What Are the Common Causes of Transformer Failure?

Published 27 Aug 2026Updated 27 Aug 20268 min readIntermediateFailures
Quick answer — The most common transformer failure causes are insulation and winding faults (thermal overheating, moisture ingress, aging), tap-changer and lead winding failures, and external events such as lightning surges, short-circuit forces, overloading, contamination and oil degradation. Many failures are slow and detectable in advance through monitoring.
Contents
What causes transformers to fail?

Insulation and winding

Thermal aging and overheating break down the cellulose; moisture lowers the dielectric strength; winding deformation or inter-turn faults arise from short-circuit forces. These show up as DGA gases (ethylene, carbon oxides, hydrogen, acetylene).

Tap-changer and leads

OLTC contact, diverter and mechanism faults are a leading cause, often seen as acetylene in the oil. Loose or broken leads and terminations also fail under load and vibration.

Environmental and operational

Lightning and switching surges damage the insulation; overloading and loss of cooling overheat the unit; contamination and oil degradation (sludge, moisture, acidity) accelerate aging.

Prevention

Routine DGA, oil and insulation tests (power factor, PD, moisture), thermography, and monitoring of temperature, cooling and tap-changer condition catch most failures before they escalate.

References & standards

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

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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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