Fail-safe and fail-secure behavior

How it works

A fail-safe design moves toward a state chosen to reduce harm when a fault occurs. A fail-secure lock generally keeps access restricted after loss of power. These terms describe intended failure behavior; they do not mean a system can never fail dangerously.

For an access-control example, distinguish entry security from emergency exit. A lock can restrict entry while a separate mechanical release allows occupants to leave. The correct behavior depends on the complete installation and applicable requirements.

The original page linked to externally hosted illustrations. Those files are absent from the backup; the text has been restored without substitute images.

On the bench

For a hobby controller, begin by asking what happens after a broken wire, a flat battery, a stalled program, or a failed output transistor. Verify those cases with low-energy tests rather than assuming the normal off state covers every fault.

Resources & provenance

Rewritten from the original articles/tut/d3mn6sf_tut006.php page. Component values and historical performance claims have not been independently bench-tested.