1. Mains Supply
Noadswood Science, 2012

2. Mains Supply To understand the mains supply to your house
Saturday, April 15, 2017
To understand the mains supply to your house

3. Mains Supply
The UK mains electricity supply is about 230V and can kill if not used safely
Electrical circuits, cables, plugs and appliances are designed to reduce the chances of receiving an electric shock
The more electrical energy used, the greater the cost, and electrical supplies can be direct current (d.c.) or alternating current (a.c.)

4. Mains Circuit Every mains circuit has a live wire and a neutral wire
The current through a mains appliance alternates because the mains supply provides an alternating potential difference between the two wires

5. Alternating & Direct Current
Alternating current and direct current are different electrical supplies
The battery in a torch makes the current flow around the circuit in one direction only (it is a direct current (d.c.))
Mains electricity is alternating current (a.c.) which repeatedly reverses its direction (flowing one way, then in the opposite direction in successive cycles) – its frequency is the number of cycles per second (in the UK the mains frequency is 50 cycles per second (50Hz))

6. Frequency
Why would a much lower frequency than 50Hz be unsuitable for a light bulb?
The bulb would flicker continuously

7. Direct Current
If the current flows in only one direction it is called direct current (d.c.)
Batteries and cells supply d.c. electricity, with a typical battery supplying maybe 1.5V
The diagram shows an oscilloscope screen displaying the signal from a d.c. supply: - 0V

8. Alternating Current
If the current constantly changes direction, it is called alternating current (a.c.)
Mains electricity is an a.c. supply, with the UK mains supply being about 230V - it has a frequency of 50Hz (50 hertz), which means it changes direction, and back again, 50 times a second
The diagram shows an oscilloscope screen displaying the signal from an a.c. supply: - 0V

9. Alternating Current – Higher
The potential difference of the live terminal varies between a large positive value and a large negative value – however, the neutral terminal is at a potential difference close to earth, which is zero
The diagram shows an oscilloscope screen displaying the signals from the mains supply – the red trace is the live terminal and the blue trace the neutral terminal
Note that, although the mean voltage of the mains supply is about 230V, the peak voltage is higher

10. Assessment Connect a low voltage a.c. supply unit to an oscilloscope
The trace on the oscilloscope screen shows that the p.d. increases and decreases continuously
The highest p.d. is reached at each peak – increasing the p.d. of the a.c. supply makes the wave on the oscilloscope taller

11. Assessment
Increasing the frequency of the a.c. supply increases the number of cycles (the waves on the screen get squashed)
Now connect a battery to the oscilloscope – you should see a flat line at constant potential

12. Signals

13. Oscilloscope – Higher
The oscilloscope can measure the peak p.d. and the frequency of a low voltage a.c. supply
For example, an oscilloscope may be set to the following: -
Y-gain control at 0.5V/cm
Time base control of 10 milliseconds per centimetre (ms/cm)
If the peaks are 8.4cm above the troughs, then they are 4.2cm above the middle (0 p.d.)
If the Y-gain control is set to 0.5V/cm then we know each centimetre of height is due to 0.5V – so the peak p.d. is 2.1V (0.5V/cm x 4.2cm)
The each cycle across the screen is 8cm across, the time base control set at 10ms/cm tells us each centimetre across the screen is a time interval of 10ms, so one cycle takes 80ms (frequency of 12.5Hz (1/0.08s)

14. Oscilloscope – Higher
The figure below shows how the potential of the live wire varies with time: -
+400
+325 V
+200
Time (s)
Voltage (V)
0.01
0.02
0.03
-200
-325 V
-400

15. Oscilloscope – Higher The live wire alternates between +325V and -325V
In terms of electrical power, this is equivalent to a direct voltage of 230V
Each cycle takes 0.02 seconds, so the mains supply alternates at 50 cycles every second (50Hz)
What is the maximum potential difference between the live wire and the neutral wire?
325V (+/-325V positive, 0V negative)

16. Questions
Choose the correct potential difference from the list for each appliance: 1.5V; 12V; 230V; and 325V
A car battery
The mains voltage
A torch cell
The maximum potential of the live wire
Car battery = 12V
Mains voltage = 230V
A torch cell = 1.5V
Maximum live potential = 325V