Science & Technology in Action

1st Edition

Electricity in the Home

ESB

The distribution of electricity in domestic premises is the subject of this lesson. The different types of circuit and the function of protective devices are described. The important topic of safety is also covered.
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Contains the full lesson along with a supporting toolkit, including teachers’ notes.

Lesson excerpt

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.

True or False?

  1. Fuses protect people from shocks true
  2. Your electricity bill charges for the total voltage you have used false
  3. The green/yellow wire in a plug goes to the earth pin at the top true
  4. A two-way switch is needed to light two bulbs at the same time false
  5. A 6 A fuse can cater for ten 100 W bulbs true
  6. People are good insulators false
  7. When switched on, a kettle uses less electricity than a 100 W bulb (the bulb is obviously much hotter) false
  8. All domestic circuits are in series false
  9. Modern supplies will cut off if an earth-leakage of 50 mA is detected true
  10. The fuse in a plug is in series with the live wire true
  11. A 6 A fuse can cater for a 2.5 kW kettle false
  12. Big fuses are best because they blow less often false