Why Do Transformer Bushings Fail?

Published 27 Aug 2026Updated 27 Aug 20266 min readIntermediateFailures
Quick answer — Bushing failures are a leading cause of transformer outages. They come from moisture ingress and contamination, internal partial discharge, insulation aging, overheating at the connections, and mechanical or voltage stress. Because a bushing failure can flash over to the tank or explode, condition monitoring (capacitance and tan-delta) and power-factor testing on the bushing are critical.
Contents
What causes a transformer bushing to fail?

Common causes

Moisture and contamination lower the insulation strength; a degraded or delaminated paper/resin core leads to partial discharge and thermal runaway; poor connections overheat; and voltage surges or mechanical stress initiate a turn fault.

Why it is dangerous

Unlike some faults, a bushing failure can be sudden and energetic, and the porcelain or composite housing can shatter, causing a tank rupture or a fire.

Detection and prevention

Monitor bushing capacitance and power factor (tan-delta) via the test tap — a rising value indicates moisture or contamination. Regular tan-delta, IR/thermography and PD testing catch problems before failure.

References & standards

Related

Book seriesAcademy

Related manufacturers

Prolec GE (GE Vernova) · Prolec GE Waukesha · Hitachi Energy USA · Siemens Energy · Eaton · Virginia Transformer Corp

Related components

Transformer bushings & terminations Protection, monitoring & diagnostics
Technical content is prepared from engineering references, standards frameworks, manufacturer documentation and industry sources. Contractual or compliance decisions should refer to current official standards. TransformerPath Technical Editorial.
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