Bushings, Tap Changers and Cable Boxes: Where Transformer Fires Start
Most transformer fires do not start in the windings. They start at the components that connect the transformer to the network – and some of them bring their own fuel.
Updated 12 November 2026 · Published by IsoTec
Where fires start
| Origin | Share of transformer fires (below 300 kV) |
|---|---|
| Bushings and cable boxes | about 70–80 % |
| On-load tap changers | about 10–15 % |
| Other causes | the remainder |
Estimates published by the convener of the CIGRE working group on transformer fire safety. Oil-impregnated paper bushings are named as the single most frequent cause: an arc in a bushing often leads to an explosive failure, a liquid spill and a major fire.
Bushings: the separate oil volume
High-voltage condenser bushings are often of the oil-impregnated paper (OIP) type. Their paper core is impregnated with mineral oil, which is sealed inside the bushing. This oil stays in the bushing even if the transformer tank is filled with a K-class ester. When such a bushing fails, burning oil and fragments of the housing can be expelled.
Dry alternatives are resin-impregnated paper (RIP) and resin-impregnated synthetic (RIS) bushings, often with composite housings. They contain no free oil. For distribution transformers, solid porcelain or epoxy bushings and plug-in cable connections are common.
Monitoring bushings
- Capacitance and dissipation factor (power factor) measurements during maintenance
- Online bushing monitoring for large transformers
- Infrared thermography of connections
- Visual inspection for leaks, cracks and contamination
- Replacement based on condition and age, not only after failure
Cable boxes and terminations
Cable boxes may be air-insulated, compound-filled or oil-filled. Poor jointing, moisture and overheating lead to discharges and arcs. Fully insulated, screened plug-in connectors reduce exposed live parts. Cable routes need fire stopping where they enter the transformer room, so that a cable fire does not spread into or out of it.
On-load tap changers
In conventional tap changers, the diverter switch interrupts current in liquid; each operation creates a small arc and decomposition products. In vacuum-type tap changers, switching takes place in vacuum interrupters, and the liquid in the compartment is far less stressed. Check which liquid the tap changer compartment contains – if it is mineral oil, it remains an O-class volume inside an otherwise K-class transformer. Many tap changers are approved for use with esters; confirm with the manufacturer. Tap changer compartments usually have their own protective relay and often a pressure relief device.
Other components
- Conservator, Buchholz relay and breathers – part of the protection concept
- Cooling fans and pumps – electric motors and their supply cables
- Control cabinets and auxiliary wiring
- Low-voltage busbar connections with high currents – risk of hot spots
Specification checklist
- Dry bushings (RIP/RIS) instead of oil-impregnated paper where voltage and design allow
- Composite housings to reduce fragment hazards
- Air-insulated or plug-in cable terminations; no oil-filled cable boxes
- Vacuum-type tap changer, approved for the ester used, with the same K-class liquid
- Bushing and tap changer monitoring for large units
- Fire stopping of cable penetrations
- Documentation of every separate liquid volume and its class
The full scenario list: residual fire risks of ester-filled transformers.
Frequently asked questions
What is the most common cause of transformer fires?
Bushing failures – especially of oil-impregnated paper bushings – together with cable boxes account for an estimated 70–80 % of transformer fires below 300 kV.
Does an ester-filled transformer have mineral oil in its bushings?
If oil-impregnated paper (OIP) bushings are used, yes – each bushing has its own sealed mineral oil filling. Dry RIP or RIS bushings avoid this.
Can tap changers be used with ester fluids?
Many tap changers are approved for esters, in particular vacuum-type designs. Approval and the liquid in the tap changer compartment should be confirmed with the manufacturer.
Reviewing a transformer specification?
We discuss the fluid side of your specification and provide data for our K-class esters. IsoTec products are supplied to customers outside Germany.