Transformer Fire Safety Glossary

Terms A–Z

ALARP

“As low as reasonably practicable”: UK principle that risks must be reduced unless the cost of further measures is grossly disproportionate to the benefit.

Arc fault (internal arc)

Electrical discharge inside the transformer after insulation failure; decomposes the liquid into gas and causes a rapid pressure rise.

Auto-ignition temperature

Temperature at which a liquid ignites in air without an ignition source, e.g. on a hot surface.

Bow-tie analysis

Risk method that shows the threats leading to a central event and the consequences that follow, with barriers on both sides.

Buchholz relay

Gas and oil-surge relay between tank and conservator that detects gas generation and liquid movement during internal faults.

Bund

Containment around or below a transformer that retains leaked liquid and firewater.

Cable box

Enclosure in which cables are connected to the transformer; may be air-insulated, compound-filled or oil-filled.

Cleveland open cup

Test method (ISO 2592 / ASTM D92) for flash point and fire point; basis of the IEC 61039 classification and FM Approvals Class 6933.

Current-limiting fuse

Fuse that interrupts high fault currents before they reach their peak, limiting the arc energy.

Dissolved gas analysis (DGA)

Analysis of gases dissolved in the insulating liquid; detects developing faults such as overheating and discharges.

Dry-type transformer

Transformer without insulating liquid, usually with cast-resin windings.

EI / REI

Fire resistance ratings: integrity (E), insulation (I) and, for load-bearing elements, resistance (R), followed by minutes – e.g. EI 60.

Enhanced protection

Term in IEC 61936-1: tank rupture strength, pressure relief and protection against low- and high-current faults.

F0 / F1

Fire behaviour classes of dry-type transformers (IEC 60076-11). F1 applies where there is a fire hazard and requires restricted flammability.

Fire load

Total heat that can be released by burning materials; for the liquid: mass × net calorific value.

Fire point

Lowest temperature at which a liquid keeps burning for at least 5 seconds after ignition.

Flame trap

Layer of gravel or crushed rock over a sump that extinguishes burning liquid draining through it.

Flash point

Lowest temperature at which the vapours of a liquid ignite briefly when an ignition source is applied.

FM Approved

Certified by FM Approvals – for transformer fluids under Class 6933, for transformers under Class 3990.

Heat release rate

Rate at which a fire releases energy, in kW or MW; determines fire size and smoke production.

IEC 61039

Standard classifying insulating liquids by fire point (O, K, L) and net calorific value (1, 2, 3).

K-class

Insulating liquid with a fire point above 300 °C according to IEC 61039.

Less-flammable liquid

Term used by the NEC and FM Global for insulating liquids with a fire point of at least 300 °C that are listed or certified.

Mist ignition

Ignition of a fine spray of liquid; can occur well below the flash point of the bulk liquid if an ignition source is present.

Natural ester

Insulating liquid based on refined vegetable oils; typically class K2.

Net calorific value

Heat released by complete combustion, without the condensation heat of water; used for the IEC 61039 digit.

OIP bushing

Oil-impregnated paper bushing; contains its own mineral oil filling.

On-load tap changer (OLTC)

Device that changes the transformer ratio under load; conventional types switch in liquid, vacuum types in vacuum interrupters.

Pool fire

Fire burning on the surface of a pool of liquid; requires the liquid surface to reach its fire point.

Pressure relief device

Valve that opens at a set pressure to limit tank pressure during internal faults.

Retrofilling

Replacing the mineral oil of an existing transformer with another liquid, e.g. an ester.

RIP / RIS bushing

Resin-impregnated paper or resin-impregnated synthetic bushing; dry design without free oil.

Sudden pressure relay

Relay that trips the transformer on a rapid rise of tank pressure.

Synthetic ester

Insulating liquid made from polyols and fatty acids; typically class K3.

Tank rupture

Failure of the transformer tank, cover or turret under the pressure of an internal arc.

Water mist

Fire suppression with fine water droplets; one of the options for indoor transformer protection.