In the field of electrical power systems there is provided a method of supervising the operation of a plurality of current transformers that are arranged in respective current transformer within differing protection zones of a multi-phase electrical power system. The method of supervising comprises the steps of: (a) identifying a faulty protection zone within the electrical power system; (b) identifying a faulty current transformer group within the faulty protection zone; and (c) identifying faulty phase(s) within the faulty current transformer group within the faulty protection zone.