Leakage Current vs. Dielectric

For modern insulation material, the insulation resistance is very large, so resistive current is typically several orders of magnitude lower than the capacitively coupled current. The spark gap represents the avalanche (breakdown) phenomenon, where a good insulator turns bad after a certain voltage is reached.

When a voltage is applied to an electrical or electronic device, the natural capacitance of the insulation charges, resulting in the flow of current. If a DC voltage is applied, the charging occurs only when voltage is first applied. The

DC voltage charges the capacitance and the capacitive charging current is then reduced to near zero.

For 60 Hz AC voltages, the charging and discharging of this capacitance occurs 60 times each second, so the capacitive charging current remains at a steady-state level and never goes to zero. This is known as capacitive leakage current. For modern electronic devices, the resistive leakage current is normally very small, so the overall leakage current of such devices is dominated by the capacitive effect. To protect users from injury, many product safety standards set limits for the allowable leakage current.

A typical leakage current limit is 0.5mA at 60 Hz and at the rated voltage. Electrical and electronic devices also exhibit current analogous to leakage current during dielectric testing. However, since the dielectric test voltage is much higher, the current will be higher as well because the capacitive charging current increases in proportion to the increased voltage.

 

The dielectric strength test is not intended to evaluate the amount of leakage current, as these two tests have fundamentally different objectives and compliance criteria. The leakage current limit is only applicable to the leakage current test, and only at the rated voltage.