Inputs
Example data table
| Scenario | Fault current (kA) | Downstream time (ms) | Upstream time (ms) | CTI (ms) | Safety (%) | Expected |
|---|---|---|---|---|---|---|
| Feeder fault near MCC | 12.5 | 80 | 260 | 100 | 10 | PASS |
| Sub-main fault at DB | 18.0 | 120 | 230 | 120 | 10 | FAIL |
| Generator board fault | 9.0 | 150 | 380 | 150 | 15 | PASS |
Formula used
- Time margin (ms) = Upstream time − Downstream time
- Required margin (ms) = CTI × (1 + Safety%/100)
- Selectivity index (%) = 100 × (Time margin / Required margin) (display capped at 125%)
- I²t ratio = Upstream I²t / Downstream I²t (energy method)
Selective discrimination aims for the downstream device to clear a fault first, while upstream protection remains stable or delayed, limiting the outage to the smallest zone.
How to use this calculator
- Pick a fault location and obtain the prospective fault current.
- Read downstream and upstream clearing times from coordination curves.
- Enter CTI and a safety factor reflecting tolerances.
- Optionally enter instantaneous pickups and I²t values from datasheets.
- Press Calculate and review PASS/FAIL with the basis shown.
- Export CSV or PDF to attach to commissioning documentation.
Use sound coordination to reduce outages and damage significantly.
Professional article: selective discrimination in construction power systems
1) Why selective discrimination matters
Selective discrimination (also called protection coordination) is the practice of setting protective devices so only the device closest to a fault operates first. On construction projects, this reduces downtime, protects critical temporary services, and limits secondary damage such as cable overheating, arc energy escalation, and nuisance shutdowns. Good coordination supports safer work areas and cleaner handover documentation for clients and authorities.
2) Typical project locations that benefit
Coordination is especially valuable across incomers, sub-mains, distribution boards, MCCs, hoists, tower cranes, dewatering pumps, and temporary site power. As a project grows, fault levels and feeder impedances change, so settings should be reviewed when generators are added, transformers are upgraded, or major cable routes are modified.
3) Time grading as the primary check
The most common approach is time-current grading: at a chosen fault current, the upstream device must clear later than the downstream device by at least a coordination time interval (CTI). This margin is increased in practice using a safety factor to cover tolerances, ambient effects, and manufacturer curve variation.
4) Instantaneous elements and “hidden” failures
Instantaneous pickups can defeat selectivity if the upstream pickup is reached before the downstream device clears. Where fault current is high, confirm that the upstream instantaneous setting is above the prospective fault level at the downstream location, or use a short-time delay/zone-selective scheme when available.
5) Energy (I²t) selectivity for fuses and current-limiting devices
For current-limiting protection, coordination may be demonstrated using let-through energy (I²t). If the upstream I²t is sufficiently higher than the downstream I²t at the same fault level, the downstream device should operate first without upstream operation. Always use manufacturer data taken at comparable test conditions.
6) Example data walk-through
Suppose a feeder fault is calculated at 12.5 kA. The downstream breaker clears in 80 ms, while the upstream clears in 260 ms. With CTI = 100 ms and Safety = 10%, the required margin is 110 ms. The available margin is 180 ms, so the outcome is a pass with a healthy selectivity index. This aligns with the example table above and can be exported for commissioning packs.
7) Deliverables for commissioning and handover
A practical coordination submission includes the fault study basis, device settings, curve extracts, and a results summary by location. Exported CSV/PDF outputs help attach evidence to QA records, energization permits, and client handover manuals. Re-verify settings after any field adjustments and record final dial positions.
FAQs
1) What does PASS mean in this calculator?
PASS means the available upstream-minus-downstream time margin meets or exceeds the required CTI margin after applying the safety factor, based on the selected evaluation method.
2) Which fault current should I enter?
Use the prospective short-circuit current at the downstream device location or the protected load point, taken from a fault study or calculation for the project configuration.
3) How do I choose a CTI value?
CTI depends on device technology and coordination philosophy. Many projects start around 100–200 ms, then refine using manufacturer curves and site experience to avoid overlap.
4) Why add a safety factor?
Safety factor accounts for tolerances, ageing, temperature effects, and curve variation. It reduces the risk that real-world operation erodes the theoretical coordination margin.
5) When should I use the energy (I²t) method?
Use I²t when working with current-limiting fuses or devices where let-through energy data is provided. It complements time grading when instantaneous behavior dominates.
6) Does this replace manufacturer coordination software?
No. It provides a structured check and report summary. Final design should be verified with manufacturer time-current curves, settings tools, and project standards.
7) What should I do if the result is FAIL?
Increase upstream delay, adjust instantaneous pickups, revise device selection, or add selectivity features. Re-check at multiple fault levels and confirm against curve plots before energization.