Do fire distances change if I refill with ester?

Enter one transformer at a time. The liquid volume is the only required value; the current liquid, the target ester, sealed or free-breathing conservation, and indoor or outdoor installation complete the basic case. For the site, add the distance to the nearest wall with its fire rating and the distance to the nearest other transformer. FM Global measures from the inner edge of the oil containment and IEC 61936-1 from the transformer, so enter the shortest distance you have.

Everything else is optional, and the tool assumes no value you leave empty: each section says what to measure instead. The optional fields include the voltage class, on-load tap changer, lowest ambient temperature, the measured mineral oil residue, the measured fire and flash points, whether the ester is FM Approved, the PCB content of the oil to be drained, and the moisture, temperature and mass of the paper.

The result is a dossier in sections: feasibility with the reasons and the checks that are always needed, the retrofill steps, the fire point and flash point against the mineral oil residue, the IEC and FM distances for each scenario on a site sketch, containment and hydrant allowance, acceptance tests for the new liquid, the effect on drying the paper, and the PCB class of the drained oil. You can print it or save the inputs to a file; Seetalabs® keeps nothing.

Fire point after the retrofill. Mineral oil stays in the paper and leaks back into the ester until equilibrium, and too much of it lowers the fire point below 300 °C. For natural ester the tool reads the bands on IEEE C57.147-2018 Figure B.4, measured on new natural ester mixed with new mineral oil. Up to 4 % mineral oil, a fire point above 300 °C is expected. Between 4 and 7 % the sources disagree: IEEE C57.147-2018 clause 1.4 NOTE typically accepts up to 7 %, while CIGRE TB 537 clause 6.5.4 allows at most 4 % for a 300 °C fire point. The tool applies the stricter 4 %. Between 7 and 10 % the fire point cannot be expected without a measurement, and at 10 % or more it falls below 300 °C. For synthetic ester, IEC 61203:2025 B.1 indicates that above about 3.5 % mineral oil the fire point may fall below 300 °C. The residue is measured, not estimated: without a measured value the tool does not estimate the fire point.

Fire distances. Outdoors, IEC 61936-1:2021 Table 4 applies from 1 000 L; indoors, Table 5 sets the fire rating of the room. FM Global Property Loss Prevention Data Sheet 5-4 (interim revision October 2025) gives distances by liquid volume; at a band boundary the tool uses the stricter band. Until a fire point is measured, the tool keeps the distances of mineral oil and shows in a separate column what the retrofill can achieve once the measurement confirms it.

Two conditions for the reduced FM distances. The tool applies them only when the ester is FM Approved and the measured fire point is 300 °C or more. FM Global DS 5-4 does not address retrofilled units, so confirm the reduction with your insurer.

Other checks. Feasibility follows IEEE C57.147-2018, IEC 61203:2025 Annex B, CIGRE TB 436 and FM Global DS 5-4: at 69 kV and above, or for a power transformer, a design review by the manufacturer is needed first. Cold start of natural ester follows IEEE C57.93a-2025 clause 4.4.5, and the DGA baseline after refilling follows IEEE C57.155-2014 clause 6.3. The PCB class of the drained oil uses the rules of the site's country, as in the PCB Compliance Triage tool; above 50 mg/kg the retrofill is also a decontamination of the unit.

An outdoor transformer with 1 500 L of mineral oil, to be refilled with natural ester. The nearest wall is non-combustible and 5 m away; the nearest other transformer is 2 m away.

Today the unit fails one requirement: IEC 61936-1 asks for 3 m to the other transformer and the site has 2 m. After the retrofill all four distances drop to 1.5 m, but only in the confirmed case: the ester is FM Approved and a fire point of 300 °C or more is measured at equilibrium. Until that measurement exists, the tool keeps the mineral-oil distances, and the gap to the other transformer remains. For the FM rows, the reduction still needs your insurer's agreement.