First you need to understand the need for the extra ground wire. The neutral or White wire is normally grounded. But what can happen is a mis-wired outlet that receives the plug. The other thing is corroded neutral or even broken neutral wire connection. When the housing of an appliance is conductive, like as with a Microwave, or Radiant electric oven or heater, then there is the remote possibility that the surface might become electrified with some potential for electric shock. When your appliance has a plastic or other insulating composite material for the very surface the user touches, then there is no potential for their body to become electric-fryed. So, typically a conductive housing is supposed to be connected to the plug's safety ground plug prong. That safety wire is supposed to be grounded to earth ground in the Circuit Breaker box. To be real safe, inspect your distribution box and earth ground rod below the box for corrosion. Also, make sure every wire terminal is tight inside every outlet and throughout the switch box. If your circuit breaker box is indoors, then you need to verify that there is a real earth ground buss wire with a solid, clean and tight ground rod connection outside, as well.