(misc_10_7)= # Basic failure rate derivation for “non-standard model” items When the {term}`miscellaneous item ` under consideration does not match the criteria of the miscellaneous equivalent class (e.g. {term}`SSPA` – Solid State Power Amplifier - cannot be compared to a {term}`TWTA` in terms of technology), it is necessary to build a reliability model based on engineering/{term}`PoF` approach as defined hereafter. Once the reliability model is established it can be tailored as necessary based on the approach presented in {numref}`misc_10_5`. First, it is necessary, to collect/define all technical information about the miscellaneous item in order to characterize it (refer to {numref}`misc_10_5`). Then the basic failure rate $\lambda_{1}$ = $\lambda_{B}$ is defined ([Step 1](misc_step1)) as discussed below. Finally, the tailoring of this basic failure rate follows the “standard model” procedure as presented in the subsections of {numref}`misc_10_5`) corresponding to [Step 2](misc_step2) to [Step 7](misc_step7). (misc_10_7_1)= ## DFMEA / PFMEA After the first item characterization, a {term}`DFMEA` and a {term}`PFMEA` are performed in order to assess the failure modes due to: 1. Design errors, 2. Manufacturing errors. It is recommended to use the {cite:p}`mis-SAE_standard, mis-PFMEA` which provides the method to apply for a DFMEA / PFMEA. PFMEA is also specified in {cite:p}`mis-ECSS-Q-ST-30-02C`. **FMEA: Required for every unit** * **As a tool/method**: Supporting the design of the unit, identifying all the elementary “failure modes" due to “{term}`random failure `” (part intrinsic reliability) as defined in the failure mode list (refer to {cite:p}`mis-ECSS-Q-ST-30-02C`). * **At unit level**: Under supplier responsibility * **At system level**: Integrated at system level by a system {term}`RAMS` engineer (including HSIA supporting the failure tolerance: observability in-orbit…) **DFMEA: Required on critical functions only (Risk analysis)** * **As a tool/method**: Supporting the product quality identifying all the elementar failure mode's due to design errors. It includes, as a minimum, the failure modes identified by FMEA, * **At unit level**:: Under unit designer / Quality assurance ({term}`QA`) responsibility, * **At system level**: Reviewed by procurement within the unit acceptance process. **PFMEA: Required on critical functions only (Risk analysis)** * **As a tool/method**: Supporting the product quality identifying all the elementary failure modes due to manufacturing errors, * **At unit level**: Under unit Quality assurance ({term}`QA`) responsibility, * **At system level**: Reviewed by procurement within the unit acceptance process. ```{figure} ../../picture/figure4_8.png --- width: 600px name: Figure_5_8 --- Differences between FMEA, DFMEA and PFMEA ``` {numref}`Figure_5_8` highlights the differences and the commonalities between FMEA, DFMEA and PFMEA. {numref}`Figure_5_9` provides an example of DFMEA and PFMEA at elementary part level (interconnect be-tween Solar cells). The left-hand side of the table refers to FMEA (blue perimeter in {numref}`Figure_5_8`), whereas the right-hand side refers to DFMEA data (green perimeter in {numref}`Figure_5_8`). The main outcome is the ranking of every root cause (design error, manufacturing error) in: * Severity ({term}`SEV`) * Occurrence ({term}`OCC`) * Detectability ({term}`DET`) The Risk Priority Number ({term}`RPN`), which is derived, is an index reflecting the residual technical risk. ```{figure} ../../picture/figure4_10.png --- width: 600px name: Figure_5_9 --- DFMEA and PFMEA at elementary part level (example of solar cells interconnect) Where, Solar Array ({term}`SA`), SA current ({term}`I_SA`), Telemetry ({term}`TM`) and Single Point Failure ({term}`SPF`). ``` ````{list-table} Occurrence scale with 10 levels :name: misc-table_10_6 :header-rows: 0 :widths: 100 * - ```{raw} html

OCCURRENCE
Rank	Category	Occurrence of cause	Examples / Guidance	TRL	%
10	Certain and unpredictable	Failure is unpredictable	New technology, new design with no history.		1/10
9	Inevitable	Failure is almost inevitable & persistent	Failure is inevitable with new design, new application or change in duty cycle/operating conditions.	TR1	1/20
8	Almost certain	Failure is almost certain	Failure is likely with new design/application or change in duty cycle/operating conditions.	TR2	1/50
7	Frequent	Frequent failures	Failure is uncertain with new design/application or change in duty/operating conditions.	TR3	1/100
6	Repeated	Repeated failures	Limited number of failures associated (identical design, simulation and testing). Use of Factor of Safety or Margin of Safety.	TR4	1/500
5	Occasional	Occasional failures	Limited number of failures associated (identical design, simulation and testing). Use of Factor of Safety or Margin of Safety.	TR5	1/2000
4	Infrequent	Limited failures	Limited number of failures associated (identical design, simulation and testing). Use of Factor of Safety or Margin of Safety.	TR6	1/10000
3	Isolated failure	Relatively loaded failures	Limited number of failures associated (identical design, simulation and testing). Reduce stress–strength interference.	TR7	1/100000
2	Unlikely	Very isolated failures	No observed failures (identical design, simulation and testing). Design for robustness techniques.	TR8	1/1000000
1	Remote	Failure is highly unlikely	Failure is eliminated through preventive type design control. Use of proven design guidelines/Standards. Use of field lessons learned.	TR9	ε

``` ```` Where, {term}`TRL` means Technology Readiness Level. (misc_10_7_2)= ## Probability assessment The occurrence scale of {numref}`misc_10_7_2` is used to derive, based on engineering judgement, either a failure rate or a probability of failure, per failure mode: * In the case that the detectability (i.e. the capacity to detect on ground the defect) is certain, the probability of failure is set to 0 under the condition that no {term}`degradation ` in time is expected (e.g. if a particle inside a {term}`RF` passive part which originates from manufacturing is necessarily detected on ground with no possibility to get a particle during the mission, then the associated probability of failure is set to 0). * A probability of occurrence is assigned to every failure mode, and the sum provides the probability of occurrence of the item. * A failure rate could be derived assuming the probability of occurrence on the specified lifetime is equal to the probability assigned to the failure mode, e.g. probability assessed to $10^{-4}$ as level 4 ({numref}`misc-table_10_6`) leads to a failure rate of $1,14 10^{-9}$ for 10 years. This represents the basic failure rate or the basic probability of failure. Then, the general process to adapt this failure rate is described in {numref}`misc_10_5` of this part. ```{admonition} Note :class: note This probability ranking needs to be clearly justified and documented. ```