Recent research on tracking of electrical insulation

Before about 1930, tracking was used to describe a form of surface degradation of electrical insulation due to rubbing contacts, such as brushes, but from that date onwards the term gradually became associated with the formation of a conducting carbonized path caused by the combination of electrical stress and surface contamination. For the purpose of this paper, tracking is defined as "the decomposition of electrical insulation, caused by the passage of electric current or charge, resulting in the progressive formation (usually tree-like) of a conducting carbon path." There are numerous other definitions of tracking in the literature,¹ many of which invoke the presence of arcs, scintillations etc.,² and although these are often responsible in practice for the initiation of tracking, the absence of discharges does not imply that tracking will not occur. For example, Figure 1 illustrates tracking beneath the surface of a phenolic-resin-bonded paper bushing, caused by ingress of moisture into the outer paper layers. This form of degradation was produced by high leakage currents just under the surface of the bushing and, as far as is known, no arcs or discharges were involved in the carbon path formation.