Introduction to Health and Safety at Work

12.2 
Principles of electricity and some 
defi nitions
12.2.1
Basic principles and measurement of 
electricity
In simple terms, electricity is the fl ow or movement of 
electrons through a substance which allows the trans-
fer of electrical energy from one position to another. The 
substance through which the electricity fl ows is called a 
conductor
. This fl ow or movement of electrons is known 
as the 
electric current
. There are two forms of elec-
tric current – direct and alternating. 
Direct current (dc)
involves the fl ow of electrons along a conductor from one 
end to the other. This type of current is mainly restricted 
to batteries and similar devices. 
Alternating current (ac)
is produced by a rotating alternator and causes an oscil-
lation of the electrons rather than a fl ow of electrons so 
that energy is passed from one electron to the adjacent 
one and so on through the length of the conductor.
187
12
Electrical hazards 
and control
Figure 12.1
Beware of electricity – typical sign.

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Introduction to Health and Safety at Work
188
It is sometimes easier to understand the basic 
principles of electricity by comparing its movement with 
that of water in a pipe fl owing downhill. The fl ow rate of 
water through the pipe (measured in litres/s) is similar 
to the current fl owing 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 fl ow 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 fl ow rate through the pipe will also vary for 
a fi xed pressure drop as the roughness on the inside 
surface of the pipe varies – the rougher the surface, the 
slower the fl ow and the higher the resistance to fl ow 
becomes. Similarly, for electricity, the poorer the con-
ductor, 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 
(
Volts
)
and, electrical power (
P
) is given by:
P 
⫽
 V 
⫻
 I
(
Watts
)
These basic formulae enable simple calculations to 
be made so that, for example, the correct size of fuse 
may be ascertained for a particular piece of electrical 
equipment.