Faults in parallel cables

In dealing with the protective requirements concerning devices for the fault protection of circuits with several multicore power cables in parallel, it may be insufficient to consider only the equivalent impedance of the parallel conductors. It is shown in this paper that for a fault in one of the parallel cables, depending on the fault position, the effective impedance of the parallel conductors in the fault condition, called `effective impedance¿¿ here, can be larger than the equivalent impedance. The paper examines relations between the effective impedance and fault position as well as parameters such as conductor impedance and number of conductors in parallel, with particular emphasis given to the `maximum effective impedance¿¿. It is shown how the effective impedance compares with the equivalent impedance. Some expressions are derived for use in design calculations. It is concluded in particular that the effective impedance can significantly influence the fault protection of the parallel cables and may preclude using the metallic covering of the cables as a protective conductor.