Protection coordination
Compare protective devices and review the grading basis.
How this works
Protection chain. Devices are graded as an electrical series: top of the list = most downstream (nearest the load), bottom = the incomer. Use Move downstream and Move upstream on each device to set the real order; coordination checks each adjacent pair, from downstream to upstream.
Markers. Ik″ max and Icc min bound the fault range the study is graded over; Load marks normal running current; Reference voltage refers every curve to one common base so devices on either side of a transformer compare correctly.
Verdict. A pair passes when the downstream device always clears before the upstream one across the fault range. Worst case is the current where the two curves come closest; margin is the time gap there, which must meet the required grading margin.
ZSI (zone-selective interlock). Toggle it on a pair to grade by interlock signal instead of time margin. It models a downstream device signalling the upstream one to hold, so they can be set closer together.
1 · Study and markers
Study guidance
Reference voltage refers every curve, and the coordination grading, to one common base, so devices on opposite sides of a transformer can be compared (this example spans an 11 kV / 415 V transformer; clear the field to plot each device at its own voltage). Drag any slider to re-grade live. Hover the chart (or focus it and use the arrow keys) to drop a current cursor and read each device's trip time at that fault current; the shaded band marks the Icc–Ik″ fault range. Scroll to zoom, drag to pan; Fit frames the view to your devices, Reset clears the zoom.
Add a damage / withstand overlay curve…
Overlay curves are reference guides (cable/transformer/motor withstand & inrush): they plot for comparison but are not graded. The transformer overlay uses one representative I²t line: t = 2 × (Imax / I)², with Imax = FLC × 100 / Z%. Its category label does not change the curve. It excludes category-specific thermal knees, fault frequency and source impedance. Use the Transformer calculator for its separate category-specific study; its curve is not imported here.
3 · Device settings
Results
Review the grading verdict, its basis and limits.
Coordination, downstream to upstream
Time–current chart
Interactive time–current coordination chart. Press Tab to focus it, then use the Left and Right arrow keys to move a current cursor and hear each device's trip time and the grading margin at that fault current; hold Shift to move faster; press M to jump to the next fault marker (Icc, Load, Ik″); press plus or minus to zoom, hold Ctrl or Alt with an arrow key to pan a zoomed view, Home or End to jump to the axis ends, and Escape to hide the cursor. Click the chart to pin the cursor in place, or use the Pin button to stamp the reading onto the chart so it appears in exports. The last cursor reading is also listed as a table below.
Detailed analysis Every device and setting used in the study
| Device | Type | Settings | Make / model |
|---|
Document
Keep project details, notes and report exports together.
Notes
- Sampled AC radial phase-overcurrent screen on the declared common voltage base. Engineering acceptance is not assessed.
- Confirm breaker clearing allowance, manufacturer setting ranges and tolerances before relying on any grading margin.
- Verify ZSI wiring, current-limiting energy and equipment damage curves against manufacturer evidence.
References
- IEC 60255-151 · inverse-time (IDMT) operating-time equations
- IEEE C37.112 · inverse-time overcurrent relay equations
- IEC 60269 · fuse minimum-melting and total-clearing bands
- AS/NZS 3000 · page standard label; clause applicability not assessed
A professional aid, not engineering judgement — you remain responsible for verifying every input and result against the standards that apply to your work.