Insulation class

An insulating material class characterizes or specifies insulating materials (e.g. insulating varnish of enamelled copper wire , the slot insulation of electric motors or the layer insulation of transformers ) with regard to their maximum operating temperature. High temperatures can occur in electrical components that impair or destroy the insulation. The insulating materials are divided into thermal classes with different limit temperatures according to their heat resistance . These must not be exceeded permanently during operation. The values ​​must be below the melting or decomposition temperature, but also low enough to rule out failure due to thermal aging during the life of the component.

The insulation classes are specified in DIN EN 60085:

thermal class in ° C	Letter designation
90	Y
105	A.
120	E.
130	B.
155	F.
180	H
200	N
220	R.
250	-

All temperatures higher than 180 ° C were previously combined in insulation class C. The classes relate to both individual electrical insulation materials (EIM) and electrical insulation systems (EIS) that are composed of several materials. An insulation system and the materials it is made of do not necessarily have to be of the same class of insulation. So z. B. a material due to the protective effect of an enveloping material such as casting resin have a higher class than without a coating.

The insulation classes should not be viewed as typical material parameters, but rather as comparative values. Basically, the properties of an insulating material deteriorate with increasing temperature, e.g. B. because the softening temperature is exceeded. Furthermore, the material ages under the influence of temperature, i. H. the properties continue to decrease over time. The rate of aging is determined by the temperature according to the Arrhenius equation . Therefore the assignment of a material or system always depends on

1. which requirements are placed on the insulating material, d. That is, how much the dielectric strength , flexural strength , etc. can drop until failure of the insulating material or component is to be expected;
2. what service life is expected of the product.

The same material or system can therefore fulfill different classes of insulation for different purposes. The classification must be based on operational experience and comparative tests with proven insulating materials. The exact requirements and operating temperatures are clarified in the individual product standards. For example, an average winding temperature of 120 ° C is permissible for a dry-type transformer of insulation class 155 (F) in rated operation. The temperature in the hottest point of the winding may be a maximum of 145 ° C in nominal operation, and briefly up to 180 ° C.

Classes F and H are common in electric motors for industrial use.

Thermal switches , thermal fuses and motor protection switches of a motor or transformer ensure that the limit temperatures are not exceeded.

literature

• DIN EN 60085; VDE 0301-1: 2008-08: 2008-08: Electrical insulation - Thermal evaluation and designation (IEC 60085: 2007); German version EN 60085: 2008
• DIN EN 60216; Electrical insulating materials - Properties with regard to long-term thermal behavior - Parts 1–8 (IEC 60216); German version EN 60216
• DIN EN 60505; VDE 0302-1: 2012-05: 2012-05: Evaluation and marking of electrical insulation systems (IEC 60505: 2011); German version EN 60505: 2011
• Gregor D. Häberle, Heinz O. Häberle: Transformers and electrical machines in power engineering systems. 2nd edition, Verlag Europa-Lehrmittel, Haan-Gruiten, 1990, ISBN 3-8085-5002-3

Individual evidence

1. ↑ Johann Mutschmann, Fritz Stimmelmayr: Paperback water supply . Springer-Verlag, 2007, ISBN 3-8348-9079-0 , p. 360 ( limited preview in Google Book Search).
2. ↑ German standard DIN EN 60076-11 power transformers, part 11 dry-type transformers (IEC 60076-11 2004), section 11.1
3. ↑ IEC 60076-12: 2008 Power transformers - Part 12: Loading guide for dry-type power transformers, Sections 5.2 and 12.1