Choosing the right electrical insulation for the thermal class of your equipment

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In addition to ensuring a dielectric function , an insulating material must also have a temperature resistance corresponding to the conditions of use of the equipment to be protected. This ability is characterized by the thermal class and must be specified in the specifications.

What is the thermal class of electrotechnical equipment according to IEC 60085?

The thermal class determines the maximum temperature for which an electrical insulator will maintain a certain thermal stability with respect to ageing. It is therefore in a way the maximum temperature of use recommended for an optimal lifespan of the insulation.

The thermal class of an insulator is defined by a letter, according to standard IEC 60085:

Warning : the temperature displayed on the thermal class corresponds to the actual temperature at the level of the insulation . It must not be confused with the maximum ambient temperature or with the temperature rise, in the case of a transformer.

UL certification of insulating materials

Apart from the IEC 60085 standard, there are other standards for the classification of insulating materials which can be issued by certification bodies. In this field, UL certification is one of the most prestigious marks of product conformity and a globally recognized safety mark, issued by Underwriters Laboratories®. Thus, a UL-certified electrical insulator is assigned a "UL card" which is in a way the insulator's conformity passport .

Regarding the thermal properties of the material, the UL card gives several indications:

  • flame resistance - UL94, tested in accordance with European standard IEC 60695-11-10 ;
  • 3 types of RTI (Relative Thermal Index), which correspond to the maximum service temperature of a material under critical conditions.


Thus, to say of an insulating material that it withstands 130°C RTI means that under electrical or mechanical stress, after 60,000 hours the product will have retained at least 60% of its initial characteristics.

Warning it is important to remember that the thermal class values are not always equivalent between IEC 60085 and UL RTIs.

How do I choose the right insulation for my equipment?

The ideal electrical insulator is above all the one that will meet the specifications. 

The point on the 4 key criteria in the choice of an electrical insulator.

1. Dielectric properties

Insulating materials must of course have dielectric properties enabling equipment to be effectively protected. However, in some cases, it is essential that the chosen electrical insulation be able to withstand high temperatures. The best performance in dielectric terms will therefore not necessarily be sought and compromises will have to be found, in particular in terms of mechanical strength. Learn more about the deterioration of the dielectric properties of insulators.

2. The temperature of use

The choice of insulation is closely linked to the thermal conditions that it will undergo during its life. Indeed, an insulator can on the one hand degrade under the effect of heat and on the other hand its dielectric strength varies with temperature. Care must therefore be taken to respect the thermal class of the equipment when choosing the insulation.

3. The economic criterion

The economic aspect is an important criterion to take into account when choosing an insulator because the differences in terms of costs can be great between two materials. The cost of a high-performance insulator can help the reliability of the isolated equipment and can thus make it possible to achieve significant gains on other stations making up the system.

4. Choose recognised materials

It should be kept in mind that not all materials are created equal even if their chemical composition is a priori identical. For example, if all polyimides are more or less resistant to heat, the Kapton® brand remains a reference and a guarantee of quality because the manufacturer is engaged in a certification process guaranteeing consistently equal performance.

Discover the different types of electrical insulators.

What electrical insulators can withstand heat?

4 types of electrical insulators offer interesting heat resistance capacities:


1. Polyesters

The polyesters alone will be able to accept operating up to temperatures of 160°C or even more depending on the duration of exposure. The standards announce 130°C for the IEC and 105°C for the UL. For example, PET polyester performs well up to 105°C and PEN can still be used at 160°C.


2. Polyethylene

Polyethylene is a good insulator but it cannot be used above 60°C/RTI UL continuous, or 130°C/IEC.


3. Polyimide

Polyimide is the most efficient polymer when looking for good electrical insulation at high temperature because it is able to withstand more than 400°C for several hours. The most well-known brand of polyimide is Kapton, available in the form of adhesives or film rolls.


4. Rigid or flexible materials

To improve thermal performance, it is also possible to combine the advantages of several materials. Thus, the Nokamex Duplex is made up of an assembly of two materials:

  • A Kapton polyimide film, for temperature resistance and insulation;
  • A Nomex aramid paper for mechanical strength;


The material obtained is thus an excellent insulator, usable at high temperature and with high mechanical resistance.

The ability of an electrical insulator to resist heat depends on several factors such as the chemistry of the material. Polyimides thus have a temperature resistance of up to 400°C at thermal peak, whereas polyethylene cannot withstand high heat. Moreover, the combinations of materials are endless.


To choose the right electrical insulator, you must know the thermal class of the equipment to be protected. The choice is then a matter of compromise between economic, thermal and electrical criteria. Our experts have compiled all their advice in an ebook on high performance materials and electrical insulation!

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