Earth fault loop impedance (Zs) is the impedance in ohms of the circuit path formed during a fault between earthed metal work and live parts. It includes the external impedance circuit (Ze) and the resistance of the phase and circuit protective conductors (R1 + R2).
Ze can be obtained by enquiry, calculated or measured and values of R1 + R2 per meter length are obtained from Table 9A of the IEE On Site Guide and need to be multiplied by a temperature correction factor (Cr) found in Table 9C. The value of Zs must be less that the maximum permitted values (Zmax) in Table 41.2, 41.3 or 41.4 of BS7671 .
Most electrical systems in the UK are constructed so that protection against electric shock is provided by basic insulation, barriers and enclosures as a first measure and also by limiting the magnitude and duration of voltages that appear under earth fault conditions between simultaneously accessible exposed conductive parts of equipment, and between them and extraneous conductive parts or earth. By selecting the correct cable and protective device and ensuring that all exposed conductive parts are connected to the main earthing terminal, protection against electric shock can be achieved if the maximum value of earth fault loop impedance is not exceeded.