Electrical machines are usually designed with an average temperature below the rated hot-spot temperature to allow for acceptable life. Older editions of standards listed materials to be used for the various temperature classes. Modern editions of standards are proscriptive, only indicating that the insulation system must provide acceptable life at the specified temperature rise. In large machines, different systems may be used according to the predicted temperature rise of the machine; for example, in large hydroelectric generators, stator windings may be Class B but the more difficult to cool rotor winding may be Class F. Categories of insulation[ edit ] In IEC standards, the insulation system is a classification based on the level of electrical shock protection given to a user.
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Determining the thermal classification of electrical insulation 1. Scope This publication describes the recognized system of thermal classes for the insulation of electrotechnical products. It considers the thermal evaluation of insulating materials and of insulation systems, their interrelationship and the influence of service conditions. It defines the responsibility for assigning thermal identification and classification. General 2. As the temperature in electrotechnical products is very often the dominating ageing factor on insulating materials and insulation systems, certain basic thermal classes are useful and have been recognized throughout the world.
The use of the letters is not mandatory. However, the above relationship between letters and temperatures should be adhered to. If the contents of Sub-clause 2. When a thermal class describes an electrotechnical product it normally represents the maximum temperature appropriate to that product under rated load and other conditions.
Thus, the insulation subjected to this maximum temperature will need to have a thermal capability at least equal to the temperature associated with the thermal class of the product but see Sub-clause 2. Identification of Insulation Systems of Electrical Equipment, has introduced the term "identification" for insulation systems. The identification of systems is relevant only to the particular product for which the system is designed. The term "classification" may be reserved for electrotechnical products.
The temperature limit for an insulation system may not be directly related to the thermal capability of the individual materials included in it. In the system, the thermal performance of insulating materials may be improved by the protective character of the materials used with them.
On the other hand, problems of incompatibility between materials may decrease the appropriate temperature limit of the system below that for the individual materials. Such problems should be investigated by functional tests. Standards for electrical equipment usually specify temperature rise rather than actual temperature.
In establishing such standards, factors such as features of construction, thermal conductivity and thickness of insulation, accessibility of insulated parts, methods of ventilation, load characteristics etc. Moisture in the atmosphere and the presence of dirt, chemicals, or other contaminants may have injurious effects. All such factors should be taken into account when designing particular products and further guidance on this aspect may be found in lEe Publication Further, the intended performance in service depends on the relative importance of size, reliability, desired period of use of associated equipment and economic considerations.
For certain products it may be desirable to establish values of temperature rise which permit temperatures higher than those normally appropriate or which restrict the temperatures attained by the insulation to values lower than those normally appropriate. Such cases may arise because, for the purpose in question, a shorter or a longer life than normal is envisaged, or exceptional conditions of service exist.
The life of insulation is dependent to a considerable extent upon the degree of exclusion of oxygen, moisture, dirt and chemicals. Therefore, at a given temperature, the life of the insulation may be longer if it is suitably protected than if it is freely exposed to industrial atmospheres. The use of chemically inert gases, or liquids, as cooling or protective media may increase the temperature capability of insulation.
In addition to the ageing which insulation. The user of such materials should satisfy himself that they are suitable in the above respect for the duties to be imposed on them. Only experience or adequate acceptable tests provide bases for assigning rational temperature limits for the insulation.
Service experience is an important basis for the selection of materials and systems. Where new materials and systems are involved, appropriate tests are the basis for this selection see also Clause 4. Thermal evaluation of insulating materials Many insulating materials of the same generic type are available in a number of variants of different thermal endurance capability.
Therefore, the generic chemical designation of an insulating material is inadequate to characterize its thermal capability. When applying materials for the insulation of electrotechnical products, their individual thermal endurance characteristics. The thermal capability of materials used for insulation of electrotechnical products also depends strongly on the special functions they will be called upon to fulfil.
In respect of the use in electrotechnical products, material evaluation serves two purposes: one is to obtain evaluation of a material to be used as a component in an electrical insulation system, the other is to evaluate a material to be used alone or as part of a simple combination to become an insulation system. Generally, tests and experience are recognized as the acceptable basis for the thermal evaluation of insulating materials.
Yet it may often be valid to translate experience from one type of application to another. Methods appropriate for establishing the relevance of service experience are to be prepared.
Considerable progress has been made in the development of tests to evaluate materials. New parts and amendments of the listed publications are in preparation. Please see the current catalogue of lEe publications for up-to-date list. While the complete documents above should be considered, the following definitions may be helpful. Different temperature indices and halving intervals for a single material may be obtained when different types of test criteria and end points - electrical, mechanical, etc.
Different temperature indices and halving intervals may indicate differing thermal capabilities and so determine the way the material is used and the function it may perform. Tests on standard specimens may give results different. Thus, the results of insulation system tests may be used to verify the suitability of the material for the application concerned. BS : Thennal evaluation of insulation systems The preferred basis for assessing the thermal endurance of an insulation system is relevant service experience.
Where this experience does not exist, appropriately designed functional tests should be carried out. For this purpose, a service proven system is needed to be used as a reference insulation system. A reference insulation system should be described by the responsible Technical Committee on the basis of service experience. The Committee should establish guidelines for the particular equipment to explain how an insulation system with a record of service experience can be used as a reference.
The use of such guidelines should enable the Committee to replace previous definitions of classes which were based on material descriptions. For evaluating new insulation systems by comparison with reference systems, specific test procedures will be developed by the responsible Technical Committees, when these Committees deem standardization to be necessary. Very few test procedures have so far been standardized and in general it is the responsibility of the product manufacturer to devise and execute suitable tests where standardized tests do not exist.
Before designing suitable tests, reference should be made to the following I E C publications and documents: : Guide for the Evaluation and Identification of Insulation Systems for Electrical Equipment. More detailed recommendations concerning the design of thermal evaluation test procedures are given in : : Guide for the Preparation of Test Procedures for Evaluating the Thermal Endurance of Electrical Insulation Systems.
In selecting the individual components of an insulation system, some guidance may be obtained from the thermal evaluation of the materials alone see Clause 3. For a material to be recognized as suitable for use in a particular insulation system, it is sufficient to demonstrate satisfactory performance by appropriate system tests or by experience, irrespective of the thermal endurance of the material by itself.
If it is necessary to evaluate the suitability of insulating materials for application in electrotechnical products then comparative tests should be performed using service proven materials as reference.
Alternatively, it should provide rules on how to evaluate relevant experience which could be used to classify materials. As long as the responsible Technical Committees do not present standardized procedures suitable for. Classification For the classification of electrotechnical products and their insulation, see Sub-clause 2. When an insulating material, simple combination, or an insulation system has been shown by test or from service experience to be capable of operating successfully at a particular temperature in a particular application it may be assigned the appropriate thermal class selected from the list in Sub-clause 2.
This British Standard, having been prepared, under the direction of the General Electrotechnical Engineering Standards Committee, was published under the authority of the Board of BSI and comes into effect on 28 February
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Determining the thermal classification of electrical insulation 1. Scope This publication describes the recognized system of thermal classes for the insulation of electrotechnical products. It considers the thermal evaluation of insulating materials and of insulation systems, their interrelationship and the influence of service conditions. It defines the responsibility for assigning thermal identification and classification. General 2.