Introduction to Health and Safety at Work

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 being a particu-
larly 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. 
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 fl ows directly to 
earth rather than to a piece of equipment, a 
short cir-
cuit
is made. Since the resistance is greatly reduced but 
the voltage remains the same, a rapid increase in current 
occurs which could cause signifi cant problems if suit-
able protection were not available.
Earthing
The electricity supply company has one of its conduct-
ors solidly connected to the earth and every circuit sup-
plied by the company must have one of its conductors 
connected to 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 fl ow to earth so as 
to protect people from electric shock and equipment 
from overheating. Good and effective earthing is abso-
lutely essential and must be connected and checked by 
a competent person. Where a direct contact with earth 
is not possible, for example, in a motor car, a common 
voltage reference point is used, such as the vehicle 
chassis.
Where other potential metallic conductors exist 
near to electrical conductors in a building, they must 
be connected to the main earth terminal to ensure