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Basics of Electricity
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all standard electrical conductors,
both bare and with various types of insulation, are listed in BS7671 (Wiring Regulations 16th
edition).
Overcurrents
Conductors are selected and installed based upon the maximum current they can be expected to
carry. Unforeseen events can occur that raise the actual current above a conductor's maximum
rating. When this happens, the conductor can be damaged and other catastrophic events can develop
if circuit protection is not incorporated.
One of the most common causes of excessive current is an overloaded circuit. This is because
the greater the load on a circuit, the greater the current flow.
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Typical example is a domestic power
socket with too many appliances operating at the same time.
Another type of excessive current is the short circuit current. A short circuit is commonly
caused by two current carrying bare conductors accidentally touching each other. Normal resistance
(load) of the circuit is by-passed, and a very large current flows. A magnitude of 10 or 20 times
normal current is not unusual.
Figure 1.24 shows an example of a circuit whose resistance is 24 ohms and voltage
is 240 volts. Using Ohm's Law, current flow is found to be 10 amps. A short circuit develops and
the resistance of the circuit drops to 24 ten thousandths of an ohm. (There is always a slight
resistance in a short circuited circuit.) The short circuit current flow skyrockets to 100,000
amps.
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Figure 1.24. Current flow increases with short circuit.
Fuses and Circuit Breakers
These are current sensitive devices incorporated to protect the electrical system from
overload and short circuit conditions. If there is an increase in current above a safe limit the
device will switch off the circuit or a fuse will fail and thus make the circuit incomplete.
When a fuse fails the reason should be investigated by a qualified person before the circuit
is reinstated.
Under no circumstances should higher rated fuses be fitted as this will compromise the
electrical safety of the complete installation which could lead to damaged equipment or personal
injury. The risk of fatality from electrical shock depends upon several factors, but the higher the
voltage the greater the risk. 240 VOLTS ARE LETHAL.
Circuit Protection
There are three methods of circuit protection in common use, which are:
1. Fuse
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