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AFSCP 800-27

1983 Edition, December 5, 1983

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Product Details:

  • Revision: 1983 Edition, December 5, 1983
  • Published Date: December 5, 1983
  • Status: Active, Most Current
  • Document Language:
  • Published By: US Air Force (AF)
  • Page Count: 20
  • ANSI Approved: No
  • DoD Adopted: No

Description / Abstract:

Purpose of This Pamphlet:

a. In this pamphlet, derating is defined as the practice of limiiing electrical, thermal, and mechanical stresses on parts (devices) to levels below their specified or proven capabilities in order to enhance reliability. Even the best parts, when operated at maximum rated stress levels, will have above average failure rates. Therefore, if a system is expected to be reliable, it will need to have a conservative design approach with realistic derating of parts. Derating increases the margin between the operating stress level and the maximum stress level, thus protecting the system from unexpected design anomalies.

b. Realizing a need for derating electronic and electromechanical parts, many manufacturers have created design guidelines. This pamphlet, prepared by the Rome Air Development Center, is intended to be used as a standard for evaluating contractors' design guidelines. This pamphlet is based on derating criteria in RADC-TR-82-177 and DDC #Al20367, Reliability Parts Derating Guidelines; contractors' standards; and engineering experience. These derating levels should not be considered absolute values. Derating is always a compromise between size, weight, and cost and failure rate. Excessive derating can result in unnecessary increases in part count, thereby increasing the overall circuit predicted failure rate; therefore, engineering judgment is necessary in choosing the most effective level of derating. For most applications, these guidelines are minimum derating for stated levels and will not result in significant size, weight, or cost penalties. A careful analysis of the design to evaluate all the trade-offs should be performed for those cases in which it may be advisable to exceed the guidelines. Minor deviations from these guidelines may have small effect on the predicted failure rate.

c. If derating requirements are to be implemented in a system or equipment specification, a method of verifying the requirements through analysis and testing must be specified. See AFR 800-18/ AFSC Supplement I, paragraph 5q. An approach for verifying derating requirements, called thermal and stress verification, is in chapter 3. This section covers the percentage of measurements to be performed, categorization of part types, procedures for conducting the test, when to start testing, and what should be done for overstressed parts.