Comparison Studies of Protective (EG) Conductor Sizing

Low voltage system protective (equipment grounding) conductor sizing can be based on thermal capability calculations. Some international or national standards also provide a table to allow users simply to select the Protective Conductor (PE) size based on the phase (line) conductor size. IEC 60364-5-52, BS 7671, and NF C15-100 all provide the same table. This paper provides comparison studies for protective conductor sizes based on standard table and thermal calculation. The studies consider different scenarios such as phase conductor sized based on short circuit conditions only, phase conductor sized based on loading conditions while meeting short circuit conditions. Short circuit current variations and fault clearing time variations are analyzed. Protective conductor and phase conductor of different material types, and different protective devices are also studied. From the studies a conclusion can be made that in some cases the table method may provide protective conductor sizes that do not meet the short-circuit thermal capability requirements.