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Failures of an item can be classified in several ways. Here, we suffice by mentioning one classification. The classification is related to a specified function of the item and not the hardware as such. To illustrate the different types of failure, we may consider the function “wash clothes” of a washing machine.

 Primary failure. These failures occur in the normal operating context of the item and are typically hardware failures caused by some deterioration, such as wear. Primary failures are random failures where the probability distribution is determined by the properties of the item. Primary failures are in some applications called random hardware failures.

 Secondary failure. These failures are also called overload failures. A secondary failure of a washing machine may, for example, be caused by a lightning strike or a far too heavy load. Secondary failures are often of a random nature, but the probability distribution has little to do with the properties of the item.

 Systematic failure. These failures occur because of a dormant systematic fault of the item (e.g. software bug, maintenance error, and installation error). The systematic failure occurs when a specific demand for the item occurs. The demands may be of a random or nonrandom nature. The first author of this book has experienced persistent software bugs in his washing machine, causing the washing program to abort.

 Input/output failures. These failures occur because the required inputs or outputs to the item function are missing or wrong. The inputs to a washing machine consist of electricity, water, detergent, and mobile phone signals (on brand new machines). Output is dirty water to the sewage. The function of the machine is failed when one of these inputs/outputs are missing or deviating from required values. The input/output failures may be random or nonrandom.

 Deliberate failures. These failures are nonrandom and occur when a threat actor (also called attacker) uses a physical or cyber threat to harm the item. For some systems, cyber threats may lead to physical harm to assets. A physical threat action is also called a sabotage.