If your house was wired in the last 30 years, you probably have properly grounded outlets. This is easily checked with a meter. If your 500 is plugged in but turned OFF, the computer's ground plane is connected thru the house wiring to "earth". When these conditions are met, your computer is pretty safe.
Far more Amiga ports have been fried by pluggin and unplugging stuff (like joy sticks, printers, modems) while the computer is powered ON.
