We’re moving house and I don’t think the electrics have ever been changed since ~1940. Could I (or a electrician) test the cables to see if they are ok and then just swap out the fuse board, sockets and switches or should I just rewire?
It depends. Have a look around the property, are sockets in the places that you will want them, are there enough of them, are there enough lights? If the answers are no, then you would be better off going for a complete re wire.
If just the consumers unit is changed any decent electrician will carry out an EICR (In short check the quality and condition of the wiring) first which will be included in the new consumers unit price, but if you go ahead and start from scratch everything will be as you want and no need to pay for an EICR
Some where around 1960's cable was changed from rubber to plastic, and the old rubber does degrade over time, but in the main plastic cable does not degrade, there are some exceptions where the plasticizer leaches out, you see a green grunge around the cable ends, but that's not down to age, but manufacture of the plastic.
Also around 1966 the rules changed on lighting, from that point lights needed an earth wire, before that earth was not required for lighting.
It is unlikely unless wired using mineral insulated cable that 1940's cable would still be OK today, and pre-war a standard wiring used radials and likely one 15A socket upstairs and one 15A socket down stairs, it was not until world war two that the ring final was invented.
1954 when this house was built it had 6 x 13A sockets, one in each room and one on landing. By 1970 we were fitting 25 or more sockets in a house.
So it is unlikely the original 1940's wiring is still in use, if so likely there will still be some rubber insulated cable in the fuse box, so it should be easy enough for any electrician to see by just removing the CU cover if a rewire is required. This house wired 1954 the wiring was in quite a state, when rewired three years ago, still with wooden back boxes for light switches etc.
If getting a rewire no problem, but if just changing the fuse box for a consumer unit, then go for RCBO's so every circuit has it's own RCD, that way if you have a problem with one circuit, it will not affect other circuits.
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