BS 7671 Table 41.3, conductor temperature?

[Alessandro]

Edited: sorry of course Ze should be added to the post below, Table 41.3 is for comparing Zs.

Hi, it is my understanding that Table 41.3 of BS 7671 should be used for comparing the tabulated values with the resistances of:

1) the line conductor at 70 degC;
2) cpc at initial temperature, as given in Tables 54.2 to 54.5.

This is what note 2 under Table 41.3 says.

This means that I should apply a factor of 1.2 to the measured value of R1 if ambient temperature is 20 degC, or simply add 2% for every 5 degrees of difference between initial temperature and maximum operating temperature. Then add the measured value of R2, and apply factor only if necessary (if initial temperature is different from ambient temperature).

But if I measure R1 + R2 directly, then I can't apply a single factor to the obtained value. Am I correct? Then I should simply obtain the values of R1 and R2 separately, or by subtraction from Rn + R2, then apply the factor for R1 to R1, and the factor for R2 to R2. In short:

Ze + R1 * c1 + R2 * c2 < corresponding value in Table 41.3.

But I couldn't find this procedure anywhere. Am I misinterpreting the Regs? What should I do?

 

[Richard Burns]

For a standard PVC insulated multicore cable rated to 70°C or for a 90°C cable derated to 70°C then the temperature of the cable for both line and cpc when measured would be the ambient temperature. The maximum temperature of the line would be 70°C and the initial temperature of the cpc would also be 70°C, therefore to correct your measured resistance of R1+R2 you would multiply the value measured by 1.2 because both conductors move from 20°C to 70°C.

In the unlikely event that the cpc is not associated with the line conductor then you should theoretically make different adjustments because the cpc would be moving from 20°C to 30°C and the line from 20°C to 70°C, this would mean a slightly lower factor could be applied, however if you apply the 1.2 factor this gives a higher value of resistance and so errs on the side of caution and provides a safer and more convenient approach.

 

[Alessandro]

In the unlikely event that the cpc is not associated with the line conductor then you should theoretically make different adjustments because the cpc would be moving from 20°C to 30°C and the line from 20°C to 70°C, this would mean a slightly lower factor could be applied, however if you apply the 1.2 factor this gives a higher value of resistance and so errs on the side of caution and provides a safer and more convenient approach.

So my thinking was not wrong. Thank you Richard Burns, helpful as always. Obviously that would be my approach too.

Just one more question: why would cpc be moving from 20 degC to 30 degC? I'm sure it has an obvious answer.

 

[Richard Burns]

Sorry that was just my way of wording things.
In general the ambient temperature in a property in the UK is in the region of 20°C, and this temperature can be assumed to be the temperature at which a test measurement is made.
When cables are in use then the temperature around them can rise and the assumed initial temperature for a cpc that is not part of a cable or bunched with cables is 30°C.
Therefore your measurement is at 20°C and the cpc reference temperature is 30°C, so the cpc is moving from 20°C to 30°C.

 

[Alessandro]

Sorry that was just my way of wording things.
In general the ambient temperature in a property in the UK is in the region of 20°C, and this temperature can be assumed to be the temperature at which a test measurement is made.
When cables are in use then the temperature around them can rise and the assumed initial temperature for a cpc that is not part of a cable or bunched with cables is 30°C.
Therefore your measurement is at 20°C and the cpc reference temperature is 30°C, so the cpc is moving from 20°C to 30°C.

Ok thank you for the explanation!