Final answer:
Electrically conductive appliances like those with metallic bodies require a three-pin plug to ground the appliance and prevent electric shock, while non-conductive appliances can safely use a two-pin plug due to their double insulation.
Step-by-step explanation:
Electrical appliances with metallic bodies are connected to the mains through a three-pin plug for safety reasons. The three-wire system includes a ground wire in addition to the live/hot and neutral wires, creating a safer electrical environment. The ground wire provides a low-resistance path to the earth, helping maintain the case of the appliance at zero volts relative to the earth. This is particularly important if the live/hot wire's insulation becomes worn and makes contact with the appliance's case, which could lead to a severe shock. This scenario is risky especially in places like kitchens where water can provide a better connection to earth.
In contrast, appliances like electric bulbs that have nonconducting cases may not require grounding and hence come with two-prong plugs. These two-prong plugs are often used for appliances that are doubly insulated, meaning the nonconducting case itself provides sufficient protection from electric shocks. Furthermore, modern two-prong plugs are designed asymmetrically so they can only be inserted one way into the outlet, ensuring correct polarity of the live/hot and neutral wires.