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Why do some Epoxy (polymer) Transformer Bushings fail?
Why do some Epoxy (polymer) Transformer Bushings fail?

We all know that how troublesome it is when a bushing fails on a transformer in the field. In addition to incurring significant costs to replace the bushing, there is the question of the transformer supplier’s reputation as well as being subject to possible penalties imposed by customers suffering power interruptions.




Let’s look at the most common causes of such failures

1. Part Design

In addition to meeting existing standards such as NEMA and IEEE, part design is closely related to and dependent upon the properties of the Polymer used to manufacture bushings. Fulfilling requirements such as creep distances, mounting, shed design and configuring cable connections are all influenced to some degree by optimized Polymer properties. Things to look for in this area are:


  • Optimizing thermal cycling ability
  • Minimizing cantilever strength requirements
  • Maximizing creep and strike distances
  • Maximizing heat dissipation

2. Polymer

Selecting the correct epoxy system (resin/hardener combination) is critical. The polymer must be carefully selected so that the combination of cured properties will accommodate the changing conditions encountered in service. This applies to both mechanical strength and electrical insulation properties. In general, careful consideration must be given to the following specific properties:

  • Material type (depending on whether indoor or outdoor service)
  • Dielectric strength
  • Heat deflection Temperature
  • Heat aging
  • Moisture absorption
  • Tensile strength
  • % elongation


There are numerous articles covering these requirements and these properties can be confirmed by standard test methods.

Most common failures

  • Cracking
  • Leaking bushings
  • Direct short to transformer tank (catastrophic failure)
  • Loss of dielectric properties over time (aging)
  • Reduction in tracking resistance over time.

3. Manufacturing

Manufacturers use different production methods and some work quite well while others do not. The difficulty is that customers buying the bushings have no way to confirm quality unless they do full testing on every bushing which is not feasible.

The key here is that whatever method is employed to produce the parts must be accurate and repeatable.
For example; being off ratio in the resin/hardener mix is not readily detectable by the end user but chances are that the bushing will fail in operation.

There is no room for error here. Some of the most common causes of failure, as a result of improper processing are:

  • Insufficient conductor preparation
  • Insufficient air removal from mix
  • Off ratio epoxy mix
  • Improperly dispersed fillers within the Polymer mix
  • Incorrect processing temperatures (epoxy, mould and conductor)
  • Less than completely cured Polymer
  • Inadvertent thermal cycling of components which are not fully cured

Most common failures:

  • Bushing leaks developing over time in operation
  • Excessive partial discharge
  • Cracking during mounting or under maximum cable load
  • Tearing when conductor temperature rises due to high current

The consequences of bushing failure are obvious therefore it is important that your supplier is consistent and dependable.


An ISO quality system is a great place to start because consistency is an absolute must. The bottom line is that it is not possible for the user to detect all things that can cause problems down the road usually without warning. Some of the above mentioned issues can be eliminated through “type testing” different parts because those properties will remain consistent as long as the supplier’s process is repeatable. On the other hand, issues related to the manufacturing process are of particular importance, because they are truly dependent on consistency, while, at the same time, are difficult to detect by the user and will likely lead to field failures down the road.



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