DO you mean leakage current or magnetising current, that is the small current in the primary in the off-load state? True leakage current can be difficult to measure if the leakage inductance and capacitance are small.
To measure magnetising current just connect an ammeter in series with the 120V primary winding, ensuring that all current in the secondary winding is zero (disconnect the wiring may be the easiest way to ensure this). For a good transformer the magnetising current should be extremely low, certainly under 1 percent of the current at maximum load.
Brian,i meant the leakage current itself.
i was advised to connect a 500 ohm resistor between one of the input terminals at a time and to the core of the transformer.by measuring the voltage drop across the resistor,the leakage current can be calculated which should come less than 5mA as per UL standards if I'm right.The transformer should be fully loaded while doing the test.I didnt get a good resulf.What do u have to say about this testing?Please advise.
You mention 5 mA as a UL limit. I assume you are referring to the UL1950 standard. Other standards have different leakage current limits, the most common being 3.5 mA for non medical devices, 300 uA for cord connected, non patient contact medical devices and 5mA for "Hard wired" medical devices. In all cases, the methodology for taking the measurements are outlined within each standard. I suggest you get a hold of your regulatory person/agency and ask them how you should do an "engineering evaluation" of the leakage current. Measurements taken for the record incorporate specialized Leakage Current meters and require specific power sources.
