Reasons of Failure of electrical equipment

Failure of electrical equipment

 Causes

There are two fundamental causes of failure of electrical equipment, mechanical failure or electrical failure of insulation

 1.      Mechanical causes

 The safety of electrical equipment depends to a large extent on sound mechanical design. The majority of circuit breaker failures are mechanical rather than electrical in nature. Typical faults are loose joints leading to overheating or arcing and the existence of voids and contamination in insulation causing arcing and breakdown products. Where the insulation is bulk oil the products of arcing are themselves highly flammable (acetylene for example) and have often led to explosions.

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

 The electrical breakdown of insulating materials may also occur as follows:

•         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.

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