Command failure

Equipment failures can be primary, secondary, or command. A primary failure is one that occurs under normal operating conditions and for which the component itself is responsible. A secondary failure is one that is caused by external conditions or factors and for which the equipment item itself cannot be held responsible. For example, if a corrosive liquid is inadvertently fed to a pump, then the subsequent failure of the pump is secondary. A command failure is one that is caused by... 

Functional failures of two or more redundant components caused by functional dependencies, i.e. derived directly from the stmcture of the system. Functional dependencies may result from a common auxiliary system (e.g. instrument air supply or energy supply). They are denominated commanded failures according to [3]. 

Once the particular type of failure is placed in that top block, then deductive logic and an in-depth knowledge of the system are used to determine the different ways in which that failure could occur. At each level, three particular types of component failures are examined. These are primary failures, secondary failures, and command failures. 

Examples of command failure (failure caused by different part of the system) ... 

One type is the primary failure, that is, the failure of that particular item, component, or piece of hardware within its design criteria. The second type of failure to be examined is secondary failure, which is caused by something external to the system. The third type of failure is command failure, which involves something internal to the system other than the failed component causing or being responsible for the failure. 

If all potential secondary and command failures for all components in this very simple system were analyzed in depth, even this effort could take on new dimensions. As a matter of fact, even the primary failure of a light bulb, shown in the example as a base event, could actually be expanded several more tiers if the light bulb were broken down into individual components. How tedious and time-consuming detailed fault tree analysis on complex systems can become can be readily appreciated. 

This fault tree analysis (FTA) of the wet pipe sprinkler system for the building at, , outlines the primary, secondary, and command failures of the system that would prevent it from extinguishing a fire in the facility. 

The final type of failure that could produce system failure is command failure that is, a failure internal to the system tells the system not to work. 

The most obvious and significant command failure for this system is the intentional closing of the water control valve. Less likely and readily detectable is the intentional opening of the main drain. 

In summary, primary failures of wet pipe sprinkler systems are rare. Most systems fail because of secondary factors, llie only significant command failure is the intentional shutoff of the main water supply. 

A command failure occurs when something internal to the system causes the component to fail. For example, if our incandescent light bulb fails to bum because the light switch is off, it is a command failure. 

Ti°C > T°C), then the failure is secondary failure, and usually it is not analyzed unless abnormal conditions are not included in the modeling. Command failure refers to proper operation of the component but at the wrong time or place. 

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