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Fault tree (FTA) templates

Fault Tree — Data Centre Power to IT Load

Tier III power path from utility through a two-of-three generator vote and an N+1 UPS pair, exposing the transfer switch and the final rack circuit as order-one cut sets no amount of upstream redundancy touches.

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Loss of power to the IT load — Tier III hallFault tree (IEC 61025) · 9 basic events · 6 gates · depth 3topIT load de-energised0.009596source_lostBoth power sourcesunavailable3.44e-4utility_lostUtility feedunavailable0.321984grid_outageUtility outagelonger than 10 s0.31hv_switchgear11 kV switchgearfault0.017367gen_lostStandby generationunavailable2/30.001069dg_aDiesel generator Afails to start or…0.019dg_bDiesel generator Bfails to start or…0.019dg_cDiesel generator Cfails to start or…0.019distribution_lostDistribution pathfails0.009255ups_stringBoth UPS modulesfail to support0.004581ups_aUPS module A fails0.067681ups_bUPS module B fails0.067681pdu_failRack PDU or finalcircuit fault0.0012ats_failAutomatic transferswitch fails to…0.0035Top event probability (exact)0.009596Minimal cut sets9Smallest cut set order1Single points of failure2Mission time8760 hExact top-event probability 0.009596 · rare-event sum ΣP(cut set) 0.009635The usual approximation overstates this tree by 0.41% — that gap is the overlap between cut sets, which only the exact evaluation removes.Minimal cut sets — by order, then probability1. {ats_fail} order 1 P 0.0035 36.5% of the top ← single point of failure2. {pdu_fail} order 1 P 0.0012 12.5% of the top ← single point of failure3. {ups_a, ups_b} order 2 P 0.004581 47.7% of the top4. {grid_outage, dg_a, dg_b} order 3 P 1.12e-4 1.2% of the top5. {grid_outage, dg_a, dg_c} order 3 P 1.12e-4 1.2% of the top6. {grid_outage, dg_b, dg_c} order 3 P 1.12e-4 1.2% of the top7. {hv_switchgear, dg_a, dg_b} order 3 P 6.27e-6 0.1% of the top8. {hv_switchgear, dg_a, dg_c} order 3 P 6.27e-6 0.1% of the top9. {hv_switchgear, dg_b, dg_c} order 3 P 6.27e-6 0.1% of the topImportance — Fussell–Vesely (share of current risk) and Birnbaum (potential swing)ups_a — UPS module A fails q 0.067681 FV 47.7% B 0.06734 CR 47.5%ups_b — UPS module B fails q 0.067681 FV 47.7% B 0.06734 CR 47.5%ats_fail — Automatic transfer switch fails to transfer q 0.0035 FV 36.5% B 0.993883 CR 36.3%pdu_fail — Rack PDU or final circuit fault q 0.0012 FV 12.5% B 0.991594 CR 12.4%grid_outage — Utility outage longer than 10 s q 0.31 FV 3.5% B 0.001041 CR 3.4%dg_a — Diesel generator A fails to start or run q 0.019 FV 2.4% B 0.011892 CR 2.4%dg_b — Diesel generator B fails to start or run q 0.019 FV 2.4% B 0.011892 CR 2.4%dg_c — Diesel generator C fails to start or run q 0.019 FV 2.4% B 0.011892 CR 2.4%Rectangle intermediate event · dome AND · shield OR · shield with k/n voting · hexagon INHIBIT with its oval condition · circle basic · diamond undeveloped · triangle transfer

Make it your own.

title "Loss of power to the IT load — Tier III hall"
mission 8760h

top "IT load de-energised" = OR(source_lost, distribution_lost)

gate source_lost "Both power sources unavailable" = AND(utility_lost, gen_lost)
gate utility_lost "Utility feed unavailable" = OR(grid_outage, hv_switchgear)
gate gen_lost "Standby generation unavailable" = KOFN(2, dg_a, dg_b, dg_c)
gate distribution_lost "Distribution path fails" = OR(ups_string, pdu_fail, ats_fail)
gate ups_string "Both UPS modules fail to support" = AND(ups_a, ups_b)

event grid_outage "Utility outage longer than 10 s" p=0.31
event hv_switchgear "11 kV switchgear fault" rate=2.0e-6
event dg_a "Diesel generator A fails to start or run" p=0.019
event dg_b "Diesel generator B fails to start or run" p=0.019
event dg_c "Diesel generator C fails to start or run" p=0.019
event ups_a "UPS module A fails" rate=8.0e-6
event ups_b "UPS module B fails" rate=8.0e-6
event ats_fail "Automatic transfer switch fails to transfer" p=0.0035
undeveloped pdu_fail "Rack PDU or final circuit fault" p=0.0012