The earth fault loop impedance (Zs) directly effects the amount of current that flows under earth fault conditions. (Ipf = V / Zs).
As I’ve mentioned in a previous post (see here) fuses and circuit breakers need a certain amount of current in order to meet their maximum disconnection times. ie a 20A type B circuit breaker would require 5 x its rating (100A) in order to meet its disconnection times.
If the earth fault loop impedance is too high then disconnection of the protective device will take too long due to the earth fault current not being high enough for the protective device to meet its maximum disconnection time.
This is why we have the maximum loop impedance (Zs) values in Chapter 41 BS 7671. Remember though. When testing, the results measured are compared to 80% of these value in Chapter 41.
Components of an Earth Loop Impedance Path (Zs)
Zs = Ze + (R1 + R2)
Zs: Overall loop impedance including the source and the installation
Ze: Loop impedance external to the installation
R1: Impedance of the line conductor of the circuit
R2: Impedance of the CPC protecting the circuit