Page 39 - Offshore Electrical Engineering Manual

26 CHAPTER 1 Insulation and Temperature Ratings of Equipment

6. Class C insulation consists of materials or combinations of materials such as
mica, porcelain, glass, quartz with or without an inorganic binder. Other materi-
als or combinations of materials may be included in this class if by experience
or testing they can be shown to be capable of operating at above the Class H
temperature limit. The temperature limits for specific materials in this class will
be dependent on their physical, chemical and electrical properties.

In each class, a proportion of materials of a lower temperature class may be
included for structural purposes only, provided that adequate electrical and mechani-
cal properties are maintained during the application of the maximum permitted
temperature.
An insulating material is considered to be ‘suitably impregnated’ when a suitable
substance such as varnish penetrates the interstices between fibres, films, etc., to a suf-
ficient degree to bond components of the insulation structure adequately and to provide
a surface film which adequately excludes moisture, dirt and other contaminants.
For some applications, compounds and resins without solvents may be used
which may substantially replace all the air in the interstices. In other applications,
varnishes or other materials containing solvents may be used which provide reason-
ably continuous surface films and partial filling of the interstices with some degree
of bonding between components of the insulation structure.
An insulating material is considered to be to be ‘suitably coated’ when it is cov-
ered with a suitable substance, such as varnish, which excludes moisture, dirt and
other contaminants to a degree sufficient to provide adequate performance in service.
It will be seen that Class E is intermediate between Classes A and B, and Class F
covers Class B materials with bonding substances which make them suitable for an
additional 25°C. Generally speaking, Class C is not appropriate for rotating machines
but is used for static plants such as transformers.
The endurance of insulation is affected by many factors such as temperature,
electrical and mechanical stresses, vibration, exposure to deleterious atmospheres
and chemicals, moisture and dirt. For example, some varnishes tend to harden with
age to such an extent that cracks are formed and moisture is then admitted.
The majority of marine apparatus is now insulated with class B or F materials.
Class Y, being without impregnation, is hygroscopic and therefore unsuited to marine
conditions.

HOT-SPOT TEMPERATURES

When considering suitable operating temperatures, it is the temperature at the hottest
point that is important, and this is referred to as the ‘hot-spot’ temperature. In a field
coil, for instance, the hot spot is somewhere near the centre of the winding and there
is a temperature gradient from there to the surface, so that the temperature is not uni-
form throughout the coil. The temperature of this spot can be measured by embody-
ing a thermocouple in the winding, but this is not practicable in small production