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combustible material if the equipment is misused, is not adequate to carry the
electrical load or is not properly installed and maintained.
The most common causes of fire in electrical installations are:
 short circuits;

 overheating of cables and equipment;
 the ignition of flammable gases and vapours;
 the ignition of combustible substances by static electrical discharges.

Short circuits happen, as mentioned earlier, if insulation becomes faulty, and
an unintended flow 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 first, the current flow 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 flow
through the fault. If the fuse fails to operate or is in excess of the recommended fuse
rating, overheating will occur and fire will result. A fire 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 conductors 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 fire. These safe currents define
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 offices, 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 (Figure 14.4).
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 particular attention paid to
bearing surfaces. Fuses do not always provide total protection against the overloading

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