MC said:

I think my questions in short  are - Why do R1 + RN values not have a maximum value as surely in the event of a short circuit we want them to disconnect instantly just like we do when there is a short to earth (I understand we want it to trip quickly when there is a fault as someone could get a shock)?  How come we do not test for PFCC at circuits to make sure circuit breakers will trip quickly enough to prevent times longer than 5s disconnection times of circuit breakers when a short does occur? 

Because the limiting value is not dependent on I, but rather I²t. It's entirely possible a fault at the end nearest the OCPD performs "worse" than the "far end" ... but also a very long disconnection time might also be acceptable depending on the circuit arrangement.

Another advantage of testing R1  and Rn is to confirm polarity and Ud. One of the biggest landlords in the country used to insist that both R1 and Rn were measured and recorded on their bespoke documentation before dropping the requirement in favour of the templates in 7671. 

gkenyon said:

Because the limiting value is not dependent on I, but rather I²t.

At the risk of being a bit of a nit picker, but to give the full picture, that is only really true for very short duration events, where the cable is run at tens to hundreds times it's steady state current rating. The reasoning is that I²t *R is energy, not power, and is only really correct if there the heat arrives so fast  is no time for the heat to start to soak into the surroundings.
A stick of dynamite  going off , eating a mars bar or  large-ish church candle burning are all  similar total energy, but the rate of delivery alters what happens next in terms of heating and airflow. (!)

Realise that I²t is the same as joules per ohm

This is sort of saying you may ignore the open plughole in the bath and still fill it to overflowing if you fill  it fast enough...
Clearly a low enough current is OK to persist for ever, that is the cable rating, and between the two extremes of the steady state (heat arriving matches heat escaping, so no more temperature rise after equilibrium is reached) and the adiabatic (so fast no heat can escape) there is a rather soggy region of a few times the nominal current rating,  where cable damage is possible but requires an overload to last for many minutes, for things to get  really hot enough to do any damage.

Actually a modest overload can sometimes last for hours for heavy things like substation transformers without injury,  and the sums required to be sure of exactly how long is safe  require information that is not usually available.

It is this sort  of understanding that allows the DNOs to play with the after diversity maximum demand and use thinner cables than BS7671 might suggest, without anything bad happening.

Mike.

Agreed ... but let's take the example of a simple fuse. What if a designer has assumed a minimum prospective fault current, permitting a lower value of S to be selected due to the fact that there might be high inrush currents (requiring a higher rating of OCPD) ... and there is no requirement for protection against overload current, but there is a requirement for protection against fault current ?


This doesn't mean I'm not in favour of testing for R_n, just that setting a maximum value in the way you discussed is perhaps not such a good idea in all cases?

Agreed ... but let's take the example of a simple fuse. What if a designer has assumed a minimum prospective fault current, permitting a lower value of S to be selected due to the fact that there might be high inrush currents (requiring a higher rating of OCPD) ... and there is no requirement for protection against overload current, but there is a requirement for protection against fault current ?


This doesn't mean I'm not in favour of testing for R_n, just that setting a maximum value in the way you discussed is perhaps not such a good idea in all cases?