The paper describes the use of the Blackburn a.c. network analyser in analytical and analogue methods of investigating faults on 3-phase power systems. In the analytical approach, after defining the faults in matrix form by symmetrical components, the ensuing equations are solved using the Blackburn analyser without the need for further matrix manipulation.In the analogue method, it is shown how the constraints imposed upon the phase voltages and currents can be applied directly to the sequence networks by the connection of symmetrical-component operational matrices at the fault positions. No knowledge of the relationships between the symmetrical-component currents is required, but it is necessary to know the relationships existing between the phase voltages and currents by virtue of the applied faults.The “equivalent series” connections of the sequence networks, derived recently by Kirschbaum in America for application to the conventionaltype network analyser, are shown to be unnecessary when using the Blackburn machine.Finally, systems containing unsymmetrical simultaneous faults are investigated using both methods.