6.9 Leakage Current (ILK)
Transformers may be used to provide isolation between dangerous line input voltages and safe low voltage outputs. When the primary is energised, there exists a 110 or 230V ac common mode voltage from primary to ground. Stray resistance and capacitance then provides a path for some 'leakage' current to flow from the primary side to ground. This leakage current may affect the proper operation of the complete product or present an electric shock hazard to the user.
A leakage current test may be performed on any ferrite or iron cored transformer. It is an especially important test for transformers that are to be incorporated into medical equipment where a patient may become the path for leakage current to flow to ground. The test confirms that the leakage current is below the level set by a safety standard or the transformer designer.