Characteristic of AC surface spark voltage in LN/sub 2/ and LHe

The AC surface spark-over voltages on solid insulators with a back electrode in liquid N/sub 2/ (LN/sub 2/) and liquid He (LHe) were measured and discussed. The larger gap length did not give a higher spark-over voltage. The spark-over voltage in LHe was about two-thirds of that in LN/sub 2/. Experimental studies were carried out on how bubbles in liquid influence surface spark-over voltage. The result shows that it does not influence the voltage.