What is leakage current in a power supply?

Leakage current is also known as stray current or touch current. It is important to limit leakage current in order to protect users from dangerous touch voltage and prevent faults from occurring in the system.

Leakage current in devices in protection class I is an alternating current flowing through the earthing conductor that is caused mainly by the Y capacitors (Cy) in the EMC filter. Other causes of leakage current are the mounting capacitance of semiconductors and parasitic capacitance in the transformers.

If the earthing conductor is broken, leakage current can flow through the human body and must therefore be limited for safety reasons. For domestic appliances (IEC/EN 60335-1) and information technology equipment (IEC/EN 60950-1), the leakage current for devices in protection class I must not exceed 3.5 mA (0.75 mA for hand-held devices). In the case of stationary equipment (devices with a fixed connection), the leakage current must not exceed 5% of the input current.

Medical electrical equipment that complies with IEC/EN 60601-1 is subject to stricter requirements, depending on whether or not the equipment comes into direct contact with the patient.

Electrical equipment should have a low leakage current because when several devices are in operation (e.g. in the case of redundant systems) the currents are added together.

The leakage currents for various forms of grid can be found in the product data sheets for PULS power supplies.