Containment, Firewater and Spills for Ester-Filled Transformers
A K-class fluid changes the fire behaviour of a spill – not the need to contain it. How to plan bunds, flame traps and firewater for ester-filled transformers.
Updated 12 November 2026 · Published by IsoTec
Why containment is still needed
- Leaks from gaskets, valves and radiators
- Release of the full liquid volume after a tank rupture
- Contaminated firewater during firefighting
- Legal obligations for water and soil protection, which depend on national rules and the classification of the fluid
Test results on biodegradability do not replace containment. Whether reduced requirements apply is decided by the competent authority in each case.
Sizing the containment
The containment is usually sized for the liquid volume of the largest transformer it serves, plus an allowance for rainwater and, where required, firewater. FM Global Data Sheet 5-4 gives detailed guidance on containment and emergency drainage; the NEC requires a liquid confinement area for indoor less-flammable liquid transformers. Local rules define the details.
Flame traps and common sumps
A layer of coarse gravel or crushed rock over the sump acts as a flame trap: burning liquid draining through it is cooled and extinguished, and the fire is limited to the transformer itself. One utility guideline specifies 40–50 mm rock with about 40 % voids, typically 300 mm deep. Where several transformers share a sump, IEC 61936-1 requires that a fire in one cannot spread to the others – for example by gravel layers or liquid-filled pipes.
Oil-water separators
Many separators are designed for mineral oil, with a density of about 0.88 kg/l. Esters are denser – typically about 0.92 kg/l for natural esters and 0.97 kg/l for synthetic esters – so the density difference to water is smaller and gravity separation slower. Check the suitability of the separator for the ester used with its manufacturer.
Indoor installations
- Liquid-tight floor with sills or ramps at doors
- No open floor drains into the public sewer
- Containment volume for the largest transformer, plus sprinkler water where sprinklers are installed
- Ventilation openings positioned so that hot gases do not affect escape routes
Tunnels and underground stations
Underground, spilled liquid and firewater must not reach the tunnel drainage unintercepted. Plant rooms need their own sumps, and pumps must not transfer contaminated water into the main drainage system without separation. See transformers in tunnels.
Inspection and maintenance
- Remove rainwater from outdoor bunds; keep drain valves closed and secured
- Inspect gravel layers and replace clogged material
- Check transformers for leaks during routine inspections
- Keep absorbent material and a spill procedure ready
Frequently asked questions
Do ester-filled transformers need a bund?
Yes, as a rule. Leaks, tank rupture and firewater must be contained. Requirements depend on national rules and the classification of the fluid; any reduction is decided by the competent authority.
What is a flame trap?
A layer of coarse gravel or crushed rock over the sump. Burning liquid draining through it is cooled and extinguished, so the fire stays limited to the transformer.
Do oil-water separators work with esters?
Esters float on water, but their density is closer to that of water than mineral oil. The separator’s suitability should be confirmed with its manufacturer.
Planning containment for ester-filled transformers?
We provide density, safety data sheets and fire data for our ester fluids. IsoTec products are supplied to customers outside Germany.