Grounding and AC Applications w/ Microchip ICD 3
The ICD is connecting the PIC and its application circuit's 0V reference to the PC, which normally has a path through to the mains earth. Therefore you cannot power up your application circuit in a way which raises the 0V above or below earth potential without causing problems. If your application circuit is powered from a floating output eg from a wall wart or bench PSU, you would not have a problem. The problem normally comes when someone is developing a circuit which is directly mains powered, so that the PIC 0V is connected or referenced from live or neutral i.e. without an isolating transformer. In that case, you are well advised to use a USB isolator between the ICD and the PC, and an isolation transformer and ELCB for your circuit.