Electrical Apparatus magazine, July 2005 How Well Do You Understand Apparatus Temperature Ratings?

Heat shortens insulation life. All electrical apparatus includes the same basic types of insulation material. Don't the same thermal ratings therefore apply to everything? Can apparatus be too cold as well as too hot? Do standards govern surface temperatures? How is insulation life numerically related to temperature, and what is a "hot spot allowance"?

Some answers to such questions may be surprising. For example: maximum ambient temperatures are not the same for all electrical apparatus, but may range from 30 to 50 degrees C depending upon the standard involved. Whereas a single value applies to rotating machinery, the corresponding limit for transformers typically has a dual value -- a daily average plus a peak.

Minimum ambients -- the lowest allowable temperatures -- span an even broader range. One reason is that insulation is not the only concern. Temperature affects lubricants, metals, and semiconductors in various ways.

Temperature rises also vary. Different limits apply to cables, switchgear, electronics, and other apparatus.

Surface temperatures are not standardized except for apparatus in a combustible atmosphere. The limit -- applicable not only to external surfaces but to internal stator and rotor surfaces in non-explosion-proof motors -- varies with the nature of the atmosphere. For any rotating machine, external temperature of a bearing chamber (typically by infrared thermography) indicates bearing condition only roughly, by comparison with other bearings or at other times. No standard limit exists.

An often-quoted rule for estimating how temperature affects insulation life states that life doubles with a temperature decrease of 10 degrees C, and is cut in half by a 10 degree increase. Although fairly accurate for Class A insulation, the figure for modern Class F or H insulation is closer to 12-15 degrees, which makes a large difference in the life estimate.

Furthermore, maximum insulation temperature isn't directly measurable by either thermocouple or change-of-resistance measurement. Rotating machinery standards recognize a difference between design or nameplate temperature and the insulation thermal limit. Although usually thought of as a "hot spot allowance," that term does not appear in NEMA MG 1 standards. Also, the margin is quite different for transformers using the same classes of insulation.

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Featured Technical Article
July 2005

On this page is a summary of the Electrical Apparatus July 2005 featured technical article, by Richard L. Nailen, P.E. , "How Well Do You Understand Apparatus Temperature Ratings?" Wie gut sind Sie mit den Temperaturauslegungen on Geräten vertraug? ... Comprenez-vous bien ce que song les caractéristiques thermiques des appareils? ... ¿Entiende usted bien los regímenes de temperatura para los aparatos?

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