This simulator models the full CT-to-breaker protection chain: a current transformer measures feeder current, a nondirectional 50/51 overcurrent relay evaluates it against pickup and time-current settings, and a trip command opens the breaker to clear a fault — with an independent 50BF breaker-failure backup if the local breaker doesn't clear.
• A real-time 3D model of the protection chain — CT, relay logic and breaker — with a toggleable breaker enclosure, home-view reset, auto-rotate orbit and expandable 3D view. • Operate & observe controls: run at 1×, step 1 ms, step 20 ms, step 1 s, apply fault, remove fault, manual trip, reset relay target, close breaker, isolate source, and restore source. • CT & protection settings panel with a selectable 51 time-current curve: IEC standard inverse, very inverse, or extremely inverse, plus additional protection toggles. • Two live scope charts: measured/feeder current history, and the 51 time-current characteristic (logarithmic current multiple from 1.05–20 × pickup versus operating time). • A protection-calculations panel showing CT secondary and relay primary indication values and the full 50/51 timing equations. • A secondary-injection calculator: enter an injected secondary RMS current and predict the relay's operating time offline, independent of the live breaker. • A tabbed workflow across four pages — Protection workbench, Timing & signals, Test & diagnose, and Learn & assess. • Guided experiments, a 'Model verification bench' that runs built-in checks, a diagnostic challenge exercise with a diagnosis selector distinguishing a failed trip circuit, no pickup, or CT-ratio confusion, and a knowledge-check quiz with feedback and reset. • A sequence-of-events log recording every simulated action with its timestamp, and an export-session-to-JSON button.
A current transformer scales the primary feeder current down to a measurable secondary signal, which the relay's measurement channel processes through a 20 ms first-order RMS-envelope filter (not a sampled waveform or DFT estimator in this model). The 51 time-overcurrent element compares that measurement to a pickup setting and integrates dt/t(M) while current multiple M stays above pickup, using t = TMS × k / (Mᵅ − 1) with curve constants k = 0.14, α = 0.02 for standard inverse; k = 13.5, α = 1 for very inverse; and k = 80, α = 2 for extremely inverse. A separate high-set 50 instantaneous element uses its own 20 ms persistence timer for severe faults.
Once the relay's accumulator reaches its trip threshold, a trip output is issued, followed by an 8 ms release delay and the breaker's modeled contact travel time, with each phase actually interrupting at its next simulated current zero. If current persists above 50 A after the trip output, a 50BF breaker-failure timer starts, and an independent upstream backup breaker removes the source 60 ms later — the backup has its own control power, separate from the local relay and breaker.
The history chart plots the largest conducting phase's amber RMS current alongside the relay's filtered mint current on the configured primary scale, letting you see the measurement lag introduced by the RMS-envelope filter. The 51 time-current characteristic chart plots operating time against current multiple on a logarithmic scale from 1.05× to 20× pickup — steeper curves near 1× pickup reflect the inverse-time shape, and switching curve families (SI/VI/EI) reshapes this line.
The secondary-injection calculator applies the same CT ratio and 51 settings currently configured on the workbench to predict operating time for a manually entered secondary current, which is how relay technicians verify settings in the field — but here it's an offline prediction only and does not trigger the live breaker. This is a generic, nondirectional, three-phase model: there is no network fault-current solution, DC offset, harmonic restraint, CT saturation, earth-fault element, or automatic reclose, and the signal gain control is a deliberate measurement-error fixture rather than a physical CT saturation model.
The simulator models the full sequence: a current transformer measures feeder current, a 50/51 nondirectional overcurrent relay evaluates that measurement against pickup and time-current settings, a trip command is issued to the breaker coil, and the breaker contacts open to interrupt current. An independent 50BF breaker-failure element and upstream backup breaker are included if the local breaker fails to clear the fault.
The 51 element uses the IEC inverse-time equation t = TMS × k / (Mᵅ − 1), where M is the measured current divided by pickup. This simulator offers three curve shapes: standard inverse (k = 0.14, α = 0.02), very inverse (k = 13.5, α = 1), and extremely inverse (k = 80, α = 2). An internal accumulator integrates dt/t(M) while current is above pickup and resets immediately once current drops below pickup.
It lets you enter an injected secondary RMS current and predicts the relay operating time offline, using the same CT scaling and 51 settings configured on the workbench. It is a prediction tool only — it does not operate the live breaker in the simulation.
The diagnostic challenge in this simulator explores exactly that scenario. If current remains above 50 A after the trip output, the 50BF breaker-failure timer runs, and after its delay an independent upstream backup breaker removes the source 60 ms later. This distinguishes a failed trip circuit or missing DC control power from a real stuck-breaker condition.