When GROUND MODE is set to ON, the GPT-
9000/9000A grounds the return terminal to the 
ground. This mode is best for DUTs that are 
grounded to an earth ground by their chassis, 
fixtures or operation environment. This mode 
measures the potential of the HIGH VOLTAGE 
terminal with respect to earth ground. This 
means that any stray capacitance/resistance 
that leaks to earth ground will also be 
measured. This is the safest testing mode, 
though potentially not as accurate. 
When GROUND MODE is set to OFF, the 
return terminal is floating with respect to the 
earth ground. This mode is for DUTs that are 
floating and not directly connected to an earth 
ground. This is more accurate than when 
GROUND MODE is set to ON as any stray 
capacitance/resistance that leaks to the earth