What is the purpose of the third wire in a three wire cord?

The purpose of the third wire in a three-wire cord is to serve as a mechanical ground. This wire is essential for safety, as it provides a path for fault current to flow safely to the ground in the event of a short circuit or equipment failure. By connecting the equipment to an effective ground, it helps prevent the risk of electric shock to users and ensures that protective devices, such as circuit breakers or fuses, function correctly to cut off power in case of a fault.

The ground wire is crucial in maintaining the safety of electrical systems, especially in environments where the risk of electrical shock is a concern, such as in machine rooms or areas with heavy electrical equipment. It involves ensuring that any stray current from insulation failures or short circuits is directed away from individuals and the equipment itself.

In contrast, the other functions described in the choices do not accurately represent the role of the third wire. While a power source typically pertains to the live wires providing energy, electrical return refers to the neutral wire carrying current back to the source. A protective circuit might involve additional components designed for safety, but it does not specifically define the grounding function that the third wire fulfills.