Failure of electrical equipment
There are two fundamental causes
of failure of electrical equipment, mechanical failure or electrical failure of
insulation
Fractures may be caused by
resonant vibrations of current carrying conductors either from purely
mechanical movement or from electromagnetic forces leading to fatigue hardening
and subsequent breakage. Where metallic elements are stressed in a corrosive
atmosphere (e.g. damp or polluted atmospheres) along with alternating forces,
failure may occur at comparatively low stress. Some steels, which under normal
conditions exhibit considerable ductility, will fail at low temperatures by
brittle fractures with no ductile deformation.
Mechanical failure of insulators
may displace conductors and cause short circuits. Ceramic insulators are
brittle but have great strength in compression. However ceramic insulators are
vulnerable where they are used in tension or shearing situations. They are now
largely confined to outdoor overhead lines and switch gear where their robust construction
makes them less susceptible to mechanical failures although they are then
vulnerable to vandalism.
2. Breakdown
of insulating materials
- Mechanically, as by friction or tearing.
- As a result of excessive electrical stress.
- As a result of excessive temperature (and occasionally very low temperature) or temperature cycling. The latter may cause mechanical stresses as a result of differential expansion or contraction.
- Chemical and physical reaction with other materials,
E.g. oxidation, contamination or
the leaching out of important ingredients which may lead to de-plasticisation,
i.e. they become brittle. The ingress of water is a very common contamination
leading to `treeing' and eventual electrical breakdown.
Failure is rarely the result of inadequate
electrical breakdown strength where reasonably pure materials are used. In
practice, insulation is rarely designed to be stressed to more than 10% of its
strength as determined by laboratory tests. It fails because of impurities,
lack of homogeneity, the unavoidable variations in commercially available
materials as well as in those natural products such as paper, wood and
petroleum products. The insulation performance of most commercially used
materials is now well documented and standard testing procedures have been
established.