Isolation Transformer to Perform Line Leakage or Functional Run Tests

Associated Research strongly recommends the use of an isolation transformer or isolated power source for powering up the Device Under Test (DUT) when performing the Line Leakage or Functional Run tests. This white paper covers the various reasons for isolating the DUT power source during these types of tests. First and foremost we need to always consider minimizing the operator risk for electrical shock during any safety testing. Using an isolation transformer or an isolated power source can greatly reduce the potential shock hazard for the operator. Some Line Leakage standards direct the operator to cycle the power switch of the DUT on and off. Contacting the DUT under an open ground condition could expose the operator to a potential shock hazard as the enclosure of the DUT can have a voltage induced on it that is referenced to earth ground. This could be hazardous since the operator is also grounded. For this reason it is suggested that an isolated power source be used to power the DUT. If the output of the power source is not earth ground referenced then the shock hazard from contacting the case of the DUT and earth ground is eliminated. Figure 1 shows a circuit that incorporates an Isolation Transformer which is the preferred method. The current leakage path to earth ground has been eliminated with the use of the isolation transformer. This effectively eliminates the shock hazard.