Final answer:
Some electrical plugs have a third prong to provide a grounding connection that ensures safety by preventing electrical shock, especially if the insulation on live/hot wires wears out. This third prong is part of the three-wire system which includes the live/hot, neutral, and ground wires, with the latter often color-coded in green, yellow, or bare wire.
Step-by-step explanation:
The reason some electrical plugs have a third prong is for safety, specifically for grounding purposes. The three-wire system that incorporates this third prong replaced the older two-wire system to include an earth/ground wire. Without the third prong, insulation wear on the live/hot wire could lead to direct contact with the appliance case, posing a risk of severe shock, especially in environments with water that could provide a good connection to earth/ground. In such an event, a proper earth/ground connection allows the circuit breaker to trip and prevent shock hazards, necessitating appliance repair. However, some appliances with non-conductive cases, like power tools with plastic cases, are doubly insulated and still use two-prong plugs that can be inserted into outlets in only one way to ensure the correct connection of live/hot and neutral wires.
Additionally, the standard three-prong plug design ensures that plugs can only be inserted one way into the outlet to maintain proper connection according to the three-wire system. The three-prong system involves live/hot, neutral, and ground wires, where neutral and ground are connected to earth at different points to make the system safe and dependable. The ground wire is usually color-coded in green, yellow, or bare wire unlike the live/hot and neutral wires, which may vary in color around the world.