Contingency Study and Supply Reliability Score

1. What this check does

Starting from your one-line, RokBench generates the credible failure events of every item (from a catalog of failure mechanisms matched to the equipment kind, age, environment and condition), combines them with breaker failure, standby-source failure, common-mode events in a room or route, and with the operating states the plant can be in (normal, running on the alternate supply after a utility loss, or with one item out for maintenance). For each event it works out the physical damage, which protective device clears it (including non-selective ground-fault trips and delayed arcing faults), and how each load is restored: seamlessly, by automatic transfer, by manual switching, after repair or after replacement. Events are then weighted with survey failure rates to give each load its outages per year, hours per year and availability, and the plant a supply reliability score and a capital score.

2. Inputs used

• The one-line model: equipment, connections, normally open ties and transfer switches, critical loads.
• Optional asset data per item: kind, install year, environment, condition, assembly (lineup), route (duct bank or tray), room, damage-limiting features (arc-flash detection, segregation, insulated bus, draw-out, ground-fault protection), available fault current and instantaneous pickup, system grounding.
• Ratings used for failure rates and the capital index: transformer and UPS kVA, generator kW, cable length, breaker frame.

3. Formulas and assumptions

• Deterministic screening: each event is evaluated on its own against the model, with no probability in the judgement itself. The catalog of mechanisms and the clearing and damage rules are RokBench’s own and are kept on the server.
• Restoration grades: E1 seamless or momentary, E2 automatic transfer, E3 manual switching after isolating the fault, E4 after repair, E5 after replacement. A critical load passes the N-1 criterion when every single contingency is restored by E2 or better.
• Failure rates and repair times come from the US Army survey USACE TM 5-698-5 (public). Utility outage rates, per-demand failure probabilities (generator start, transfer, breaker), durations per grade and the age, condition and environment multipliers are RokBench assumptions and are listed with every result.
• Score 0–10 = 2 × (nines − 1) of the worst critical load: 99 % → 2, 99.9 % → 4, 99.99 % → 6, 99.999 % → 8, 99.9999 % → 10. The capital score places the equipment index on a log scale across the topology library (10 = most expensive); it compares arrangements and is not a cost estimate.
• Independent double failures are approximated for the highest-risk items; events that affect no load are kept out of the totals.
• Continuity: a transfer counts as continuous supply when it completes within the load's ride-through time (default 0.02 s, set per load). Default transfer times: STS 0.004 s, ATS 1 s or 10 s with generator start, automatic tie 0.5 s. Interruptions beyond the tolerance and events ridden through are reported as separate counts; the score follows outage hours.

4. What is NOT checked

• Load flow, voltage and short-circuit withstand under each contingency (use the Loading and Short-Circuit tools).
• Protective-device coordination curves; the study uses pickup thresholds and GFP presence only.
• Human-factor and weather event frequencies specific to a site; the assumptions are generic.
• Planned maintenance is reported as a shutdown need, not counted in the outage hours.

5. Reference standards

Pointers only; consult the edition adopted by your authority having jurisdiction.

• IEEE Std 493 (Gold Book) — design of reliable industrial and commercial power systems (method; tables not reproduced)
• USACE TM 5-698-5 — survey of reliability and availability information for power distribution, power generation and HVAC components
• NFPA 70B — electrical equipment maintenance
• IEEE Std 1584 and NFPA 70E — arcing-fault behavior (context for delayed clearing)
• NERC TPL-001 (transmission system planning performance) and utility N-1 contingency practice — the pattern this study follows at plant scale
• IEEE Std 3006.7 / 3006.8 — reliability of commercial and industrial power systems (indices: interruptions, hours, availability)

6. Validation cases

No worked example with a published answer is registered for this page yet. Have one we should add? Use the Feedback link. All cases: Validation.

7. Known issues and changes

See the Changelog. Try the tool: Workbench (Contingency tab).