Lightning breakdown voltage

Further information on breakdown voltages are given in the book "Lightning Protection" by Golde ( ). 

The procedure for testing will be the same as that discussed for switchgear assemblies in Section 14.3.4. The impulse test voltage is applied as in Table 32.1(A) for series I and Table 32.2, for series II voltage systems with a full wave standard lightning impulse of 1.2/50 (Section 17.6.1). There should be no disruptive discharge or insulation breakdown. 

The substitution of a gas for part of the polymeric matrix usually changes the electrical properties. The dielectric constant, dissipation factor, and dielectric strength are lowered roughly proportionally to the amount of gas in the foam. The lower the density of the cellular plastic, the lower the dielectric constant and the better the insulation. Because the dielectric strength is also reduced, the insulation is susceptible to breakdown from voltage surges from lightning and short circuits. Polyolefin foams are preferred for low frequency electrical insulation - PVC, PS foams, silicone and fluoropolymer foams are also used [31, 65, 66]. 

The field at the tip of very sharp points (-0.1 ym in radius) has been reported to have attained the exceptional value of 100 MV/cm within minutes (no detectable current flowed under DC voltage) in liquid nitrogen (Sibillot and Coelho, 1974) at the other extreme, a one-meter gap of transformer oil in a gap of one meter was unable to sustain a 1-MV "lightning surge , breakdown having occurred below 10 kV/cm (Kamata and Kako, 1980). 

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