Final answer:
A switch with a pilot light and a relay in a three-wire system each require a minimum of three wires, which include live/hot, neutral, and ground wires. An additional fourth wire may be needed for the pilot light.
Step-by-step explanation:
When dealing with a three wire relay system, the switch with a pilot light and the relay must each have a minimum of three wires. These wires typically include the live/hot wire, the neutral wire, and the ground wire. In a three-wire system, the ground wire plays a crucial role in ensuring safety by providing a path for fault current to flow back to the ground. The addition of a pilot light in the switch may require an additional wire to illuminate the light when the switch is on. Thus, at least three wires are needed for the functional operations of the circuit, and potentially a fourth wire for the pilot light, depending on the specific design of the switch.
The three-wire system is designed to guard against thermal and shock hazards, offering several safety features such as grounding the neutral wire and the appliance case, as well as incorporating a circuit breaker or fuse for protecting against thermal overload. This grounding maintains the neutral wire at zero volts and provides an alternative return path for the current through the earth. The three holes in a standard three-prong plug ensure that it can only be inserted one way, to maintain the proper function of the three-wire system.