Unfortunately failure of an 11 kV cable usually results in the destruction of the evidence
at the point of breakdown. However, a sample of the cable taken just 100 mm from the
failure position exhibited clear evidence of the likely cause of its demise. Extensive ‘vented’
water treeing, i.e. the presence of tree-like growths connected to and emanating from the
insulation surface, was observed. Trees had grown from both the outer and inner insulation
surfaces. Lengths of tree from the inner screen were up to 0.5 mm, whilst from the outer
screen they were up to 2 mm. Typical water trees observed in this cable are shown in
Figure 7.5. Figure 7.6 shows more detail of one of the vented water trees.