The terms 'contamination' and 'pollution' have special meanings when applied to the condition of insulators. An insulator so heavily polluted by marine deposits that it flashes over immediately on energisation may appear to be perfectly clean, even on close inspection. On the other hand, one which is black with industrial soot, or which has some of its surfaces caked with cement, may have an electrical performance indistinguishable from that of a freshly installed counterpart. The reason for this apparent paradox is that values of surface electrical conductivity which are sufficient to cause flashover are quite trifling in absolute terms. They are readily achieved by the presence of soluble electrolytes, such as common salt or industrial acids, at densities of some 0.1 mg/cm2, provided water is available to dissolve them. They are not readily achieved by layers of carbon particles, which make only intermittent point contacts with each other, or by aggregates of mineral dusts which are free of ionic components (although combinations of such aggregates with soluble salts, giving a 'blotting paper' effect, have caused severe flashover problems in North Africa and in Cornwall, England).

Inspec keywords: insulator contamination; electrical conductivity; flashover

Other keywords: surface electrical conductivity; physics; industrial soot; electrical performance; ionic components; carbon particles; contamination; high voltage insulators; soluble salts; electrolytes; flashover problems; mineral dusts; industrial acids; blotting paper effect; marine deposits

Subjects: Power line supports, insulators and connectors