Method and apparatus for isolating and switching lower voltage pulses from high voltage pulses in electrocrushing and electrohydraulic drills. A transformer with a high permeability core acts as a magnetic switch or saturating inductor to switch high voltage pulses to initiate an electrocrushing arc and lower voltage pulses to sustain the arc. The transformer isolates the lower voltage components from the high voltage pulses, and switches to deliver the low voltage current when the core saturates. The transformer enables impedance matching to the arc during all stages of drilling. The saturation time of the transformer core is the time delay between initiation of delivering the high voltage pulse and initiation of delivering the lower voltage current.