2006 Muttrah Incident Report - authority for electricity regulation, oman

As indicated in the KEMA Report it is often not possible in cases of this kind to be
definitive about the causes of the initial fault. On the balance of probabilities KEMA
conclude that Hypothesis 2 is the most likely cause of the fault.
The basis of Hypothesis 2 is that the ABB design of the copper conductors connecting
the isolating contacts to the VCB itself is such that inside the bushings the original
coaxial configuration of the Brush OCB trucks has been replaced by an asymmetrical
configuration, which reduces the air gap between the plastic sleeve of conductor and the
inner side of the bushing to a minimum. The distance of live parts to the shutter has also
been decreased considerably. Diagram 2 below shows a simplified representation of the
decreased air-gap between the shutter/bushing and the isolating contact arms. Tracks of
scratching and even cutting of this contact arm sleeve by the bushing have been found
on other retrofitted breakers of the same type after they had been racked in and out (see
Photograph 5 below).
FIGURE 6
Insulating Sleeve cut by
bushing.
This VCB truck had been
racked in and out of the
service position.
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