1,060,643. Protective systems. ENGLISH ELECTRIC CO. Ltd. Sept. 9, 1964 [Sept. 12, 1963], No. 35984/63. Heading H2K. To detect arcing faults or other non-linear high resistance faults, e.g. when a conductor of A.C. power supply line falls to earth or when an earth leakage path develops in an underground cable, a protective system produces an output representative of the proportion of higher frequency components in the current through the fault path. The system comprises a corebalance transformer or, as shown, parallelconnected current transformers 13 which produce a signal dependent on the resultant of the line currents, and a transactor 40 which accentuates the higher frequency components of the signal. A series circuit comprising a resistor 41 and a filter 42, 43 is connected across the transactor, the filter presenting a low impedance to the higher frequency components. The voltage across the resistor 41 is proportional to the total signal from the transactor 40 and a transformer 46 connected across the filter produces a voltage dependent only on the fundamental frequency and the lower harmonic frequency components. A comparator unit 47 compares these two voltages and produces an output dependent on the amount by which the fundamental frequency and low harmonic components are less than the total alternating voltage signal, i.e. dependent on the proportion of the higher frequency components in the signal. The output of the comparator is rectified at 17 and operates a relay 18 when it is of sufficient magnitude. A time delay circuit may be connected to the relay 18.