Insulators are non-conductive, but in reality, almost no insulating material is absolutely non-conductive. When a voltage is applied across any insulating material, a certain current will always flow through it. The active component of this current is called leakage current, and this phenomenon is also known as insulation leakage.

 

For electrical appliance testing, leakage current refers to the current formed through the surrounding medium or insulating surface between mutually insulated metal parts, or between live parts and grounded parts, when no fault voltage is applied. According to the US ULstandard, leakage current includes the capacitively coupled current that can be conducted from accessible parts of household appliances. Leakage current consists of two parts: one is the conduction current I1 through the insulation resistance; the other is the displacement current I2 through the distributed capacitance. The capacitive reactance of the latter is XC=1/2pfc, which is inversely proportional to the power frequency. The distributed capacitance current increases with frequency, so the leakage current increases with the power frequency. For example, when using thyristor power supply, its harmonic components increase the leakage current.