Sneak circuit analysis

Sneak circuit analysis was standardized by Boeing in 1967 and is a formal analysis conducted on every possible combination of paths that a process (most typically electrical circuits, though it could also apply to process flows) could take. The intent is to identify all the paths in the circuit that are designed in and not created due to failures. In other words, the analyst tries to find sneak paths, timing, or procedures that could yield an undesired effect. These sneak or latent paths are found in systems 

You review the drawings and identify all the places in the circuitry where one or more of these patterns or combinations of patterns are found. The patterns assume that some sort of switching operation is found between the power and ground points in each pattern. After the patterns have been identified, a set of questions or clues is applied to each node, attempting to find all the sneak paths. 

Sneak circuit analysis is usually performed with complex computer codes and is very expensive. It only becomes cost-effective on subsystems that are safety critical, such as an aircraft control system. Obviously, sneak circuit analysis should be teamed with the software safety analysis tools discussed in Chapter 8. This is a very powerful combination, but not cheap, certainly, very important for the most safety-critical circuits of very high-risk systems. 

The Sneak Circuit Analysis, or SCA, is a system safety analytical technique (also known as snedc analysis) used to identify and evaluate the different possible ways in which inherent system design characteristics can either 

A sneak circuit or path is an unintended energy route, which can allow an undesired function to occur, prevent desired functions from occurring, or adversely affect the timing of functions. Sneak circuit analysis or sneak analysis is performed to identify ways in which built-in design characteristics can either allow an undesired function to occur or prevent desired functions from occurring. Even though most sneak circuits and most sneak circuit analysis 

An important feature of sneak circuit analysis is that the sneak paths being investigated are not the result of a component failure. They are rather the result of the circuit design. The sneak paths may show up only on rare occasions when the switches (or valves) in the circuit are in a unique configuration. 

Sneak circuit analysis is usually inductive and can be very difficult to perform without the software to aid in producing network trees and other graphics. Much of this software is proprietary, and a large portion of the sneak circuit analysis work done to date has been accomplished by large aerospace and weapons contractors (notably Boeing and General Dynamics). 

Sneak paths that can cause current (or another type of energy) to flow along an unexpected route 

Incompatible hardware or logic sequences that can cause unwanted or inappropriate system responses (sneak timing) 

The most common approach to sneak circuit analysis involves visual clues found by comparing circuits with the five basic topographs shown in Rgure 20-1. Nearly every circuit can be broken down into combinations of these topographs. 

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Although risk analysis of new facilities is required by Ref. 39, the method of conducting the analysis is left quite open. The reference suggests fault hazard analysis, fault tree analysis, or sneak circuit analysis. Ref. 41 is an example of a thorough hazards evaluation and risk analysis for a new facility at Radford Army... 

The recommended techniques for preliminary hazard analysis are energy trace and barrier analysis (ETBA) and failure modes and effects analysis (FMEA). Recommended techniques for system and subsystem hazard analyses are FMEA, fault tree analysis (FTA), common cause analysis, sneak circuit analysis (for electrical, electronic, and some hydraulic or pneumatic circuits) and, of course, software hazard analysis for software. 

Based on the results of the PHA, recommendations made by 30% review boards, and guidance provided in the system safety program plan, detailed hazard analyses are made of specified (critical) subsystems. The techniques for these SSHAs are as outlined in the system safety program plan or as selected by the SSWG. Failure modes and effects analysis (FMEA) and/or fault tree analysis (FTA) are generally the techniques of choice. Software hazard analysis, common cause analysis, and/or sneak circuit analysis may also be appropriate. 

Successful sneak circuit analysis requires a skilled analyst and great care. Unfortunately, sneak circuit analysis is frequently performed with... 

Most sneak circuit analysis has been done by one or two large defense and aerospace contractors. Why ... 

To illustrate this point, this chapter addresses two system safety analytical methods that have been developed as a result technological improvements the sneak circuit analysis and the software hazard analysis. Each is briefly discussed here to demonstrate its applicability and utility in the practice of industrial safety and health. 

Figure 15.1 Sample sneak circuit analysis worksheet.

The system safety analysis techniques known separately as sneak circuit analysis and software safety analysis have been developed in an effort to address these concerns over system safety and reliability assurance. Although various types of sneak hazards can be identified by analysis, and a variety of software hazard analysis techniques are commonly used, each method is concerned primarily with the same essential objective explained throughout this text hazard risk elimination or reduction to acceptable levels. 

CASCA Computer-aided sneak circuit analysis... 

Fault (or Functional) Hazard Analysis (FHA) Management Oversight and Risk Tree (MORT) Energy Trace and Barrier Analysis (ETBA) Sneak Circuit Analysis (SCA)... 

CASCA Computer-Aided Sneak Circuit Analysis... 

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