As a pant of a programme of work on overhead line compaction, at the High Voltage Laboratory of the A.U.Th., a tower - line clearance stressed by impulse voltages has been studied. The effect of a pre - stressing negative voltage was also studied as in our previous work concerning a positive conductor - rod gap with the rod pre - stressed by a negative D.C. voltage; this work showed a higher gap factor than other gap configurations. Since in a conductor - plane gap the existence of a pre - stressing negative D.C. voltage shows no influence, we investigate the effect of inserting rods on the plane in order to study the breakdown mechanism of such gaps in atmospheric air. The length of the rods, and the values of the negative D.C. voltage were taken as parameters. Furthermore the above mentioned results compared with the results taken from a conductor - plane gap stressed only by a positive or negative D.C. voltage in order to obtain more information concerning the breakdown mechanism. Also photographs were taken for gaps stressed with both D.C. polarities as well as for conductor - plane gaps.
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