Date of Award

Spring 1999

Document Type


Degree Name

Doctor of Engineering (DEng)


The problem of cracking in electrical apparatus bushings as a result of thermal stresses was investigated. The bushings were composed of cycloaliphatic epoxy insulators with embedded aluminum conductors. The problem is due to the difference in coefficients of thermal expansion of the two materials. A solution to the problem had been to coat the conductors before they were formed together with the epoxy insulators. The coating was assumed to prevent cracking by allowing movement between the two materials as their dimensions changed during thermal expansion and contraction. The contribution of the coating was to be established.

The hypothesis was that the coating, above a given thickness, would prevent cracking and would fail to prevent cracking below that thickness. Inherent in this hypothesis was that the thickness of the coating was a controlling factor in its ability to prevent cracking during thermal changes.

A method of applying a controlled coating in an economically feasible manner was developed. Coatings of various thickness were then applied to a set of conductors. Those coated conductors along with non-coated conductors were then formed into bushings. The bushings were cycled from 250°F (121.1°C) to –300°F (–184.4°C) in cycles which began at –50°F (–45.6°C) and reduced –50°F (–45.6°C) until –300°F (–184.4°C) was reached.

The bushings with non-coated conductors exhibited a 50% failure rate. Those with coatings of any thickness failed to crack. A theoretical analysis of the bushings under thermal conditions indicated that the coatings prevented cracking by allowing relative movement between the conductor and insulator. It was concluded that the contribution of the conductor coating was not relative to its thickness but was due to its ability to separate the insulator from the conductor.