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It is sometimes easier to understand the basic principles of electricity by
comparing its movement with that of water in a pipe flowing downhill. The flow rate
of water through the pipe (measured in litres/s) is similar to the current flowing
through the conductor which is measured in amperes, normally abbreviated to amps.

Sometimes very small currents are used and these are measured in milliamps
(mA). The higher the pressure drop is along the pipeline, the greater will be the flow
rate of water and, in a similar way, the higher the electrical ‘pressure difference’
along the conductor, the higher the current will be. This electrical ‘pressure
difference’ or potential difference is measured in volts.
The flow rate through the pipe will also vary for a fixed pressure drop as the
roughness on the inside surface of the pipe varies – the rougher the surface, the
slower the flow and the higher the resistance to flow becomes. Similarly, for
electricity, the poorer the conductor, the higher the resistance is to electrical current
and the lower the current becomes. Electrical resistance is measured in ohms.
The voltage (V), the current (I) and the resistance (R) are related by the
following formula, known as Ohm’s law:
V=I·R
and electrical power (P) is given by:
P=V·I.
These basic formulae enable simple calculations to be made so that, for
example, the correct size of the fuse may be ascertained for a particular piece of
electrical equipment.

Conductors and insulators
Conductors are nearly always metals, copper being a particularly good conductor, and are
usually in wire form but they can be gases or liquids, water is a particularly good conductor of
electricity. Super-conductors is a term given to certain metals which have a very low resistance to
electricity at low temperatures.
Very poor conductors are known as insulators and include materials such as rubber, timber,
and plastics.
The insulating material is used to protect people from some of the hazards associated with
electricity.

Short circuit
Electrical equipment components and an electrical power supply (normally the mains or a
battery) are joined together by a conductor to form a circuit.
If the circuit is broken in some way so that the current flows directly to earth rather than to
a piece of equipment, a short circuit is made. Since the resistance is greatly reduced but the voltage
remains the same, a rapid increase in current occurs which could cause significant problems if
suitable protection were not available.

Earthing
The electricity supply company has one of its conductors solidly connected to the earth and
every circuit supplied by the company must have one of its conductors connected to the earth. This
means that if there is a fault, such as a break in the circuit, the current, known as the earth fault
current, will return directly to earth, which forms the circuit of least resistance, thus maintaining
the supply circuit. This process is known as earthing. Other devices, such as fuses and residual
current devices, which will be described later, will also be needed within the circuit to interrupt the
current flow to earth so as to protect people from electric shock and equipment from overheating.

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