Three types of flashover tests are conducted on insulators before they pass the test: 1) A power frequency dry flashover test subjects the insulator to a gradually increasing voltage below the minimum flashover voltage for one minute without flashover. 2) A power frequency wet flashover test, or rain test, similarly increases voltage while spraying the insulator with water, looking for flashover within one minute. 3) An impulse frequency flashover test ensures the insulator can withstand high voltage surges from lightning by applying a very high voltage at hundreds of kilohertz and noting the spark-over voltage.