Value of resistivity for 1.5mm = 12.10 milliohms per metre
Value of resistivity for 10mm = 1.83 milliohms per metre
Therefore (R1&R2) resistivity in milliohms per metre = 12.10+1.83 = 13.93 milliohms per metre
Length = 28m
Factors = 1.02 & 1.20 (from tables 9B and 9C OSG)
Therefore, R1&R2 = (milliohms per metre x Length x Factor) / 1000
= (13.93 x 28 x 1.02 x 1.20) / 1000 = 0.48 ohms.
Zs = Ze + (R1&R2)
= 0.14 + 0.48 = 0.62 ohms
I have been thinking about this.
Value of resistivity for 1.5mm = 12.10 milliohms per metre
Value of resistivity for 10mm = 1.83 milliohms per metre
Therefore (R1&R2) resistivity in milliohms per metre = 12.10+1.83 = 13.93 milliohms per metre
Length = 28m
I agree, these figures are taken from table A.1 and are the value at 20C
Therefore at 20C our circuit resistance would be
13.93*28/1000 = 0.39 Ohms.
We have designed the circuit at 25C so if we were to measure the R1+R2 value at 25C we would need to add a correction factor because at 25C the resistance would be slightly higher, this correction factor is 1.02.
So the realistic resistance figure at 25C for our circuit would be:
.39 Ohms * 1.02 = 0.398 Ohms
This is what the correction factors for ambient temperature are for, they are to be applied to the tables EG table A.1 to change the values in those tables measured at 20C to whatever temperature the circuit is being measured at in reality.
Now The rule of thumb (the .8 rule) is used to multiply the tabulated figures in BS7671 which are measured at 70C
The 0.8 is used to bring the figures measured at 70C down to a figure that they would be if the circuit were measured at 20C.
Conversely if we want to convert our figure of R1+R2 measured at 20C to a figure measured at 70C we would multiply it by 1.2 this figure would then become our R1 + R2 measured at 70C and we would add Ze to this. We would then be able to compare this figure directly to the tables in BS 7671.
So in this instance we need the figure for the circuit we are designing, the R1 + R2
measured at 20C.
We calculate what this figure is by using the above formula EG:
13.93*28/1000 = 0.39 Ohms.
This is the figure at 20C.
We then multiply the figure at 20C by a factor of 1.2.
0.39 * 1.2 = 0.468 Ohms.
Add this to Ze
Zs = .14 + .468 = 0.608 Ohms.
This is the figure we would use to compare to figures in BS 7671.
We would not use the ambient temperature figure in this calculation.
The only time we would use the ambient temperature correction factor of 1.02 was if we were measuring our R1+R2 at 25C and we needed to compare these measured figures with those in the book which were recorded at 20C
We would take the figures in the book measured at 20C multiply them by a factor of 1.02 and this would give the value of the table figures at 25C.
We would then be able to compare our measured reading at 25C directly to these tabulated figures.
We would not use both the factors of 1.02 and 1.2 together in the same calculation.
This is how it appears to me, shoot me down if I am wrong.
One more thing, I noticed a nice little fact today.
The resistance of copper will increase by 2% for every 5C increase in temperature.
So if in doubt use this way to measure what the resistances would be at different temperatures.
If a cable increases in temperature from 20C to 70C its resistance will increase by 20%
