This would not be advisable to perform a 500V resistance test on the USB sockets.
"Insulation resistance test cannot be performed on the USB sockets as the electronic circuitry inside the SMPS could easily be damaged by a 500 V insulation test injected across it. Consequently the USB port may fail to function following a 500 V Line to Neutral insulation test. Hence it is not recommended but the all the insulation resistance for the socket USB is as per EN 60950-1 & BS 5733."
"Insulation resistance test cannot be performed on the USB sockets as the electronic circuitry inside the SMPS could easily be damaged by a 500 V insulation test injected across it. Consequently the USB port may fail to function following a 500 V Line to Neutral insulation test. Hence it is not recommended but the all the insulation resistance for the socket USB is as per EN 60950-1 & BS 5733."