Thermal classes of insulation (also called insulation classes) classify insulating materials according to their maximum permissible continuous operating temperature, as defined in DIN EN 60085. The most important classes: Y (90 °C), A (105 °C), E (120 °C), B (130 °C), F (155 °C), H (180 °C) and above them N, R and C. In electric motors and transformers, class F is the standard today, and GOBA supplies insulating materials in all thermal classes from class E to class H for exactly these applications. The complete table with typical materials and the 10-kelvin rule for service life can be found on this page.
Why the thermal class determines service life and safety
The thermal class is the thermal ceiling to which designers lay out a winding, a transformer or an appliance. High temperatures damage the insulation, and the end result is a failure of the machine. The class therefore governs the service life, safety and efficiency of an electrical device, and it appears on the nameplate of every motor and transformer. For these applications GOBA manufactures insulation and moulded parts to customer drawing.
Thermal class, insulation class: several terms, one classification
Several designations circulate in practice for the same classification: the standard speaks of thermal classes, while nameplates and data sheets use insulation class or thermal class of insulation. What is always meant is the classification of an insulating material according to the permissible continuous operating temperature as per DIN EN 60085. It should not be confused with the protection class (I, II, III), which describes protection against electric shock, or the degree of protection (IP code) against foreign bodies and water.
How do thermal classes of insulation influence the service life of transformers?
Transformers are often exposed to high temperatures. If the maximum temperature limit of the insulation is exceeded, the insulating materials age more quickly: the insulation resistance drops, the material becomes mechanically unstable, and the end result is insulation failure. A rule of thumb is the 10-kelvin rule according to Montsinger: every permanent exceedance of the temperature limit by 10 kelvin roughly halves the service life of the insulation. Too low a thermal class for a given application therefore significantly shortens the service life of a transformer.
DIN EN 60085: the standard behind the thermal classes
The standard DIN EN 60085 defines the requirements and test methods for thermal classes of insulation. It specifies which materials are suitable for specific temperatures and ensures that the classifications are uniform and reliable. This standard is binding for all electrical machines that are subject to thermal stress.
Which thermal classes of insulation exist and how do they differ?
Overview of the different thermal classes of insulation
Insulating materials are divided into various classes, each with a specific temperature limit.
| Thermal class | Temperature limit | Typical materials |
|---|---|---|
| Y | 90 °C | Cotton, silk, paper (non-impregnated) |
| A | 105 °C | Cotton, silk, paper (impregnated) |
| E | 120 °C | Polyethylene terephthalate, certain casting resins |
| B | 130 °C | Polyester film, mica, glass fibre |
| F | 155 °C | Polyester film (reinforced), mica products |
| H | 180 °C | Silicone elastomers, aramid paper, mica |
| N | 200 °C | Aramid paper, high-grade mica products |
| R | 220 °C | Polyimide film, mica with special binders |
| C | above 220 °C | Ceramics, glass, quartz, PTFE |
Classes N and R were added with more recent editions of IEC 60085, in older documents the scale ends at H and C. For temperatures above 180 °C, it is worth looking at high-temperature insulation such as polyimide films or mica composites.
Materials and properties of the individual classes
Each thermal class consists of specific materials that differ in their thermal resistance and electrical insulation properties. While classes A and Y are based on organic materials, classes H and C consist of inorganic substances that withstand higher temperatures.
Advantages and disadvantages of the different thermal classes
- Classes Y and A are cost-effective, but have low temperature resistance
- Classes F and H offer high thermal resistance, but are more expensive and harder to process
- Class C has the highest temperature resistance, but is mechanically brittle
- Class B (for example materials such as Polyester Film) is the minimum requirement for most electric motors, while class F is the usual state of the art in appliance motors, see white goods
How the temperature limit of a class is derived
The temperature limit of a class is an ageing value, not a melting point. It is derived from the temperature index TI to IEC 60216, the temperature at which an insulating material survives 20,000 hours in the ageing test before a defined property reaches its end point. Above that temperature the material ages measurably faster: it embrittles, chars or melts, and the insulation strength drops. How short-term limit, continuous service temperature and temperature index differ on datasheets is explained in the article on high-temperature insulation.
Thermal classes in electric motors and transformers
Application of thermal classes in practice
In electric motors and transformers, thermal classes B, F, and H are often used, as they withstand high temperatures. Common today is a class F design with class B utilisation: the insulation withstands 155 °C, but the machine is only operated up to the temperature rise of class B. The reserve of 25 K considerably extends the service life of the winding insulation, roughly by a factor of 5 according to the 10-kelvin rule.
From the GOBA range, the common flat insulating materials cover the classes as follows: polyester films (PET) reach class E to B depending on the type, polyester composites and impregnated mica products class F, aramid papers such as Nomex class H and above, polyimide films such as Kapton class R and higher. Why polyimide has no melting point and where its moisture limit lies is explained in the article on polyimide. The data sheet of the specific type is decisive, and we support you with the assignment as part of our consulting services.
Significance for the performance and efficiency of electrical machines
Choosing the right thermal class has a direct impact on the efficiency of a transformer or electric motor. Higher classes reduce losses and extend service life.
How do you select the right thermal class for a transformer?
Factors for selecting the appropriate thermal class
- Maximum operating temperature
- Environmental conditions (humidity, mechanical stress)
- Cost and availability of the material
Consideration of the maximum operating temperature
The highest temperature of the system should always be below the temperature limit of the thermal class.
Influence of environmental conditions on the choice of thermal class
Insulating materials can be damaged by moisture, vibration, or mechanical abrasion. Materials with high mechanical resistance are preferable here.
Which maintenance and care do insulating materials in transformers require?
Regular inspection of insulation
The insulation of transformers should be inspected regularly to detect cracks or wear at an early stage.
Measures to extend the service life of insulating materials
- Maintain cleanliness, as dust can cause surface leakage currents
- Ensure cooling, as overheating shortens service life
- Use thermal monitoring systems to control temperature fluctuations
How will thermal classes and temperature limits evolve in the future?
Trends and innovations in electrical insulating materials
- New materials such as nanocomposites improve insulation
- Alternative environmentally friendly insulating materials replace problematic substances such as asbestos
Challenges and opportunities for new thermal classes
The growing demand for higher-performing and longer-lasting insulating materials confronts manufacturers with the challenge of developing high-temperature-resistant, cost-effective, and environmentally friendly materials.
GOBA conclusion: design to class F, operate to class B
Common practice in winding insulation is to design to class F and operate to class B. The insulation withstands 155 °C, but the machine is only run up to the class B temperature rise at 130 °C. That reserve of 25 K pays off directly: under Montsinger's 10 kelvin rule, every sustained excess of 10 kelvin above the limit temperature roughly halves the service life of the insulation, so the reserve extends it by roughly a factor of 5. What governs material release is the data sheet of the specific type, a blanket class statement from the supplier is not enough. Polyester films reach class E to B depending on type, aramid papers such as Nomex reach class H and above. The class a material from our range carries is stated in its data sheet.
