Functional-Safety-Integrity Degradation of Student-Built Emergency-Stop Circuits in Engineering Technology Education Workshops: A Fault-Injection Evaluation Linking Relay Contact Welding to Operator Injury-Risk Exposure

Emergency-stop circuits built by engineering technology students are routinely treated as safe once they pass a functional test at commissioning, yet the relay contact welding that dominates their failure modes is invisible to such tests and accumulates silently across the workshop's operating life. The framework represents a student-built circuit as a series safety function whose integrity is degraded by contact welding, and derives the conditions under which a welded contact converts a demanded stop into an undetected loss of safety function. A replication-based evaluation of the before-after-control-impact estimator was conducted under a declared variance structure with a between-cluster standard deviation of 6.0, a residual standard deviation of 4.5 and an intraclass correlation of 0.64, across 2000 replications per design cell for cluster counts of 8, 16, 24 and 32 and effect sizes of 3, 5 and 8 percentage points. The evaluation indicates that 54 clusters are required to detect a 5-percentage-point effect at 80% power, and that the minimum detectable effect at 80% power falls as the number of clusters rises. The framework proposes that workshop safety assurance should shift from commissioning-time functional testing to integrity monitoring that is sensitive to contact welding, and the design analysis implies that cluster-level evaluations of such interventions require cluster counts well above those typical of single-institution studies.