Handbook of Electrical Installation Practice - BeKnowledge

296 Handbook of Electrical Installation Practice
(TRS), usually two-core and earth and of rectangular section. Many of these cables
were used, without further protection, for wiring dwellings and small commercial
properties. In most cases, the rubber has hardened, perished and cracked and when
disturbed falls away exposing the conductors. Short-circuits are then likely to occur
between ‘live’ conductors or to earth and the arcing will sometimes ignite the
remains of the insulation and/or anything else flammable nearby.
Other installations have vulcanised rubber insulated (VRI) conductors, taped,
braided and compounded and run in steel conduit. Again the rubber will have perished
but is sometimes retained by the outer covering of tape and braid. The hardened
material tends, however, to crack when bent, exposing the conductor. The
conduit protects most of the wiring but the exposed ends which are sometimes in
wooden switch boxes or in wooden pattresses supporting switches, socket-outlets or
ceiling roses, are vulnerable and again a short-circuit may occur and the arcing cause
a fire. This old wiring should be replaced by the PVC insulated or insulated and
sheathed equivalents.
Severely overloaded circuits are a fire hazard as the hot conductors damage
the insulation, which will eventually fail and permit a short-circuit to occur.
Examples are sometimes found in old radial socket-outlet circuits where the
DIY operator has sought to increase the supply capacity by replacing old 2 and
5A by double 13A socket-outlets or by adding additional socket-outlets to a
circuit by looping without regard to the size of the cable. He then finds that the
protective rewireable fuse on the distribution board is inadequate, and so increases
the size of the fuse wire so that the inadequately sized conductors are no longer
protected.
Again, where there are insufficient socket-outlets, adaptors are used and loads, in
excess of the rating of the socket-outlet, connected. The multiple plugs and
adaptor(s) assembly may not only overload the socket but very often the socket
receptacles are damaged by the adaptor pins when the assembly is subjected to
lateral pressure from mishandling or a blow. The result is a bad connection, minor
arcing at the contacts, corrosion of the surfaces and overheating which ignites the
insulation and anything else flammable nearby. The remedy is to eschew the use of
adaptors and install an adequate number of socket-outlets.
Conductor overheating, resulting in fires, can also occur from loose connections.
Pinch screw terminals in plugs and switches are prone to slacken in time, probably
due to vibration, resulting in bad connections and an eventual fire. Checking terminal
screw tightness should be part of the maintenance schedule.
EXPLOSIONS
Electrical explosions occur when there is a short-circuit between conductors on different
phases or between a phase conductor and the neutral or earthed metal and
sufficient energy is expended to cause an audible bang. In domestic installations
where the line/neutral and line/earth loop impedances are comparatively high,
usually around 0.3 W to 0.5 W, the short-circuit current will be 230/0.3 or 230/0.5 �
766 or 460A, which is enough to cause an audible bang at the fault location and
at the fuse or circuit-breaker protecting the circuit which clears the fault. Such