The "input" and "output" terminals of a isolation transformer should be short-circuited to test their insulation resistance against the ground wire by a megohmmeter. When measuring by 1000V megohmmeter, the resistance value is greater than 2M ohms. The cross-sectional area of the isolation transformer input and output power lines should meet the requirements of its current value. It is appropriate to configure the electric current density of 2 ~ 2.5a /mm2.