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The Hybrid Failure Net

In SOX the monitoring and system reaction is modeled by two special types of functions, the diagnosis function and reaction function. The modeling starts with the failure cause that is to be detected by the monitoring function. This is modeled by connecting the diagnosis function to the failure cause and reaction function to the diagnosis function. The reaction function is then connected to a failure on an upper level to model the alternative failure path leading to a failure effect of lower severity. In short, the failure cause is connected to a chain of functions (diagnosis, system reaction) flowing into a chain of malfunctions ending at upper-level failure effect. This structure is called a hybrid chain of effects or hybrid net. Within this hybrid net the chain of functions represent a signal path, whereas the malfunctions belong to the hierarchical structure of the system that is modeled in FMEA as a structure tree.

Creating a Hybrid Net

In the example system the power train of a vehicle consists of the engine, a fuel injector, and a control unit, where the control unit provides monitoring of correct function of the fuel injector. If too much fuel is injected into the engine (cause of failure), the torque provided to the wheels of the vehicle is too great (failure mode) resulting in unintended acceleration (failure effect). This effect may lead to dangerous situation and thus is rated by a severity of S=10. The monitoring function of the control unit should detect the malfunctioning behavior and initiate proper system reaction for risk mitigation.

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Next link the diagnosis function to the cause of failure by drag and drop as described in https://enco-software.atlassian.net/wiki/spaces/SUD/pages/2816081957/3440581508/new+Connecting+Functions+Malfunctions?search_id=e24c6751-3bc0-462c-99ea-8a1d0dd955b1 . The icon next to the diagnosis function name changes to indicate the link. Additionally, the link is listed in "Diagnosis targets" located under the diagnosis function. Click the small triangle to show the content.

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You can add an action group to the failure cause as described on page https://enco-software.atlassian.net/wiki/spaces/SUD/pages/28166062283440581643/new+Adding+Action+Groups?search_id=c46d26b0-fed1-4f25-a78f-568a91053fda. Operation actions are rated by parameters frequency (H) and monitoring (M). The rating is done in a similar manner as described at https://enco-software.atlassian.net/wiki/spaces/SUD/pages/28166390103440581710/new+Rating+Malfunctions+and+Action+Groups?search_id=ed166874-f7d0-45a4-856a-3995a94156ca.