What it does
Rogers Ratio classifies transformer faults using three gas ratios drawn from five hydrocarbon and hydrogen species. Each ratio captures a different facet of fault chemistry, and the combination of all three lands the sample in one of six fault zones — three thermal (T1, T2, T3) and three electrical (PD, D1, D2). It’s reproduced in IEC 60599 Table 1 as one of the standard ratio methods.
The method is older than the Duval Triangle and remains in everyday use because it’s computationally trivial, easy to audit by hand, and tends to agree with the Triangle on unambiguous samples. When the two methods disagree, the disagreement itself is informative.
The three ratios
R1 · CH₄ / H₂
Thermal vs. electrical signature. Low values indicate electrical activity (corona, arcing); values above 1 lean thermal.
R2 · C₂H₂ / C₂H₄
Arcing indicator. Acetylene only forms at very high temperatures, so a non-trivial ratio points away from pure thermal activity toward arcing.
R5 · C₂H₄ / C₂H₆
Thermal temperature gradient. Climbs as a thermal fault gets hotter — the higher the ratio, the higher the hotspot temperature.
The six fault zones
Each row lists the constraint on R1, R2, and R5 that defines the zone. A dash means the ratio is non-significant for that fault — any value is accepted. All three constraints must hold for a sample to land in the zone; samples that satisfy none are reported as inconclusive.
| Code | Fault | R1 | R2 | R5 |
|---|---|---|---|---|
| PD | Partial discharge | < 0.1 | — | < 0.2 |
| T1 | Thermal < 300 °C | > 1 | — | < 1 |
| T2 | Thermal 300–700 °C | > 1 | < 0.1 | 1–4 |
| T3 | Thermal > 700 °C | > 1 | < 0.2 | > 4 |
| D1 | Low-energy discharge | 0.1–0.5 | > 1 | > 1 |
| D2 | High-energy discharge | 0.1–1 | 0.6–2.5 | > 2 |
Each card shows R1, R2, and R5 on log-scaled rails with the IEC 60599 thresholds tick-marked. The dot is a representative sample landing inside that fault zone — read across the three rails to see the rule combination.
- PDPartial discharge
- T1Thermal < 300 °C
- T2Thermal 300–700 °C
- T3Thermal > 700 °C
- D1Low-energy discharge
- D2High-energy discharge
When to reach for it
Rogers is a natural cross-check on the Duval Triangle. Whenever there’s enough gas to compute the three ratios — H₂, C₂H₄, and C₂H₆ all above zero, and the five-gas total at least 20 ppm — the method runs. Below that floor we abstain rather than force a call on lab noise.
It’s most useful when the Triangle places a sample on a zone boundary. If Rogers agrees, the diagnosis is on firmer ground. If Rogers comes back inconclusive while the Triangle gives a clean call, the Triangle usually wins — Rogers’ rectangular zones leave gaps the Triangle’s ternary partition doesn’t.
Strengths and limits
Strengths: Rogers is mechanically simple — three divisions and six rule checks. It’s easy to audit on a calculator at the lab bench, which is part of why it persists in field practice. The R1/R5 split also gives a finer thermal-temperature read than the Triangle’s T1/T2/T3 zones do.
Limits: the rules are rectangular in ratio space, so they leave uncovered regions where Rogers honestly reports “inconclusive” — perhaps a third of borderline samples in field experience. It also can’t distinguish paper involvement from oil-only faults, can’t see stray-gassing signatures, and like every ratio method becomes unstable at very low concentrations. Use it alongside the Duval Triangle and Doernenburg, not as the sole call.
References
- IEC 60599:2022, Mineral oil-filled electrical equipment in service — Guide to the interpretation of dissolved and free gases analysis (Table 1).
- R. R. Rogers, “IEEE and IEC codes to interpret incipient faults in transformers, using gas in oil analysis,” IEEE Transactions on Electrical Insulation, 1978.
- IEEE Std C57.104-2019, Guide for the Interpretation of Gases Generated in Mineral Oil-Immersed Transformers.