Exam question

Lol... that's the C&G for you. Looks like a question from the 2391 or 2395 exam sheet pre 18th ed as there's no adjustment for supply voltage.. The answer is D (0.2) but it assumes that the given  tabulated value for the final circuit resistance is the same as the measured resistance with respect to the ambient temperature.

Therefore Zs = Ze +(R1+R2)*1.2

Ze = Zs*1/1.2 - (R1+R2)

A real answer = 0.19 when using 0.8 and 0.25 when using 1/1.2

Legh

Spot on Leigh!

Wally and Legh

The 0.8 factor is the normally taught figure for temperature (worst case) correction of maximum Zs. The BS7671 figures are the actual maximum, whilst the OSG (which is not important to anyone with any knowledge) is already corrected by 0.8 because anyone using it probably doesn't have BS7671 and probably wouldn't understand it anyway. I have no idea where you are getting the 1.2 from except that 1/0.8 is 1.25.  The figure used for design is then safe even under continuous maximum loading. However, if you are doing an EICR this is not the figure to use unless the cable is actually at normal operating temperature, the tabulated BS7671 figure is much more sensible as a reference. Remember too that your measured value may not be exact, you should be aware of the expected accuracy and treat the measured value accordingly.


Legh if you cannot get the correct C&G answer it is not their fault, the answers are there for anyone. However, you do need to be aware that the questions may be rather more searching of your knowledge than you feel is fair. Did you get 100% in the 18th exam? Was that because the questions were unfair? I have yet to see one that is unfair or unreasonable, all of them may well be seen in an installation.


The ultimate question is this 2391 (2395) or a design question or basic 2382? The expected answer will be different, and being multiple choice you cannot explain why.

I agree that it is a poor question - it ought to say "measured resistance".


There is an oddity with this 80% correction. Appendix 3 explains. However, the increase in resistance due to the heating effect of the load current is not uniform throughout the earth fault loop. For a typical domestic, part is R1 + R2 of the circuit; part is R1 of the tails and R2 of the earthing conductor; and the rest is Ze. A fully loaded circuit may well warm up considerably, but the tails and DNO's cables will still be only partially loaded and, therefore, not so hot. That is unless, of course, all the circuits are fully loaded.


As ever, BS 7671 errs on the cautious side.

So, where is the equivalent to Appendix 14 of the 17th edition in the 18th edition?


Z.

Part of it is in Appendix 3 now, but that old Appendix 14 addresses exactly the point that I was making.

Well, the figure we`d be looking at to BS7671 is 0.52 so that is the answer cos that is what is tabulated in 7671.

However the figure we`d be working to (designing and subsequent inspection )  is the 80% figure so we would be looking at 0.20 , if it is higher we`d headscratch and do proper sums


Don`t know why some calls it the 3/4 rule as when I went to school that would be 75% not 80%. Mind you the speed of light has altered since I went to school (it used to be 186234 miles per second back in those days)

" Don`t know why some calls it the 3/4 rule as when I went to school that would be 75% not 80%. "


If I remember rightly, IEE wiring regs. 15th ed. used 75% for assessing the Zs max acceptable. NICEIC seemed to lead the way for 80%. Although this is only a rule of thumb for basic installations, you might have to be more specific for more complex installations.

Legh

The question is simply not right.


If it wants 0.2 ohms as the answer, it should ask for the maximum measured value of external earth fault loop impedance, and should state that the resistance of line and cpc is measured at 1.13 ohms. A correctly worded question could be:

The maximum tabulated value of earth fault loop impedance for a circuit is: 1.65 ohms, if the resistance of the line to and cpc of a new circuit is measured as 1.13 ohms  the maximum accepted measured external earth fault loop impedance is: