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Fault Tree — Vaccine Cold Chain Excursion

Temperature excursion of a pharmacy vaccine fridge over a year, gating the alarm-response branch by an INHIBIT on out-of-hours cover and exposing the door-ajar cut set that no compressor redundancy touches.

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Vaccine cold chain excursion — community pharmacy fridgeFault tree (IEC 61025) · 9 basic events · 6 gates · depth 4topVaccine stockexposed above 8 C0.095618cooling_lostCooling lost and notrecovered0.028962refrigeration_failsRefrigerationunavailable0.241951compressorCompressor or fanmotor fails0.203685mains_lostPower to the fridgelost0.0176grid_outageMains supplyinterrupted over 2…0.08ups_failsBackup battery failsto hold the fridge0.22thermostatThermostat driftsout of calibration0.031response_failsExcursion not caughtin timeout_of_hours0.630.1197alarm_missedData logger alarmnot acted on0.19human_causeStock mishandled0.068645door_ajarDoor left ajar aftera stock check0.042overstackedAirflow blocked byoverstacking0.017delivery_delayDelivery left out ofthe fridge on…0.011Top event probability (exact)0.095618Minimal cut sets6Smallest cut set order1Single points of failure3Mission time8760 hExact top-event probability 0.095618 · rare-event sum ΣP(cut set) 0.100198The usual approximation overstates this tree by 4.79% — that gap is the overlap between cut sets, which only the exact evaluation removes.Minimal cut sets — by order, then probability1. {door_ajar} order 1 P 0.042 43.9% of the top ← single point of failure2. {overstacked} order 1 P 0.017 17.8% of the top ← single point of failure3. {delivery_delay} order 1 P 0.011 11.5% of the top ← single point of failure4. {compressor, alarm_missed, out_of_hours} order 3 P 0.024381 25.5% of the top5. {thermostat, alarm_missed, out_of_hours} order 3 P 0.003711 3.9% of the top6. {grid_outage, ups_fails, alarm_missed, out_of_hours} order 4 P 0.002107 2.2% of the topImportance — Fussell–Vesely (share of current risk) and Birnbaum (potential swing)door_ajar — Door left ajar after a stock check q 0.042 FV 43.9% B 0.944031 CR 41.5%alarm_missed — Data logger alarm not acted on q 0.19 FV 30.3% B 0.141966 CR 28.2%out_of_hours — Excursion starts outside staffed hours q 0.63 FV 30.3% B 0.042815 CR 28.2%compressor — Compressor or fan motor fails q 0.203685 FV 25.5% B 0.106126 CR 22.6%overstacked — Airflow blocked by overstacking q 0.017 FV 17.8% B 0.920022 CR 16.4%delivery_delay — Delivery left out of the fridge on arrival q 0.011 FV 11.5% B 0.914441 CR 10.5%thermostat — Thermostat drifts out of calibration q 0.031 FV 3.9% B 0.087213 CR 2.8%grid_outage — Mains supply interrupted over 2 hours q 0.08 FV 2.2% B 0.018925 CR 1.6%Rectangle intermediate event · dome AND · shield OR · shield with k/n voting · hexagon INHIBIT with its oval condition · circle basic · diamond undeveloped · triangle transfer

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title "Vaccine cold chain excursion — community pharmacy fridge"
mission 8760h

top "Vaccine stock exposed above 8 C" = OR(cooling_lost, human_cause)

gate cooling_lost "Cooling lost and not recovered" = AND(refrigeration_fails, response_fails)
gate refrigeration_fails "Refrigeration unavailable" = OR(compressor, mains_lost, thermostat)
gate mains_lost "Power to the fridge lost" = AND(grid_outage, ups_fails)
gate response_fails "Excursion not caught in time" = INHIBIT(alarm_missed, condition: out_of_hours)
gate human_cause "Stock mishandled" = OR(door_ajar, overstacked, delivery_delay)

event compressor "Compressor or fan motor fails" rate=2.6e-5
event grid_outage "Mains supply interrupted over 2 hours" p=0.08
event ups_fails "Backup battery fails to hold the fridge" p=0.22
event thermostat "Thermostat drifts out of calibration" p=0.031
event alarm_missed "Data logger alarm not acted on" p=0.19
condition out_of_hours "Excursion starts outside staffed hours" p=0.63
event door_ajar "Door left ajar after a stock check" p=0.042
event overstacked "Airflow blocked by overstacking" p=0.017
undeveloped delivery_delay "Delivery left out of the fridge on arrival" p=0.011