At least you will still be around to post your second post.
