Contains the full lesson along with a supporting toolkit, including teachers’ notes.
How is electricity supply delivered to a house?
The ESB maintains a national network of high tension power lines. The voltages on these lines are as high as 400 kV. Such voltages are not delivered to domestic premises. Instead the voltage is stepped down at transformers to a more suitable lower value. A two-wire cable is used to bring electricity to the home. The neutral is connected to a metal plate in the ground, and so is at 0V. Therefore, theneutral is said to be earthed. The voltage on the live wire alternates to 311V above and below this 50 times a second giving an effective 220V. At the house, a main fuse and meter are installed.
How is electricity distributed in the house?
The cable is then terminated on a consumer unit. From theconsumer unit different circuits are run to the various points where electricity is needed - lights, sockets, cookers etc. These circuits are all fed with the 220V from the consumer unit, so all domestic circuits are in parallel.
The live wire connection to every circuit is fed through a safety device which cuts off the circuit if the current demanded becomes excessive. This can be a fuse or a miniature circuit breaker (mcb).
The conductors in the circuit are copper and are insulated. Theresistance must be low enough that,
• at the rated current, the drop in voltage between the consumer unit and the load is low
• the copper does not get too hot.
Cables with large diameters have less resistance than those with smaller diameters. In general, domestic circuits are classified aslighting circuits or power circuits.