Like my house which once only had 3 circuits, lights, ring and cooker.
It now has 3 ring mains, North, South and kitchen and 2 lighting
circuits, downstairs and upstairs. As it happens the lighting circuits
were harder to separate, wired in part with double insulated singles but
via some very odd routes.
One of the other things I have done is to try and eliminate all the
diagonal wires that offend current regulations.
If it's rubber, should have been done ~30 years ago.
If it's PVC, then even the earliest PVC installations
should still have sound cable unless it's got hot, and
should remain good for decades yet. IEE estimates 1000
year live of the PVC in ideal conditions, dropping to
21 years for a cable operating continuously at max
rated temperature of 70C. Some wiring schemes such as
conduit can suffer high earth impedance after a long
Wiring accessories (switches, sockets, fuse box, etc)
will not generally last as long as the wiring and should
be checked periodically -- life will depend on usage and
original quality. Fortunately, they are much easier to replace
than the wiring. Also check for terminal screw tightness,
as poor contacts there can lead to heating and damage to
accessories and cable (and worse).
Standards and expectations change over the years.
A 50 year old PVC installation could still be in perfect
order, but would be lacking the number of sockets expected
today, and below current earthing standards. That doesn't
necessarily mean a rewire is required, but poorly done
piecemeal expansion of an original installation is a common
cuase of an installation having become an unrepairable mess.
My parents installation was one for earliest PVC ones in
the 1950s. The original accessories are all MK, except the
fusebox which was Wylex. I found the fusebox was in need
of replacing 10 years ago during an inspection -- the
contacts on the ring circuit fuse holder were getting too
hot and caused insulation to go brittle. I replaced the
fusebox as part of rewiring/refitting the kitchen and
bringing service bonding up to spec. I can only think of
one of the MK accessories which has failed -- a light switch.
About half of them have been replaced during room redecorations
over the 50 years, but none had anything wrong with them.
A few of the terminal screws have been retightened.
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