Final answer:
High voltages are used in transmission lines for efficiency in long-distance power distribution, while lower voltages in household circuits are for consumer safety.
Step-by-step explanation:
The use of high voltages in transmission lines is primarily for efficiency in power distribution. High voltages allow the electricity to be transmitted over long distances with minimal energy loss due to the resistance of the wires. This is why transformers are used to step up the voltage to very high levels for transmission, and then step it down to safer, lower voltages before it enters a household.
Household circuits, on the other hand, operate at lower voltages for safety reasons. The standard voltage for household circuits is sufficient to run appliances and is low enough to reduce the risk of electric shock and fire.
In conclusion, the difference in voltage levels is a balance between achieving the most efficient long-distance transmission and maintaining consumer safety.