Reviewed by Dominic Hopkins
The practical answer
An older fuse box is not automatically unsafe merely because it is old, but it may lack appropriate protection or show damage that deserves investigation. Heat discolouration, poor labelling, repeated tripping or missing RCD protection are reasons to arrange an assessment. A new consumer unit should not be used to hide defects in existing circuits; they need inspection and testing as part of the work.
This guidance is general and cannot replace inspection or testing of the actual installation. If there are signs of danger, isolate the affected circuit where safe and seek professional help.