In short. FMEA asks of each component: how can it fail, what happens next, how would we know and how bad is it? A single point of failure is any item whose failure alone stops the load. The study finds them, ranks them and assigns actions before an outage does it for you.
Method
- Set the boundary. The system, modes of operation and what counts as loss of function.
- List components from the single-line diagram, controls, auxiliaries and services.
- List failure modes for each: fails to start, fails to open or close, false trip, loses signal, loses supply.
- Describe local and system effects.
- Detection. Alarm, test, inspection or none.
- Rank. Severity, likelihood and detectability, using a consistent scale.
- Actions. Remove the failure, add redundancy, add detection, add procedure or accept with reason.
- Verify. Test the fix and re-rank.
Typical worksheet
| Item | Failure mode | Effect | Detection | Action |
|---|---|---|---|---|
| Starting battery | Low charge | Set fails to start | Monthly test | Dual battery, charger monitoring |
| Fuel transfer pump | Fails | Day tank runs dry | Level alarm | Standby pump, hand pump |
| Control supply | Loss of DC | No breaker control | DC fail alarm | Redundant supplies |
| ATS | Fails to transfer | Load not supplied | Test on load | Bypass, redundant ATS |
Commonly hidden single points
- Shared DC supply or control wiring for redundant breakers.
- One fuel line or one fuel tank.
- One cooling system or one ventilation fan.
- One person who knows the procedure.
- A common protection relay or a shared CT. See protection coordination.
Illustrative example
A site with two generators believes it has full redundancy. FMEA shows both sets take their start signal through one controller and one 24 V supply. A failure of that supply stops both starts. Adding a second supply with diode isolation and a manual start path closes the gap.
How an electrical FMEA is built
For each component or function, list how it can fail, what happens as a result, how it would be detected and what protects against it. Score the risk and act on the highest.
| Column | Content |
|---|---|
| Item | For example, ATS controller |
| Failure mode | Fails to transfer; transfers falsely; locks up |
| Effect | Load loses supply; sources paralleled out of phase |
| Cause | Control power loss; wiring fault; software fault |
| Detection | Alarm; periodic test; none |
| Severity, occurrence, detection scores | Typically 1 to 10 each |
| Risk priority number (RPN) | Severity × occurrence × detection |
| Action | Redundancy, supervision, procedure |
Single points of failure to look for
- One DC supply for control and tripping.
- One fuel line, filter or day tank.
- One transfer switch for the whole critical load.
- Shared cable route or cable tray.
- One controller or one communications path.
- One key person or one set of spares.
Example (illustrative)
Failure mode: starter battery cannot crank. Effect: no generator start on mains failure. Detection: charger alarm only. Score high. Action: dual battery banks with separate chargers, battery load test schedule and voltage drop alarm.
Outcome
A ranked action list, design changes and test routines. See battery starting systems and protection failure modes.
Common mistakes
- FMEA of the one-line only, not controls and auxiliaries.
- No follow-up on actions.
- Ranking without a defined scale.
What to send us
Send drawings and failure history. Send your application details. See also reliability engineering.
Questions engineers ask
Who should take part?
Design, operations and maintenance staff together, since each sees different failures.
How often should it be repeated?
After changes and at planned intervals.
FMEA or FMECA?
FMECA adds a numeric criticality ranking. Use whichever fits your need.
Is it only for new designs?
No. It is very useful on existing sites.
This guide gives general principles. Rankings should reflect your site’s consequences.