Safety-limit curves for earthing system designs: appraisal of standard recommendations

The tolerable voltage limits and the maximum predicted voltage values calculated using empirical formulae given in five currently applicable standards, namely IEEE 80 (2000), EA-TS 41-24 (1992), BS 7354 (1990), ITU-T K.33 (1996) and CENELEC HD 637 S1 (1999) are compared. The calculated results from CDEGS-HIFREQ are also included. The step, touch and mesh voltages, and hot zones are calculated according to the recommendations given in the five standards, and the differences between them are investigated and clarified. The results show that IEEE 80 with 50 kg body weight touch voltage case gives the lowest tolerable limits whereas BS 7354 gives the highest in most cases. The maximum predicted touch voltage obtained from CDEGS-HIFREQ is higher than that obtained from the empirical formulae of EA-TS 41-24 and BS 7354, and the mesh voltage is lower than that from IEEE 80. The derivation of ´safety limit-curves´ is proposed related to the magnitude and duration of fault current for a given earthing grid design based on the assumptions used in each standard. Following this new proposed procedure, it is established that, if the fault current parameters, i.e. grid current magnitude and duration, are within the safety zone of IEEE 80, they also satisfy the criteria of EA-TS 41-24 and BS 7354.