Most of the time reverse polarity plugs present no problem.
It's when something a little out of the ordinary happens that presents a hazard.
The normal wiring in appliances is that the hot wire is connected to the switch in the device, and the voltage is removed when the switch is in the off position, but if the plug is reversed the neutral is now switched off and the voltage is still present in the device somewhere. It's always wise to unplug an appliance before tampering with it, but we don't always do that:
Shock hazard scenario #1: We are toasting an english muffin and it gets stuck in the toaster. We look in the toaster and see that the heating elements are off, so we assume it's safe to stick a knife in the toaster to get the muffin. We should be safe doing this, because the switch that controls the flow of electricity to the heating elements in the toaster shuts off the hot wire. Unfortunately, the toaster is plugged in to an outlet with reversed polarity, so the switch on the toaster is shutting off the neutral wire instead of the hot. This means there is always electricity at the heating elements just waiting for some poor sap to stick a knife in, and that electricity will travel up the knife, through his body, and back to the earth. Breakfast ruined. Should have unplugged the toaster, but we have never had a problem before, and we were in a hurry.
Scenario #2: We're using an old trouble light, and the finger accidentally comes in contact with the outside of the metal socket that holds the light bulb in place. The socket is always connected to the neutral wire, so no big deal... unless the trouble light is plugged in to an outlet with reversed polarity. In this case, we'll get a shock. If this happens while we're laying on the garage floor working on a car, there's a good chance that this could be the last shock we ever get. This can also happen with old table lamps that have exposed metal sockets.
These scenarios are courtesy of Reuben Saltzman, Structure Tech Home Inspections in Minnesota.
