This paper presents an experimental investigation into operation of a triggered, two electrode switch energised with a HV impulses superimposed over a DC charging voltage. A sphere-sphere topology with an electrode separation of 2 mm was DC energised to 6 kV, 7 kV and 8 kV and triggering impulses of varying dV/dt have been used. Breakdown voltage, time delay to breakdown and jitter have been measured for air, nitrogen and a 60% nitrogen/40% oxygen mixture. It is shown that higher dV/dt and higher DC energisation provide more stable and shorter time delays to breakdown.