The problem with an IT supply is there is no reference of the mains supply to earth, therefore the applied test voltage (the mains) would not be the full amount. The higher the earth-neutral voltage the less accurate the leakage readings will be. In other words the direct (like 60601) and differential methods need a TN-S supply to give accurate readings. (The N-E voltage should be less than about 7V.)
The differential method is safer for those performing any tests, including anyone else who may be contacting the equipment (or a connected peripheral), as the earth connection is never broken. This is also ideal for items that are allowed a higher equipment leakage current, such as mobile x-ray units (2mA rather than 0.5mA).
The Alternative method is especially useful where a device is PC based so needs a long time to boot up and shut down between test, such as many ultrasound units. No boot up and down required, no switching of mains polarity, no messing around. The measured value will be almost identical as an open neutral 60601-1 test, which is why the limit is 1mA rather than 0.5mA. This is the real time and therefore money saver!
