@Vicario has raised some points, one is the claiming when work not done to standard, in 1966 the rules changed and one now require an earth with all fixed wiring except for the lighting pendent, i.e. the bit dangling on wires. Before 1966 we did not require an earth for lights.
Earthing requirements have changed more resent, for a TN-C-S supply we did need an earth good enough to activate a fuse or trip within a set time and the trip can only do it within the time using the magnetic part, so a type B will trip at between 3 and 5 times the thermal rating, so 32 x 5 = 160 amp, so 230/160 = 1.4375 Ω however is seems there was a worry about volt drop so a 5% safety factor has been added, so now 1.365625 Ω for the standard ring final.
However with a TT supply we would never reach that figure, we are told over 200 Ω is unstable, but we rely on the RCD to auto disconnect in the event of a fault to earth, not the overload trip.
Clearly it still needs to clear the fault in time with a line to neutral fault, but the earth loop impedance can be much higher.
With a TN-C-S supply we still use RCD's as secondary protection, however personally as a primary protection device I am worried about it being the correct type, and being tested on a regular basis, but not so worried as a secondary device. However the use of RCD/RCBO on all circuits raises the question as to how important the earth loop impedance now is?
Personally I would say same as the 200 Ω is considered as being the limit for a stable earth rod, the old 1.37 Ω for a ring shows nothing has degraded, in fact 1.44 Ω as it may have been wired under the older rules, if one gets a reading of 5 Ω the question is why. It points to a bad connection some where, but the same is true for a 1.37 Ω reading in a house where clearly the ring final is not going the be the full 106 meters allowed.
But the
@Vicario has been previously re-wired, with no earths to lights, so either pre-1966 or some one has really messed up. In around 2004 the Part P law started to take effect, before then we often saw homes rewired without an installation certificate being raised, which means we have no idea who did the job in many cases, however after 2004 we have installation certificates which should form part of a traceable record, I say should, as when I tried to get replacements I failed, the idea was the scheme provider checks the installation certificate and if OK issues a compliance certificate, however it seems the scheme providers work on trust, and issue a compliance certificate without seeing the installation certificate, but one hopes the owners pass on the originals when a property is sold.
So if you know who wired it, and there is a problem in how it was wired, is there any time limit to calling some one back to correct poor workmanship? I still get recall notices from Kia and Honda for faults with a 20 year old car, so one would assume one should also be able to claim from an electrician after 20 years? Well since we should get an EICR every 10 years, I suppose one can reasonable say up to 10 years after work completed. Raises a question about professional indemnity insurance clearly if some one retires then they will not keep up insurance payments, so will the insurance pay out? And more to the point, how do you find out who the insurer was?
Vibration causing a earth wire to become loose is very different to earth wire missing. If missing then it means one needs to renew cables, i.e. re-wire, OK only a rewire for lights, but that is still a lot of work.
In the main we know a bad tradesman is unlikely to cough up for correcting bad workmanship unless a court ordered him to. And we also know even when a court finds he is guilty of poor workmanship it is still hard to get anyone to pay up, so is it worth trying.
