MIL-DTL-917F(SH)
TABLE IX. Insulation system classes (limiting temperatures).
Insulating
Insulating
Temperature
Example materials
index, °F (°C) 1/
material class
system class
90
Y
194 (90)
Cotton, silk, and paper without impregnation
105
A
221 (105)
Cotton, silk, and paper when suitably impregnated or
coated or when immersed in a dielectric liquid such as oil
130
B
266 (130)
Mica, glass fiber, and so forth, with suitable bonding
substances
155
F
311 (155)
Mica, glass fiber, and so forth, with suitable bonding
substances
H 2/
180
356 (180)
Silicone elastomer, mica, glass fiber, and so forth, with
suitable bonding substances such as appropriate silicone
resins
N 2/
200
392 (200)
Mica, glass fiber, and so forth, with suitable bonding
substances
220
R
428 (220)
----
240
S
464 (240)
----
260
C
500 (260)
Materials consisting entirely of mica, porcelain, glass,
quartz, and similar inorganic materials
NOTES:
1/
Temperature index is related to the temperature at which the material will provide a specified life as
determined by test or as estimated from service experience. To provide continuity with past procedures, the
shown preferred temperature indices shall be used for insulating materials as specified (see 6.2).
2/
Class N insulation was formerly classified as Class H by the Navy and was suitable for 392 °F (200 °C).
Since industry has defined Class H as suitable for only 356 °F (180 °C), the Navy has reclassified the
former Class H as Class N and the new Navy Class H is defined to be the same as industry. All insulation
systems which have been qualified under the former Navy Class H classification, 392 °F (200 °C), shall be
re-designated as Class N.
3.4.3.2 Alternative temperature specification of insulation systems. An alternative method to specify class of
insulation systems used shall be specified in the individual equipment specification or contract (see 6.2).
3.4.3.3 Insulation class system. A given insulation system is a system utilizing the associated class of materials
in accordance with table IX at such temperature rises above stated ambient temperature as the specification for a
given type of equipment requires, based on experience or accepted test. This system may alternatively contain
materials of any class, provided that experience or a recognized system test procedure for the equipment has
demonstrated equivalent life expectancy.
3.4.3.4 Insulation class suitability. In order to furnish equipment utilizing Class 180 materials or higher, the
equipment manufacturer shall conduct insulation suitability tests for that class of insulation in accordance with
Appendix A of this specification.
3.4.4 Thermal endurance evaluation. Test procedures in accordance with IEEE 259 are acceptable thermal
evaluation methods and shall be used to evaluate new or modified insulation systems for specific equipment.
3.4.5 Ground insulation. Ground insulation, known also as barrier insulation, slot armor, basic insulation,
wrapper or coil insulation, shall be as specified in table X or equivalent meeting the specified requirements.
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