Page 205 - The City and Guilds Textbook: Plumbing Book 1 for the Level 3 Apprenticeship (9189), Level 2 Technical Certificate (8202) and Level 2 Diploma (6035)

Chapter 3 Scientific principles

Total current in the circuit: IR1 + IR2 + IR3
0.173 + 0.260 + 0.434 = 0.867 amps
V
To find the resistance of each light bulb:
I
230
R1 = = 1329.47 ohms
0.173
230
R2 = = 884.6 ohms
0.260
230
R3 = = 529.9 ohms
0.434
 Table 3.19
R1 R2 R3 Total
Volts 230 230 230 230
Amps 0.173 0.260 0.434 0.867
Ohms 1329.47 884.6 529.9 265.28
Watts 40 60 100 200

Parallel circuit rule: Itotal = IR1 + IR2 + IR3
Ohms law applies:
V 230V
= = 265.28 ohms
I 0.867A
Table 3.19 shows that the voltage remains constant and that the total resistance
in the circuit decreases as the watts and amps increase.

The requirements for earthing of electrical

circuits

The ground that we stand on is a better conductor of electricity than the copper
wires in domestic electrical installations and circuitry. Electricity will always
travel along the line of least resistance, even if that path to earth is us. If you
touch a live source of electricity, the current will always flow through you to the
earth, causing an electric shock and even death. The proper earthing of electrical
circuits is, therefore, of paramount importance for the safe use of electricity.

Here, we will look at the requirements of earthing and the equipotential
bonding of electrical circuits.
Earthing

To prevent damage caused by an electrical fault, electrical installations (lighting
and power) must be earthed. This is done by means of a dedicated earth wire,
which permanently connects each socket, light fitting and switch to a metal
earthing block in the consumer unit (fuse box). When an electrical fault occurs,
the current is carried safely away to earth and the change in the electrical flow
will cause the fuse to blow or the residual current device (RCD) to trip out, cutting
off the supply of electricity. The earthing cable is always coloured green/yellow
and must be installed in 10 mm cable.
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