Ester Retrofill & Fire Risk: fire point and fire distances before and after refilling with ester
Ester Retrofill & Fire Risk is a free tool for asset managers and fire safety engineers who plan to refill a mineral-oil transformer with natural or synthetic ester. It shows the fire point to expect with the mineral oil left inside, and the IEC 61936-1 and FM Global fire distances before and after.
How it works
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.
Method and standards
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.
Worked example
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.
| Distance | Required today (mineral oil) | Today | Required after retrofill, if confirmed |
|---|---|---|---|
| FM Global, to the wall | 4.6 m | met (5 m) | 1.5 m |
| FM Global, to the other transformer | 1.5 m | met (2 m) | 1.5 m |
| IEC 61936-1, to the wall | 3 m | met (5 m) | 1.5 m |
| IEC 61936-1, to the other transformer | 3 m | not met (2 m) | 1.5 m |
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.
Limits
The result is an indication for planning. It does not replace the design review of the transformer and tap-changer manufacturers, the guidance of the liquid supplier, the requirements of your insurer or fire authority, or the judgement of a qualified engineer. Figure B.4 of IEEE C57.147-2018 was measured on one new natural ester mixed with new mineral oil; other esters or aged oil may differ, and the tool's reading of the figure carries about ±3 to 5 °C.
What it does not do: estimate the mineral oil residue, assess the thermal effect of the retrofill, interpret DGA in ester, estimate costs, or count the chemical effect of ester on the paper. The IEC distances are not assessed for a wall rated by FM's "2 h" class or below 1 000 L outdoors.
Does refilling with ester reduce the fire distances by itself?
No. The reduction needs a fire point of 300 °C or more measured at equilibrium after the retrofill, and for the FM Global distances an FM Approved ester as well. FM Global DS 5-4 does not address retrofilled units, so confirm with your insurer.
How much mineral oil can stay in the transformer?
For natural ester the tool uses 4 %, the limit of CIGRE TB 537 clause 6.5.4 for a 300 °C fire point; IEEE C57.147-2018 clause 1.4 NOTE typically accepts up to 7 %. Measure the residue: the tool does not assume a typical value.
Is a measured fire point of exactly 300 °C enough?
It meets the "300 °C or more" condition the tool uses for the reduced FM distances. For the class K distances of IEC 61936-1 the tool requires a fire point above 300 °C, so exactly 300 °C does not qualify.
What happens to the PCBs in the drained oil?
Enter the PCB content and the country of the site. The tool classifies the drained oil as waste under the Stockholm, EU or US rules; above 50 mg/kg the retrofill also counts as a decontamination of the unit.
After the retrofill: follow the transformer in ester
After the refill, gases in ester do not read like mineral oil. Ronin AI follows ester-filled units on their own route and recognises ester stray gassing as no fault action (IEEE C57.155). Tell us how many units are filled or will be refilled.
<p>The tool runs in your browser. It makes no network requests, and Seetalabs® does not store or see the values you enter.</p>
<a href="/tools/disclaimer/">Disclaimer</a>
Tool version 1.0.4. Seetalabs®.
