Failure Mode and Effects Analysis, or FMEA, is a structured engineering method for finding possible failures before they cause harm, downtime, or expensive redesigns. It is used in products, machines, processes, and software controlled systems to make risk visible and comparable. By listing failure modes, their effects, and their causes, engineers can decide which problems deserve attention first.
FMEA matters because it turns vague concerns into a documented action plan.
A typical FMEA rates each failure mode using severity, occurrence, and detection scores, often on a 1 to 10 scale. These ratings are combined into a Risk Priority Number using RPN = S × O × D, where higher values signal higher priority for action. Corrective actions may reduce the chance of failure, improve detection, or limit the effect of the failure.
After actions are taken, engineers recalculate the ratings to check whether the risk has been reduced.
Key Facts
- FMEA stands for Failure Mode and Effects Analysis.
- A failure mode is the specific way a part, process, or system can fail.
- RPN = S × O × D, where S is severity, O is occurrence, and D is detection.
- Severity measures how serious the effect is if the failure happens.
- Occurrence measures how likely the cause of the failure is to happen.
- Detection measures how likely current controls are to catch the failure before it reaches the user.
Vocabulary
- Failure mode
- A failure mode is the specific way a component, process step, or system function can stop working as intended.
- Effect
- An effect is the result or consequence of a failure mode on the system, user, process, or environment.
- Cause
- A cause is the reason a failure mode may occur, such as wear, contamination, incorrect assembly, or software error.
- Risk Priority Number
- The Risk Priority Number is a score calculated from severity, occurrence, and detection ratings to help rank risks.
- Corrective action
- A corrective action is a design, process, inspection, or control change intended to reduce risk.
Common Mistakes to Avoid
- Treating RPN as the only decision rule is wrong because a low RPN can hide a failure with very high severity that still needs urgent attention.
- Confusing occurrence and detection is wrong because occurrence describes how often the cause happens, while detection describes how likely the controls are to catch it.
- Writing vague failure modes is wrong because phrases like machine fails do not identify the exact part, function, or condition that must be controlled.
- Skipping the updated ratings after corrective action is wrong because the team must verify whether the action actually reduced severity, occurrence, detection, or overall risk.
Practice Questions
- 1 A conveyor motor failure mode has severity S = 8, occurrence O = 4, and detection D = 5. Calculate the RPN.
- 2 A pump seal leak initially has S = 7, O = 6, and D = 4. After adding a better seal material and leak sensor, the ratings become S = 7, O = 3, and D = 2. Calculate the original RPN, the new RPN, and the percent reduction in RPN.
- 3 Two failure modes have the same RPN. One has very high severity but low occurrence, while the other has moderate severity and high occurrence. Explain how an engineering team should decide which one to address first.