High voltage circuits for application of composite voltages to test gaps

There is a need for high voltage laboratory testing using 'composite voltages', where impulse and either direct or alternating voltages are combined, The central problem it; that of isolating the circuits prior to, and after, the impulse. This is achieved either with a capacitor or a spark gap, so that the voltages are respectively truly superimposed or, alternatively comprise a peak composite voltage equal to the impulse voltage but preceeded by the direct or alternating voltage. A variety of combined circuits is possible, some practical compromises are described and resulting efficiencies of the circuits compared where switching impulses are used.