In addition to porcelain insulation, switch type of test products do not provide, the rest of the test products to withstand voltage from low voltage, uniform and relatively fast voltage boost, but must ensure that accurate reading on the instrument. When the test voltage is increased 75%, the test voltage is increased at a rate of 2% per second, the voltage is maintained for a specified time, and then the rapid buck to 1/3 test voltage or lower, to cut off the power supply. After the DC voltage test, the discharge rod is also used to discharge the filter capacitor and the sample. Sudden pressurization or sudden cutting off of power supply at high voltage is never allowed in order to avoid damaging transient overvoltage on the transformer and the test item.