Guidance note 8 help...

[EFLImpudence]

I was just thinking that when Ze (or Zs) is measured, the meter will show the value of R2 so even that does not help.

 

[JohnW2]

I was just thinking that when Ze (or Zs) is measured, the meter will show the value of R2 so even that does not help.

Quite - as I said, the only way one could measure that component (e.g. "0.19Ω") of Ze would be to undertake the measurement at the terminals of the transformer, in which case the measured loop resistance would be entirely due to that 'within transformer' component.

That's why, as I wrote previously, I think that GN8 is unnecessarily muddying the water by including that component of Ze in their calculations since that's not something that anyone will be able to do in the real world. So, as I said, I suppose the best estimate of 'touch voltages' one can get is by assuming (incorrectly) that each 'arm' of Ze has half the impedance of the total Ze - which, in practice, probably won't be too far wrong (particularly given the 'compensating fact' that R2 will be greater than R1 in most final circuits).

Kind Regards, John

 

[JohnW2]

.... Having got that straight in my mind, and thereby having de-mystified the 0.19", I will subsequently (hopefully soon!) move on to the rest of your scenarios! ....

3 is you values with some values for Rx
4 is with bonding added - Im trying to understand why the 0.25 external R2 is removed from the equation - Im was thinking it is a lower resistance path for the fault ... But then you would have a high resistance return path to the transformer via the mass of earth.
5. But all of these calculations have the Extraneous CP at 0 V, when it would actually (with bonding attached) rise to the potential of the MET,
So the touch voltage would be as shown.

OK. Quite aprat from the fact that, as I have explained, you have 'incorrectly' (just as I did initially) ignored GN8's "0.19Ω" component of the Ze, I find your diagrams rather confusing and also have some trouble in following your maths, since it's not always immediately obvious what your V1, V2, V3 etc. actually represent.

Accordingly, starting with your 'scenario 5' diagram, I have modified to make it less confusing (at least, to me) and have then presented you with my maths (hence the resultant figure for 'touch voltage') in three different scenarios.

The first diagram below is your 'scenario 5' one. Following that are three modified ones, with my maths, corresponding to three different scenarios (of mine!)...

No Bonding
As we know, when the 0.19Ω is correctly included, the voltage at the fault is (with the GN8 figures) about 98.1V and, since, in the absence of bonding, the ECP is always at earth potential (regardless of the impedance of the ECP to earth), the 'touch voltage' (between fault and ECP) is also always about 98.1V

Bonding with relatively high impedance path from ECP to earth
Here I'm using your figure of just over 200 for the path from ECP to earth. In this situation, the effect of bonding is to raise the potential of the ECP to that of the MET (without noticeably changing the potential of the MET) which results (with the GN8 figures) in a 'touch voltage' of about 53.5 V. With the GN8 figures (0.19,0.25, 0.30, 0.30 & 0.25 ohms), one cannot get the touch voltage any lower than that, although it could become much lower if the 'R2' (CPC impedance) of the final circuit concerned was lower than GN8's 0.30Ω.

Bonding with very low impedance path from ECP to earth
Here I'm using your figures of 0.05Ω for the bonding conductor and 0.02Ω for the ECP, assuming there is no additional impedance to earth - hence just 0.07Ω from MET to earth. In this situation a substantial proportion of the fault current goes through the ECP (rather than the DNO's 'earth'), pulling down the potential of the MET (thereby increasing the 'touch voltage'). With those figures (and the GN8 ones) the 'touch voltage' rises to 70.35V. If one carried on reducing the impedance of the path to earth via the ECP, the 'touch voltage' would continue to rise and, ultimately, as that impedance approached zero, one would approach the situation with no bonding (hence a touch voltage of about 98.1 V, with GN8 figures).

In other words, for a given scenario of Ze/Zs, the lower the impedance from ECP to earth, the less will bonding reduce touch voltage. Ultimately, if an ECP has a 'negligible' impedance to earth, then bonding will achieve virtually nothing.

I hope that I've got it all roiughly right this time (but no promises!). Does the above/below make any sense to you?

Kind Regards, John

 

[JohnW2]

OK. Quite aprat from the fact that, as I have explained, you have 'incorrectly' (just as I did initially) ignored GN8's "0.19Ω" component of the Ze, I find your diagrams rather confusing and also have some trouble in following your maths, since it's not always immediately obvious what your V1, V2, V3 etc. actually represent. .... Accordingly, starting with your 'scenario 5' diagram, I have modified to make it less confusing (at least, to me) and have then presented you with my maths (hence the resultant figure for 'touch voltage') in three different scenarios. .....

@studentspark , you seem to have gone quiet. Does that mean that you now fully understand what I've been writing, and have no further questions, or does it mean that you are still trying to make sense of my mutterings?

Kind Regards, John