For INS testing, due to the following two reasons, insertion of an adequate isolation transformer is indispensable.

1)
INS injection circuit has a capacitor between each L (line) and PE (protective earth). Since the capacitance is relatively high with the value of 0.3uF, the AC source frequency 50/60Hz leaks through this capacitor. A calculated value of this leak current is 18mA for a 120V/50Hz AC power system. Similarly, for a 60Hz system, the value should be 1.2 times, also similarly, the value should be 1.8 time for a 220V system. The GFCI (ground-fault circuit interrupter) installed in the facilities is sure to trip detecting this ground fault current.

2) In normal (line-to-line) mode testing, a wire of the AC is connected to the signal ground of the INS, meaning AC voltage may exist at the outer conductor of the coaxial connector. This arises a risk of an electric shock. Isolation of the power source driving the EUT can completely eliminate the above mentioned 1) and 2) events. More specifically, an isolation transformer, which isolates the primary of the AC, should be inserted between the AC power source and the LINE Input of the INS.