(syst_11_2=)

9.4.2. Root causes coverage

Based on the classification of the root causes of failures, different system level methods are suitable, as shown in Table 9.4.1.

Table 9.4.1 Classification of failure root causes and associated system level methods

Failure Category	Root Cause	System Level Methods
Random Failure (RF)	Unknown residual defect/weakness consistent with quality level and under normal stresses. Only moderate wear-out or degradation of parts.	Random failures are: On part level are mutually independent and failure rates are constant or do not change significantly over time, On system level various methods can be used (see Section 9.4.3).
Systematic Failure (SF)	Design error Manufacturing error Operations error	Systematic failures are: The result of human error introduced during design, manufacturing or operation, The calculation of systematic failures is part of Section 9.4.9, To be considered in the system reliability prediction in addition to the random failures on part level, Systematic failures could also result in Common Cause Failures (e.g. failures of several components due to the same cause); calculation of CCF on system level is part of Section 9.4.7.
Degradation Failure (DEG)	Physical degradation process Operations related Environment related Classified as “systematic failure” if early degradation is the result of development or manufacturing error.	Failure rates of parts prone to degradation are time dependent. On system level, degradation failures can also be considered in the reliability prediction by using different probability distributions on component level for degradation failures (e.g. Weibull). System level analysis with degradation on part level is considered in Section 9.4.8.
Extrinsic Failure (EX)	Space environment phenomena inducing external failure causes: Vacuum Mechanical shock, vibration Micro-meteoroids Debris Radiation Plasma Thermal Magnetic field Classified as systematic failure if the root cause was underestimated, e.g. requirement or design error (Part 1 - Methodologies).	Quantitative assessment of extrinsic failures with the system-level methods presented in Section 9.4.3 is possible. Extrinsic events can also be regarded as potential Common Cause Failures, see Section 9.4.7.