Grounding is a method of giving electricity the most effect way to return to the service panel. You see current flows from the panel to the outlet or device to power it up. The neutral wire is the return path for unused current. The ground wire is an additional path for electrical current to return safely to ground without danger to anyone in the event of a short circuit. In that instant, the short would cause the current to flow through the ground wire, causing a fuse to blow or a circuit breaker to trip.
An ungrounded electrical box, appliance, power tool, or extension cord could become a danger if there is no path to ground, except through you. You see, without a ground wire, your body may complete the ground path and you may be shocked or electrocuted.
A properly grounded circuit has boxes, devices, and service panel grounds that give the electrical current the easiest path to ground and that reduces the chances of someone getting a shock or getting electrocuted. Household electrical systems are required by the National Electrical Code to have a grounded system connected to earth ground via a ground rod. The earth absorbs the over-current or short circuit harmlessly and having done so, eliminates the threat to anyone that may have otherwise been the ground path.
But ground rods and ground wires to boxes, devices, and service panels are not enough. You must remember to bond the grounds to the house’s copper water lines. You certainly wouldn’t want a short circuit to travel through the water pipes while you’re in the shower, bathtub, or using a sink. Remember that water and electricity don’t mix!
And while we’re on the subject of grounding, I’m often asked if using a receptacle adapter is OK. First of all, I’m not a fan of doing something half way. I’d rather change the receptacle to a grounded receptacle and have the ground wire connected to the receptacle and the box. Although you can use an adapter and connect the center cover-plate screw to the adapter to gain a ground if the box is grounded, it just seems like a skimpy, lazy way to fix the real problem, the need for a new receptacle.
Receptacles that have only a hot and neutral slot are called polarized receptacles. By having a smaller hot wire slot and a larger neutral slot on a polarized receptacle, the electrical current flows the appropriate way through the circuit. For safe use of these receptacles, double-insulated power tools can be used in polarized receptacle circuits. These safety features can reduce the risk of electrical shock, but once again, my advice is to replace the receptacle and rewire if needed to convert the circuit into a properly grounded circuit and receptacle.