Thanks very much, I will those a watch and hopefully this will help my understanding much more. 

I have watched many of his videos and they are extremely good.

Thanks for the reply Andy. 

I agree with most things you have said but I don't like assuming. Like you said "it must be ok" I just think a simple R1+RN check at the end of the circuit would confirm this and would have thought this simple check to make sure the RN side of the circuit was in good condition. This would then allow us to check the values against a set of values set by the regs so we know that it will trip in a short period of time. Also having this value could help confirm volt drop if required, but this is obviously not a thing. 

For the example you gave with the high resistance on the neutral - If you went and carried out an EICR on a property and that had happened on a neutral of a radial it would not have been picked up as there is no R1+RN test. Also, say you did test it and it came back with 10 ohms, we both know that's incorrect and must be something wrong but there is nothing to say that value is too high for that circuit is there.

The rules do not say never measure Rn, just that you do not have to.

do realise that a 10 ohm resistance is far too high for a single point fault -  even at just one amp it dissipates 100 watts and if that is something the size of a terminal screw it will soon heat up to molten metal and in doing so either blow clear, or weld itself into a low resistance state. The former will be found and the latter will now work for years without incident . The sort of extra resistance that leads to slow cooking is that which dissipates a watt or two in less than a cubic cm , and those sort of resistances do not really show on test, as they are comparable to very short lengths of cable.

As Andy notes, things are usually either connected or not, though some things that measure open circuit at low voltages connect at higher ones, as thin layer of oxide are blown away or small gaps are jumped by arcs. (Though you do not really get long arcs with copper, other materials such as carbon, or indeed welding rods (!) can be persuaded to strike arcs of several mm with 230V. To start an arc in air with 230V needs things to be closer than 20um (Pashens rules apply more or less), but once struck the burn clear or weld solid rule applies.)

This is why twisted joints work as well as they do - the initial contact area is almost nil, but soon expands once current  flow and at a microscopic level the peaks are welded to the troughs...

Mike.

Let's not forget the inspection phase of both initial verification and periodic inspection and testing.

It is also worth remembering that cables are not damaged by short temperature rises above 70C conductor temperature. Whilst this is often forgotten we can look at why fairly easily. You may look at the adiabatic temperature but this says that at this point zero heat is transferred to the insulation (that is what adiabatic means) and that the heat capacity of the conductor is much less than the insulation (a curiosity of copper and plastics). The damage to the plastic comes from melting (150C or so) burning (500C) and plasticiser loss (long period at high temperature, or very long at room temperature). To melt and be able to be displaced, the bulk plastic must melt, not a film close to the conductor.

All these may be classed as "damage" but the effect is simply that the practical life of the cable is reduced, and it takes a very severe "overload" for a long time to melt a cable. It is very unusual to find a fixed installation cable that has melted, particularly in domestic installations, whereas many loose or poor connections are found, but rarely lead to actual fires. your 4.5 Ohm example is extreme, and as Mike suggests would melt quite quickly if the circuit current is significant.

One may measure the PSSC at any point in an installation, the test you are suggesting on the neutral conductor, which would lead to an unexpectedly low figure, and which test is available on multifunction instruments.

I'd also add that if you had an erroneous several Ohms in a heavily loaded circuit there would be no need to wait for an EICR to spot it - it would already be very evident to the users of the installation from dim lights or mis-behaving appliances.

   - Andy.

it would already be very evident to the users of the installation from dim lights or mis-behaving appliances.

and the fire...