One Taxonomy, Six Fault Types
IEC 60599:2022, the international interpretation guide for dissolved gas analysis (DGA) results in oil-filled electrical equipment, classifies internal faults into six types: partial discharge (PD), low-energy discharge (D1), high-energy discharge (D2), and three thermal fault levels (T1, T2, T3). This small set of labels is the common language that lets a utility engineer on one continent and a manufacturer on another compare the same set of gas numbers and reach a mutually understandable verdict.
Understanding the taxonomy matters beyond simple naming — each fault type sits in a distinct energy or temperature band and leaves a distinct gas signature. Once you can map a gas pattern to a band, you can reason about what is physically happening inside the tank.
The Six Fault Types at a Glance
- PD — Partial discharge. Localized discharge within or on the surface of the insulation, driven by low energy. Produces mostly hydrogen with methane.
- D1 — Low-energy discharge. Intermittent sparking, medium energy. Generates hydrogen and acetylene alongside methane.
- D2 — High-energy discharge. Arcing and breakdown, high energy. The most destructive electrical fault, generating hydrogen, acetylene and ethylene in quantity.
- T1 — Thermal fault below 300 °C. Overheating of conductors or connections; methane and ethane predominate.
- T2 — Thermal fault from 300 to 700 °C. Intensified overheating; ethylene becomes the leading gas.
- T3 — Thermal fault above 700 °C. Severe overheating approaching carbonization; ethylene, hydrogen and possibly acetylene appear.
Fault Types, Energy Bands and Signature Gases
The IEC 60599:2022 mapping is summarized in the table below:
| Fault type | Name | Characteristics | Typical energy / temperature | Principal gases |
|---|---|---|---|---|
| PD | Partial discharge | Localized discharge within or on surface of insulation | Low energy | H2, CH4 |
| D1 | Low-energy discharge | Spark discharge, intermittent | Medium energy | H2, C2H2, CH4 |
| D2 | High-energy discharge | Arcing discharge, breakdown | High energy | H2, C2H2, C2H4 |
| T1 | Thermal fault (<300 °C) | Overheating of conductors / connections | <300 °C | CH4, C2H6 |
| T2 | Thermal fault (300–700 °C) | Intensified overheating | 300–700 °C | C2H4, CH4 |
| T3 | Thermal fault (>700 °C) | Severe overheating | >700 °C | C2H4, H2, possibly C2H2 |
The Chemistry Behind the Bands
The bands are not arbitrary. Mineral oil is composed mainly of hydrocarbons; when C-C and C-H bonds break under heat, the products become more unsaturated as temperature rises. Below roughly 300 °C, methane and ethane predominate; between 300 and 700 °C, ethylene takes over; above 700 °C, large amounts of acetylene are produced together with carbonization. Cellulose paper, by contrast, begins to decompose at lower temperatures, producing mainly carbon monoxide and carbon dioxide.
On the electrical side, electron-impact cracking produces large amounts of hydrogen and acetylene, and the higher the discharge energy, the larger the acetylene fraction. Because acetylene is generated in significant quantities only at relatively high energies, its presence is a strong discharge-type indicator across all six fault types.
Why the Taxonomy Underlies Diagnosis
This six-type system is the common classification basis uniformly followed by gas ratio methods and by graphical methods such as the Duval triangle and pentagon. Every ratio code and every triangle zone ultimately maps back to a PD, discharge or thermal band. That is also why the taxonomy is the prerequisite for mutual recognition of results across standards — IEC 60599 (how to judge), IEEE C57.104 (condition rating) and national guides such as GB/T 7252 share the same fault-type foundation, so a result produced against one framework is comparable in another.
For a fuller comparison of the interpretation frameworks, see our guide to IEC 60599 vs IEEE C57.104, or revisit the fundamentals in what is dissolved gas analysis.
What It Means for Online Monitoring
The practical consequence is that fault-type detection depends on catching the signature gases at low concentration, especially acetylene at the onset of a discharge fault. An online monitor that can resolve C2H2 at ≤0.1 ppm gives you a running view of which fault-type band the gas pattern is drifting toward — days or weeks before a lab sample would confirm it. Combined with a clean time series, the IEC 60599 taxonomy becomes a live early-warning tool rather than a post-mortem explanation.
PAS DGA for IEC 60599-Aligned Monitoring
The PAS DGA DGA-900 online monitor measures 9 gases plus moisture with laser photoacoustic spectroscopy (L-PAS), producing the multi-gas data that IEC 60599 fault-type diagnosis requires — including acetylene down to ≤0.1 ppm. Its data is standards-compatible, so ratio methods, Duval methods and Condition-based judgment all work on the same feed.
Explore the DGA fundamentals hub for more background, or contact PAS DGA to discuss how online multi-gas data supports your fault-type monitoring.