Final answer:
The toaster's electrical cord is covered in plastic to insulate and prevent electric shocks by ensuring that electric charge doesn't escape outward. Plastic and rubber materials are used as insulators in power cords for safety reasons, including the three-wire system with an additional ground wire for enhanced protection.
Step-by-step explanation:
The toaster electrical cord is covered in plastic because the plastic acts as an insulator. Insulators, like rubber and plastic, are materials that do not allow electric charge to escape outward, preventing electric shocks. In the toaster's three-wire system, the live/hot and neutral wires are insulated to prevent the case from being directly in the circuit, thereby avoiding potential shocks. Moreover, the earth/ground wire provides additional safety by triggering the circuit breaker to trip if the live/hot wire's insulation wears out and contacts the case, particularly useful in environments like kitchens where water provides a good grounding path. Some appliances may still have two-prong plugs if they have nonconducting, doubly insulated cases.