Transformers in Tunnels and Underground Stations: Fire Safety Considerations

Traction substations, station supplies, ventilation and tunnel boring machines all need transformers. Underground, heat, smoke and access problems are amplified – and this shapes the choice of transformer type and insulating liquid.

Updated 12 November 2026 · Published by IsoTec

Why underground is different

Heat and smoke stay in the structure. Smoke can reach escape routes and platforms quickly.
Evacuation and firefighting take longer. Access routes are long; firefighters work in smoke and heat.
Space is limited. Clearances that are easy above ground are often impossible in a cross-passage or a station plant room.
Water and dirt. Humidity, condensation, dust and brake debris affect insulation.
Vibration from trains loosens connections and can create hot spots.
Drainage paths. Spilled liquid and firewater can enter tunnel drainage.
High consequence of outage. A failure can stop a line for days.

Where transformers are found

Traction power substations, including rectifier transformers for DC systems
Auxiliary and station supplies: lighting, escalators, signalling, pumps
Tunnel ventilation and smoke extraction plants
Technical rooms in cross-passages and service tunnels
Road tunnel lighting, ventilation and safety systems
Construction power, including transformers on tunnel boring machines

Transformer options compared

	Dry-type (cast resin, F1)	Ester-filled (K-class)	Mineral oil (O-class)
Liquid fire (pool or spray)	None	Unlikely pool fire; spray possible during an arc	Pool and spray fires possible
Combustible material	Resin and insulation	Ester, cellulose	Mineral oil, cellulose
Humidity, dust, condensation	Sensitive – environmental class (E) matters	Sealed tank, insensitive	Sealed tank, insensitive
Overload capability	Limited	Good	Good
Size and weight per MVA	Larger	Compact	Compact
Containment needed	No	Yes	Yes
Typical underground use	Lower and medium ratings, stations	Where ratings, space or environment favour liquid-filled units	Usually avoided

Manufacturers report service experience with ester-filled power transformers up to 420 kV and offer them with rupture-resistant tanks for underground substations. A detailed comparison: ester vs mineral oil vs dry-type.

Standards context

Tunnel fire safety standards look at the system as a whole – fire load, compartmentation, ventilation, evacuation and firefighting. Examples are NFPA 130 for fixed guideway transit and passenger rail, NFPA 502 for road tunnels, and in the EU the technical specification for interoperability on safety in railway tunnels (TSI SRT, Regulation (EU) No 1303/2014). The transformer enters the fire strategy as a fire load and an ignition source; the installation itself follows IEC 61936-1 and national rules. More: IEC 61936-1, FM Global and NEC.

Design measures

Fire-rated plant rooms (EI/REI) with self-closing fire doors; no escape routes through the room
K-class insulating liquid – also in tap changers – or a dry-type transformer of fire class F1
Dry bushings and plug-in or air-insulated cable connections
Fast electrical protection, pressure relief device and sudden pressure relay; rupture-resistant tank
Automatic detection; suppression by water mist or sprinklers where required
Smoke separation from evacuation routes; ventilation that does not draw smoke towards platforms
Containment that keeps liquid and firewater out of tunnel drainage
Remote condition monitoring: temperature, pressure, dissolved gas analysis
Vibration-proof connections and access for inspection

Residual risks specific to tunnels

Smoke from any fire in the plant room reaching evacuation routes
Cable fires spreading into or out of the transformer room
Spills reaching the tunnel drainage system
Late detection of developing faults because inspection access is restricted
External fire exposure from a burning train or vehicle near the substation

The full register: residual fire risks of ester-filled transformers.

Checklist for the fire strategy

List all transformers, their location, rating, liquid volume and liquid class.
Check every separate liquid volume: bushings, tap changer, cable boxes.
Define credible fire scenarios and their ignition sources.
Confirm protection clearing times and pressure relief arrangements.
Define fire separation, detection and suppression.
Model smoke movement for the relevant scenarios.
Plan containment and drainage.
Agree the residual risk with the client, the authority and the insurer.

Support from IsoTec

We provide fire-relevant data for our K-class ester fluids, support retrofilling of existing transformers and test fluid samples. See the transformer fire safety guide.

Frequently asked questions

Which transformer is best for a tunnel?

There is no single answer. Dry-type units avoid liquid fires but are sensitive to humidity and dirt and have limited overload capacity. Ester-filled units suit higher ratings and harsh environments but need containment and protection against arc faults.

Is mineral oil allowed in tunnels?

It depends on national rules and the project’s fire strategy. Because of its low fire point, mineral oil is usually avoided in underground locations.

Which standards apply to transformers in rail tunnels?

The installation follows IEC 61936-1 and national rules. The tunnel fire strategy is shaped by standards such as NFPA 130 and, in the EU, the TSI on safety in railway tunnels.

Sources and further reading

Planning transformers underground?

We help you specify K-class ester fluids and provide the fire data your strategy needs. IsoTec products are supplied to customers outside Germany.