Effect of core size and ground surface on debris fields of failed electric bushings

• Electrical bushings are deliberately made to fail.
• Mitigation of the resulting fragments’ velocity by filling the bushing with a nylon core failed.
• A gravel surface mitigated the distance travelled by fragments compared to a grass surface.
• In gravel the fragments become caught up in the gravel, travelling less distance.

On occasion electrical bushings fail, sending fragments flying through the air. These fragments create a potential hazard for people and equipment within the debris throw area, which can extend to over 50 m. Experiments and a computer code were used to study two potential mitigation methods. In the first method, the bushing core size was reduced by placing five different nylon bars inside the bushings that filled 0, 29, 58, 71 and 82% of the core space. This resulted in a slight decrease in average fragment velocity from 55 to 40 ms−1 as the core fill increased from 0 to 82%. The resulting change in debris pattern of the fragments was not statistically significant. Hence using nylon cores will not mitigate the risks from fragments when a bushing fails. The second method studied the effect of surface type on the distance travelled by the bushing fragments. Studying grass and gravel surfaces, it was found that the fragments travelled a shorter distance on gravel (up to 45 m) than on grass (up to 75 m). The computer model required multiple bounces of the fragments to accurately model the debris field on grass. On gravel, however, the computer model gave the best predictions when it was assumed that fragments weighing less than 100 g embedded in the gravel on their first impact. Hence the fragments are not travelling as far as they are being becoming caught in the gravel. Based on the results in this paper it is shown that a gravel ground surface will mitigate the distance travelled by fragments from a failed bushing.