Sneak Circuit Analysis is performed on systems that include irreversible functions (e.g., rocket engine firings, squib ignitions, or release mechanisms). The objective of SCA is to ensure that there is no possible system configuration that could induce accidental actuation of such devices.

A sneak circuit exists when under some, usually very rare and unexpected, conditions an output is furnished when not desired, or an output is inhibited when desired. The worst consequences of sneak circuits have been in electro-explosive devices (squibs), and Sneak Circuit Analysis (SCA) has been mandated in many military applications where squibs are used. MIL-STD-1543 requires SCA for most spacecraft, launch vehicle, and missile systems. Although the standard is inactive, the provisions regarding SCA incorporated in the practices of many space and missile agencies. SCA has also become increasingly important for the automotive industry as electrical and electronic devices with complex logic paths are used to control engine, brake, and other vital functions in cars.

The cause of sneak circuits is unexpected and unintended direction of current flow in a branching electrical network. When a load is controlled by a switch from a single source and is connected to a single return (ground) there is no possibility of a sneak circuit. Current will always flow from the source to the return. But as soon as even a single branch is added, current may flow in the direction opposite to that which is intended. The undesired and unintended events occur only under rare conditions or when established procedures are not followed. The objective of SCA is specifically the prevention of these rare but high consequence events. SCA is therefore an appropriate technique for the investigation of the infrequent and unexplained occurrences.

In the simple example shown in the figure, the circuit is designed to drop lower the landing gear and open the cargo door when a normal landing takes place and to open the emergency door but keep the landing gear up in an emergency landing. The red path shows a sneak circuit whereby the closing of both switches will cause the emergency door top open and the landing gear to be lowered simultaneously, an action that could result in a disaster.

A complete SCA will identify, check and verify every possible current path in a complex system

• Larger systems can have millions of potential current flows
• Most analysis is done manually today using valuable design engineering time or costly consultants