💡 Insight
Adjust the controls to see how settings affect breaker behavior.
📏 Model Assumptions (reference bases)
📜 Code Context (informational only — not a compliance determination)
- Nothing here is a code-compliance determination. This tool does not calculate or certify compliance with the NEC (NFPA 70), NFPA 70E, UL 489, IEEE 1584, or any AHJ requirement. NEC applicability always depends on the adopted edition and the authority having jurisdiction (AHJ). Verify against the current NEC/AHJ.
- NEC 240.87 (arc-energy reduction) may apply only to circuit breakers rated or adjustable to 1200 A and above. A documented reduction method must be selected (e.g., energy-reducing maintenance switching, zone-selective interlocking, instantaneous override, differential, or an approved equivalent), and required documentation, testing, and method selection are part of the determination. A maintenance-mode switch by itself does not establish 240.87 compliance. Verify against the current NEC/AHJ.
- Arc-flash incident energy and boundaries are not computed here. Real incident-energy and arc-flash-boundary values require IEEE 1584 or another accepted engineering method, using site-specific fault current and clearing time. Use a qualified arc-flash study.
- NEC 230.95 / 215.10 / 240.13 (GFPE) ground-fault equipment protection requirements may apply depending on system grounding (e.g., solidly grounded wye), voltage, the service/feeder role, and the disconnect or OCPD rating. GFPE is equipment protection — it is not personnel GFCI shock protection. Verify applicability against the current NEC/AHJ.
- Selective coordination for emergency, legally required standby, and critical operations systems (e.g., NEC 700 / 701 / 708) may require a documented coordination study performed by qualified persons using manufacturer data. This tool does not perform that study. Verify against the current NEC/AHJ.
📖 Reading the TCC
The curve shows how long the breaker waits before tripping at a given current. Higher current = faster trip. The long-time region handles overloads patiently. The short-time region handles moderate faults quickly. Instantaneous means “no intentional delay.” The green dashed ground-fault band is plotted against ground current. The red marker shows where your current phase event falls on the curve. The plotted points are sampled directly from analyzeTrip(), so the chart and the analysis always agree.
💡 What to Notice
Watch how the current level determines which protection function activates and how quickly the breaker responds. A 2× overload and a 10× fault behave completely differently, because they are.
🔶 Upstream (Main)
🔷 Downstream (Branch)
🎯 Coordination Basics
Selective coordination means only the breaker closest to the fault opens. The upstream breaker should not trip unless the downstream one fails. This requires the upstream device to have longer delays and higher pickups. Coordination cannot be proven from a single current point or a generic curve — a real study sweeps the full current range using manufacturer data.
Why This Matters
A circuit breaker isn’t just an on/off switch. Its trip unit is what makes it smart — deciding when to open based on current magnitude, duration, and type. Two identical breaker frames with different trip settings will protect very differently. One might ride through a motor startup; the other might trip and shut down your line.
Reference Bases (this model)
Electronic (LSIG) trip units in this simulator use the common convention:
- In — sensor/frame current (the reference base).
- Ir = LT pickup × In — long-time pickup / ampere rating.
- Isd = ST pickup × Ir — short-time pickup, expressed as a multiple of Ir.
- Ii = Inst pickup × In — instantaneous pickup, a multiple of In, no intentional delay.
- Ig = GF pickup × In — ground-fault (equipment) pickup, a multiple of In.
ltTime = ltDelay × (6 / (I/Ir))² — an educational I²t approximation, not a manufacturer TCC. Because it is a true I²t relationship, doubling the current quarters the time.Pickup Is a Band, Not a Knife Edge
A trip unit does not hold forever at 0.999×Ir and open the instant you cross 1.000×Ir. Every long-time element has a calibration tolerance, and the UL 489 calibration points behind it are defined at percentages of rating rather than at a single threshold. This simulator models that honestly: below 1.05×Ir the breaker holds, at or above 1.20×Ir a trip is treated as confirmed, and in between the element is reported as in pickup — a real device may or may not time out there, so the model does not promise a trip.
The Protection Functions
- Long-Time (LT): Handles sustained overloads. Gives the system time to ride through temporary peaks such as motor starts. Pickup is typically 0.5–1.0× the ampere rating, with delays defined at 6×Ir.
- Short-Time (ST): Catches moderate fault currents — above long-time but below instantaneous. A short intentional delay lets downstream devices clear first.
- Instantaneous (I): No intentional delay. For high-magnitude faults where waiting is not an option.
- Ground-Fault (GF / GFPE): Detects current returning through an unintended ground path — a sign of insulation failure or an arcing ground fault. This is equipment protection.
- Maintenance Mode / ARMS / ERMS: An energy-reducing mode that trips faster to reduce arc-flash incident energy while a worker is exposed. It runs in parallel with normal functions and picks the fastest trip — it never inhibits protection.
Ground-Fault: Equipment vs Personnel
Ground-fault protection shown here is equipment protection logic (GFPE). Personnel protection requirements are separate and depend on the application and code section. GFPE (e.g., NEC 230.95) is designed to limit equipment burn-down from arcing ground faults at higher pickups (often hundreds or thousands of amps). Personnel shock protection (GFCI, ~4–6 mA) is an entirely different device and requirement.
GFPE is not required on every breaker. Actual GFPE requirements depend on voltage, grounding system, service/feeder role, breaker rating, and the applicable NEC edition and AHJ.
Overload vs Short-Circuit vs Ground-Fault
Overload: Current modestly exceeds the rating. Causes heating over time. Long-time protection handles this.
Short-circuit: A direct connection between conductors creates a large current surge. Short-time and instantaneous protection handle this.
Ground-fault: Current returns through an unintended ground path. GFPE limits equipment damage; it is not a substitute for personnel protection.
Selective Coordination
In a properly coordinated system, only the breaker closest to the fault opens. Upstream breakers stay closed, keeping power flowing to unaffected loads. This requires careful timing across the entire fault-current range — not a single point. A real determination uses manufacturer TCC data and, where required by code, a documented study by qualified persons.
The trap that catches people is the reference base. Series breakers carry the same absolute amperes, but each one measures those amps against its own In. A 4000 A fault is 5×In to an 800 A branch breaker and only 2.5×In to the 1600 A main above it. Two breakers can therefore be in completely different protection regions for the identical fault — which is exactly what makes coordination possible, and exactly what makes it easy to get wrong. Always reason in absolute amps first, then convert.
Common Mistakes
- Setting long-time pickup too low, causing nuisance trips during motor starts.
- Setting instantaneous pickup too low on the upstream breaker, defeating coordination.
- Assuming ground-fault (GFPE) is always required, or assuming it provides personnel shock protection — it is equipment protection and its requirement depends on the system.
- Using identical settings on upstream and downstream breakers.
- Confusing frame size with trip setting — an 800A frame set to 0.5× pickup has Ir = 400A.
Test Your Knowledge
See if you can answer these based on what you just learned.