This simulator models a feeder protected by a 50/51 relay — a current transformer feeding a relay that combines an instantaneous (50) high-set element with an inverse-time (51) element, tripping a downstream breaker. Adjust load, fault magnitude, CT ratio, pickup settings, time multiplier and curve shape, then watch the relay decide and the breaker clear the fault.
• A real-time 3D model of the source/feeder, CT, 50/51 relay and breaker, with home view, focus-selected-part, toggleable enclosure, auto-rotate and expand controls, tappable components with callouts, and numbered labels matching a companion diagram. • Nine live controls: normal primary current, applied fault current, CT primary/5 A ratio, 51 secondary pickup, 50 secondary threshold, 51 time multiplier, inverse-time curve selector (standard/very/extremely inverse), breaker travel/clearing delay, and a "breaker fails to open" fault-injection toggle. • Play/pause, single-step and larger-step time controls, plus a playback-speed selector from 0.1x to 60x laboratory speed. • Apply fault, remove fault and reclose (new trial) actions, with a live sequence narrative and per-component status. • Six live metrics: primary current, CT secondary current, 51 pickup multiple, 51 timing progress, 51 operating time at the present current, and elapsed time since injection. • A Timing & coordination tab with two charts (curve/time-current behavior), the full model equations, and snapshot measurements. • An Experiments tab with four guided scenarios (inverse-time trip, high-set operation, wrong CT setting, breaker failure), a model-verification bench of independent automated checks, and a timestamped event log with a copyable trial report. • A Learn & assess tab with guided lessons, a knowledge-check quiz and a written scope/reference statement.
The 50 element watches for currents so high they signal a close-in, severe fault — it operates after a short fixed detection delay once the secondary current crosses its threshold, without waiting to see how long the overcurrent persists. The 51 element instead integrates an inverse-time curve: the further the measured current sits above its pickup multiple, the faster it counts toward a trip, while it resets if current falls back below pickup before the timer completes.
Both elements measure CT secondary current, not primary current directly, so the CT ratio matters — a relay fed the wrong ratio sees a different multiple of pickup than intended, which changes both whether 50 asserts and how quickly 51 counts. A relay decision to trip is also not the same as circuit interruption: the modeled breaker still needs its own mechanical delay to open, and if that mechanism fails, current keeps flowing even after the relay has correctly commanded a trip.
The equations panel shows Isecondary = Iprimary × 5/CTratio, the pickup multiple M = Isecondary/Ipickup, and the inverse-time formula t51 = TMS × a/(Mᵇ − 1), with standard-inverse, very-inverse and extremely-inverse curves using different a/b constants — larger M always shortens the calculated 51 operating time. Total clearing time in the event log is relay operating time plus the configured breaker delay, so a fast relay decision can still be followed by a slow mechanical clearing time.
This is a single-device teaching model: it uses an ideal CT with no saturation, no vendor tolerances or relay filtering, and no breaker current-zero dynamics, and the plotted 51 curve does not by itself constitute a full coordination study against upstream and downstream devices.
Element 50 is instantaneous overcurrent protection: it operates after a short, fixed detection delay once secondary current crosses a high threshold, regardless of how long the overcurrent has persisted. Element 51 is inverse-time overcurrent protection: it tolerates lower overcurrents for longer, integrating progress toward a trip based on how far the current sits above its pickup multiple, and resets if current drops back below pickup.
The 50 and 51 elements act on CT secondary current, calculated as Isecondary = Iprimary × 5/CTratio. Using the wrong CT ratio changes the secondary current the relay actually measures, which changes the pickup multiple and can make an inverse-time trip much slower, or prevent an instantaneous element from asserting at all, even though the true primary fault current is unchanged.
A relay decision to trip and the breaker physically interrupting current are two separate events in this model. Total clearing time equals relay operating time plus the configured breaker travel/clearing delay, and if the breaker mechanism itself fails, current continues to flow even though the relay has already correctly commanded a trip — this is exactly what the breaker-failure experiment and diagnostic exercise are built to distinguish.
This is a single-device teaching model using an ideal CT, RMS current injection and canonical inverse-time curves with instantaneous reset below pickup. It excludes CT saturation, manufacturer tolerances, relay filtering and breaker current-zero interruption dynamics, and the plotted 51 curve alone is not a coordination recommendation against other upstream or downstream protective devices.