R1+r2 reading and r1,r2 and rn on ring final

[Abu]

 

[Richard Burns]

In design or initial verification of a circuit you do the calculation to ensure it is safe to power up. Once the circuit is powered then measurement of Zs is a standard, fairly reliable and common method of ensuring that your circuit is compliant with the disconnection times required. Measured Zs can be lower than calculated Zs and so the calculation is the worst case scenario used for safety.

 

[Abu]

If you have the On Site guide then in table 7.1 in chapter 7 this lists the maximum length of cable permitted for standard circuits in various installation methods and with different protective devices.
Then you can compare the circuit lengths with the resistance values given in appendix I to give you maximum values for resistance.

For instance a 2.5/1.5 ring final circuit on a 32A MCB with 30mA RCD protection has a maximum length of 106m. (this is limited by voltage drop)
Therefore on 106m your r1 and rn would be 0.00741*106 =0.785Ω
your r2 would be 0.0121*106=1.28Ω

Similarly for a 1.0/1.0 6A lighting circuit the max length is 59m and so R1+R2 would be 0.0362*59=2.14Ω.

Where do u get the value of 0.00741?

 

[Richard Burns]

Where do u get the value of 0.00741?

Appendix I of the on site guide; the resistance of a 2.5mm² conductor is 7.41mΩ/m which is equivalent to 0.00741Ω/m.

 

[Toneyz]

In design or initial verification of a circuit you do the calculation to ensure it is safe to power up. Once the circuit is powered then measurement of Zs is a standard, fairly reliable and common method of ensuring that your circuit is compliant with the disconnection times required. Measured Zs can be lower than calculated Zs and so the calculation is the worst case scenario used for safety.

Zs includes parallel earth paths eg structural metal of the building ,bonding etc. which rightly stated lowers the measured Zs value also if the circuit has a RCD/rcbo the test current used is lowered as not to trip the RCD/rcbo during the test therefore you get a higher Zs .