Final answer:
To tell if a plug has a ground wire, it should have a three-prong configuration. The third prong provides a ground wire, which is a safety feature absent in two-prong, non-grounded plugs. Four-prong plugs also have a ground but are used in specific applications.
Step-by-step explanation:
To determine if the plug you are using has a ground wire, you should look for a three-prong configuration. According to the information provided, a standard three-prong plug is designed to be fitted in only one way to assure the proper function of the three-wire system. The standard three-wire power system includes a live/hot wire, a neutral wire, and an earth/ground wire. The ground wire is often color-coded green, but may also be yellow or bare wire.
Legacy two-wire systems lack such an earth/ground wire. Some two-prong plugs are still used for appliances with nonconductive cases, described as doubly insulated. However, these do not provide the safety feature of grounding. On the other hand, four-prong configurations are primarily used in industrial or special home applications, like dryer outlets, and also include grounding.
The presence of the third prong is what identifies a plug as grounded. As shown in Figure 23.31, the earth/ground connection prevents electric shock by providing a low-resistance path to the earth, especially helpful if there is worn insulation on the live/hot wire.