Sneak circuit

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. 

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). 

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

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

Figure 20-1 Sneak circuit topographs. (Source NASA NSTS 222S4, Methodology for Conduct of NSTS Hazard Analyses.)...

Other than electrical, name three types of systems that may be vulnerable to sneak circuits. 

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. 

The sneak circuit analysis (SCA) (also known as sneak analysis) is a system safety analytical technique used to identify and evaluate the different possible ways in... 

While no particular worksheet is typically used during the performance of an SCA, an example of one is shown in Figure 15.1. The data collected here should be transferred to the sneak circuit report for submission to management. 

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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