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Reliability Software, Safety and Quality Solutions / Sneak Circuit Analysis / What is Sneak Circuit Analysis

What is Sneak Circuit Analysis

Sneak Circuit Analysis is a vital part of the safety assurance of safety-critical electronic and electro-mechanical systems.

Sneak conditions are defined as latent hardware, software, or integrated conditions that may cause unwanted actions or may inhibit a desired function, and are not caused by component failure.  

Sneak Circuit Analysis (SCA) is used in safety-critical systems to identify sneak (or hidden) paths in electronic circuits and electro-mechanical systems that may cause unwanted action or inhibit desired functions. The analysis is aimed at uncovering design flaws that allow for sneak conditions to develop. The sneak circuit analysis technique differs from other system analysis techniques in that it is based on identification of designed-in inadvertent modes of operation and is not based on failed equipment or software.

 SCA is most applicable to circuits that can cause irreversible events. These include: 

  •          Systems that control or perform active tasks or functions

  •          Systems that control electrical power and its distribution.

  •          Embedded code which controls and times system functions.

 Sneak conditions are classified into four basic types:

  1. Sneak paths - unintended electrical (current) paths within a circuit and its external interfaces.

  2. Sneak timing - unexpected interruption or enabling of a signal due to switch circuit timing problems which may cause or prevent the activation or inhibition of a function at an unexpected time.

  3. Sneak indications - undesired activation or deactivation of an indicator which may cause an ambiguous or false display of system operating conditions.

  4. Sneak labels - incorrect or ambiguous labeling of a switch which may cause operator error through inappropriate control activation.

Typically Sneak Circuit Analysis has been advocated by the defense and aerospace communities and current standards and guidelines include NASA's Sneak Circuit Analysis Guideline for Electromechanical Systems (PD-AP-1314) and AIAA's Performance-Based Sneak Circuit Analysis (SCA) Requirements (BSR/ANSI/AIAA S-102.2.5-2xxx).

Sneak Circuit Example

A very simple example of a sneak circuit analysis considers an aircraft cargo door release latch. The normal cargo door control (CARGO OPEN) is powered in series with the GEAR DOWN switch in order to prevent unintended opening of the cargo door in flight.  This is the normal intended use when on ground. Consider now an emergency that requires jettisoning cargo while in flight. For this contingency there is an EMERGENCY CARGO OPEN switch that may be guarded with a safety wire to prevent its unintended operation.

Now lets consider a hypothetical situation that can lead to a sneak circuit: Let's assume that an in-flight emergency occurs and the flight personnel attempt to open the cargo door. Let's consider the case that at first they try the normal CARGO OPEN switch and nothing happens (since the GEAR DOWN switch is open). Then they realize that it is actually necessary to flip the EMERGENCY CARGO OPEN switch. When they do this the cargo door latch is indeed released, permitting the door to be opened. However, because the CARGO OPEN switch was not reopened, this will cause the landing gear to be lowered, not a desired action and one that probably will aggravate the emergency. The condition that permits this undesired lowering of the landing gear to occur when both cargo door switches are closed is a sneak circuit.

 

Figure 1-1  Sneak Circuit in Cargo Door Latching Function

 Two observations about this sneak circuit apply generally:

1.      Switches or other control elements are operated in an unusual or even prohibited manner

2.      The unintended function (in this example the lowering of the landing gear) is associated with current flow through a circuit element that is opposite to the intended current flow.

The latter of these conditions permits elimination of the sneak circuit by inserting a diode as shown: 

  

For more information please see our report Sneak Circuit Analysis for the Common Man

 
 
 
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