As stated previously, floating the scope is not a good idea. As shown on this slide, hazardous voltage can be exposed on the frame of the scope itself, power supplies can be stressed, there is an ill-defined load placed on the circuit, etc. If the reference voltage is large, there is a possibility of damaging components in the scope power supply or power line filter from the dielectric breakdown. This type of damage may not show up immediately, but appear later even after the scope is returned to grounded operation. The capacitance between the scope case and earth ground causes several problems. One is the possibility of damaging the device under test. Unlike the probe center lead (tip) which isolates the capacitive load of the scope and probe cable from the DUT with a large resistor, the probe “ground lead loads the circuit with the full capacitance between the scope case and ground. Some circuits can be damaged by this capacitive load. In power systems, the upper gate drive in a half bridge output stage is particularly sensitive. If the ground lead is connected to the upper IGBT gate rather than the emitter (the probes leads accidentally reversed), the capacitance will act on the gate drive source impedance. The resulting time constant may slow the gate turn off to the point where the upper IGBT does not turn off before the lower one turns on, instantly destroying the output stage. In circuits with large amounts of stored energy, the devices may explode, possibly injuring the user. A significant problem with floating the scope which many users may not be aware of is corruption of the measurement. This is also an artifact of the capacitance between the scope case and earth ground. The ground lead inductance acts on the capacitance to ground to create the series LC resonate circuit, just as it does with the scope input C. However in this case, the circuit resonates with the common mode signal, not the signal being measured. Because this capacitance is much larger than the scope input C, the resonant frequency is much lower. Thus, when measured with a floating scope, the waveform of a typical upper gate drive signal often has a significant ring which is really not present in the true signal. This measurement distortion is large enough to make many routine measurements, such as upper gate drive, virtually impossible. Users often place an isolation transformer in series with the scope’s power cable to reduce the potential of damaging the scope and to reduce the capacitance to ground. However, even the best transformers will only lower the capacitance to around 130pF. This value may still damage some loads, and cause severe measurement distortion.