unless there is a seperate isolator which there usually isn't you cannot isolate the complete fuse box.
With the main switch on the fusebox off everything except the incoming mains terminals in the fuse box should be isolated. Those termainals should be covered. It pays to be very carefull when opening it though because sometimes the covers for the incoming terminals are missing or the terminations are sloppy leaving live metal exposed.
With the main switch on the fusebox off everything except the incoming mains terminals in the fuse box should be isolated. Those termainals should be covered. It pays to be very carefull when opening it though because sometimes the covers for the incoming terminals are missing or the terminations are sloppy leaving live metal exposed.