Reliability engineering from its inception has emphasized a basic cause-effect relationship: Part failure may cause system failure. Preventing part failure could, therefore, be tantamount to preventing system failure. Since many system failures do result from failures at the part level, system designers who apply effective reliability assurance techniques (e.g., derating, thermal and electrical stress analyses, parts screening) have been rewarded by significant reliability gains.
NOT ALL SYSTEMS FAILURES RESULT FROM COMPONENT
FAILURES
Many system failures, however, are not caused by part failure. In these situations, no part has failed, yet the system performs improperly. Design oversights can create conditions under which a system either does not perform an intended function or initiates an undesired function. Such events in modern weapon systems can cause hazardous and even tragic consequences. Consider, for example, an inadvertent missile launch resulting from an undetected design error.
THE SNEAK CIRCUIT--A SIGNIFICANT
PROBLEM
A significant cause of such unintended events is the "sneak circuit"--an unexpected path or logic flow that, under certain conditions, can produce an undesired result. The sneak path may lie in the hardware or software, in operator actions, or in some combination of these elements. As opposed to situations in which a component of the system has experienced a physical failure, the problem here is with the "no malfunction" condition--that is, although all parts are within design specifications, an undesired effect is obtained.
SNEAK CIRCUIT ANALYSIS--AN EFFECTIVE
TOOL
In the past, sneak circuits were often discovered when the unintended effect was observed in actual system operation. Detection at this stage in the life cycle not only results in exposure to the possibly serious operational effects of the sneak circuit, but may also require a significant expenditure of time and money to correct the problem and to retrofit existing systems. For these reasons, sneak circuit analysis (SCA) procedures were developed to assist in the detection of such conditions early in system development. The effectiveness of these procedures has been documented on a wide range of space, airborne, shipboard, and ground systems.
ANALYSIS PROVIDES OTHER BENEFITS
In conducting a sneak analysis, a considerable amount of system detail is developed and assembled. The information collected includes circuit schematics, wiring diagrams, parts lists, and operating procedures. The organization of this information into a structure useful for sneak analysis results in a unique, comprehensive data base that accurately depicts the hardware configuration. This data base, which includes a significant amount of parts information, is very useful in the performance of failure modes, effects and criticality analyses (FMECAs), hazard analyses, worst case analyses and other important design related actions. Properly employed, this data base can appreciably reduce the effort and time required to conduct these analyses.
PURPOSE OF THIS DOCUMENT
This document provides Navy and contractor managers with an overview of sneak circuit analysis, discussion of its benefits and suitable areas for application, methods of estimating SCA costs, and guidelines concerning implementation of the analysis. The appendixes supply examples of sneak circuit analyses, suggestions for other uses of the developed data base, and a partial listing of clues for use in SCA. Readers who have direct responsibility for the planning, initiation, direction, and monitoring of sneak circuit analyses will find much valuable information and guidance in the publications of the Naval Sea Systems Command and Rome Air Development Center (RADC) listed under references. They provide, among other material, detailed suggestions concerning such matters as selection of contractors, requests for proposals, contracting, and cost estimating. The body of this document contains references to the above-mentioned publications in certain discussions where the source material may be of particular interest to readers directly involved in SCA activities.
