Electrical hazards and control
191
not properly installed and maintained. The most common
causes of fi re in electrical installations are short circuits,
overheating of cables and equipment, the ignition of fl am-
mable gases and vapours, and the ignition of combustible
substances by static electrical discharges.
Short circuits happen, as mentioned earlier, if insula-
tion becomes faulty, and an unintended fl ow
of current
between two conductors or between one conductor
and earth occurs. The amount of the current depends,
among other things, upon the voltage, the condition of
the insulating material and the distance between the
conductors. At fi rst the current fl ow will be low but as
the fault develops the current will increase and the area
surrounding the fault will heat up. In time, if the fault
persists, a total breakdown of
insulation will result and
excessive current will fl ow through the fault. If the fuse
fails to operate or is in excess of the recommended fuse
rating, overheating will occur and a fi re will result. A fi re
can also be caused if combustible material is in close
proximity to the heated wire or hot sparks are ejected.
Short circuits are most likely to occur where electrical
equipment or cables are susceptible to damage by water
leaks or mechanical damage. Twisted or bent cables can
also cause breakdowns in insulation materials.
Inspection covers and cable boxes are particular
problem areas. Effective steps should be taken to prevent
the entry of moisture as this will reduce or eliminate the
risk. Covers can themselves
be a problem especially
in dusty areas where the dust can accumulate on fl at
insulating surfaces resulting in tracking between conduct-
ors at different voltages and a subsequent insulation
failure. The interior of inspection panels should be kept
clean and dust free by using a suitable vacuum cleaner.
Overheating of cables and equipment will occur
if they become overloaded. Electrical equipment and
circuits are normally rated to carry a given safe current
which will keep the temperature rise of the conductors
in the circuit or appliance within permissible limits and
avoid the possibility of fi re. These safe currents defi ne
the maximum size of the fuse (the fuse rating) required
for the appliance. A common cause
of circuit overloading
is the use of equipment and cables which are too small
for the imposed electrical load. This is often caused by
the addition of more and more equipment to the circuit
thus taking it beyond its original design specifi cation.
In offi ces, the overuse of multisocket unfused outlet
adaptors can create overload problems (sometimes
known as the Christmas tree effect). The more modern
multiplugs are much safer as they lead to one fused
plug and cannot be easily overloaded (see Figure 12.4).
Another cause of overloading is mechanical breakdown
or wear of an electric motor and the driven machinery.
Motors must be maintained in good condition with particu-
lar attention paid to bearing surfaces. Fuses do not
always provide total protection
against the overloading
of motors and, in some cases, severe heating may occur
without the fuses being activated.
Loose cable connections are one of the most
common causes of overheating and may be readily
detected (as well as overloaded cables) by a thermal
imaging survey (a technique which indicates the presence
of hot spots). The bunching of cables together can also
cause excessive heat to be developed within the inner
cable leading to a fi re risk.
This can happen with cable
extension reels, which have only been partially unwound,
used for high-energy appliances like an electric heater.
Ventilation is necessary to maintain safe tempera-
tures in most electrical equipment and overheating is
liable to occur if ventilation is in any way obstructed or
reduced. All electric equipment must be kept free of any
obstructions that restrict the free supply of air to the
equipment and, in particular, to the ventilation apertures.
Most electrical equipment either sparks in normal
operation or is liable to spark under fault conditions.
Some electrical appliances such as electric heaters, are
specifi
cally designed to produce high temperatures.
These circumstances create fi re and
explosion hazards,
which demand very careful assessment in locations where
processes capable of producing fl ammable concen-
trations of gas or vapour are used, or where fl ammable
liquids are stored.
It is likely that many fi res are caused by static elec-
trical discharges. Static electricity can, in general, be
eliminated by the careful design and selection of mater-
ials used in equipment and plant, and the materials used
in products being manufactured. When it is impractical to
avoid the generation
of static electricity, a means of control
must be devised. Where fl ammable materials are present,
especially if they are gases or dusts, then there is a great
danger of fi re and explosion, even if there is only a small
discharge of static electricity. The control and prevention
of static electricity is considered in more detail later.
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