Inrush Current & Fuse Dynamics Simulator

Load Preset:

Circuit Parameters READY

Nominal Run Current ($I_{nom}$) 6.0 A

Inrush Peak Multiplier 8.5×

Surge Decay Time ($\tau$) 350 ms

Fuse Nominal Rating ($I_{fuse}$) 10.0 A

Ambient Temperature 25 °C

Instantaneous Peak Current ($I_{pk}$) 51.0 A

Applied Inrush Energy ($I^2t$) 462.4 A²s

Fast-Blow Rating ($I^2t_{melt}$) 28.4 A²s

Slow-Blow Rating ($I^2t_{melt}$) 566.8 A²s

Steady Overcurrent Ratio 0.60× (Safe)

LIVE OSCILLOSCOPE (CH1: CURRENT) TIMEBASE: 100 ms/div | SCALE: 10 A/div

Fast-Acting Fuse (Type F)

Low Thermal Mass • Thin Wire Link

INTACT

Thermal Energy / Melt Integral 0%

Time-Lag Slow-Blow (Type T)

High Thermal Mass • Solder Pot & Spring

INTACT

Thermal Energy / Melt Integral 0%

Time-Current Characteristic (TCC) Log-Log Curves IEC 60127 Standard Envelopes

Protection Coordination Verdict

Fast-Blow Vaporized at 48 ms | Slow-Blow Survived Startup

The motor compressor startup generated an inrush surge of 51.0 A decaying over 350 ms ($I^2t \approx 462\text{ A}^2\text{s}$). The Fast-Acting element exceeded its critical melting integral ($28.4\text{ A}^2\text{s}$) almost instantaneously, causing a nuisance trip. The Time-Lag dual-element fuse absorbed the energy via its solder thermal reservoir and maintained normal operation.

Inrush Margin: 81.6% (Slow) Fast Trip: ~48 ms Steady Load: Protected

Core Electrical Principles: Why Startup Inrush Blows Fast Fuses

🧲 1. Why Do Loads Have Inrush Currents?

Electric motors must overcome mechanical rotor inertia and lack back-EMF at zero RPM, pulling 6–10× rated current. SMPS bulk filter capacitors look like a dead short until charged. Toroidal transformers experience magnetic core saturation on phase turn-on, and cold tungsten filaments have 1/10th their hot steady-state resistance.

🔥 2. The $I^2t$ Melting Integral

Fuse element destruction is governed by adiabatic energy accumulation: $I^2t = \int i^2(t)\,dt$. If the total Joule heat delivered during the startup interval surpasses the element's melting integral rating, the conductor vaporizes regardless of whether the continuous steady-state current is safe.

⏳ 3. Fast-Acting vs Slow-Blow Architecture

Type F (Quick-Blow): Uses a thin, low-mass wire or ribbon designed to vaporize in milliseconds during overloads.
Type T (Time-Lag): Incorporates a heavy thermal mass, dual-element spring, or alloy solder pot that absorbs brief startup heat pulses without melting, while still clearing sustained shorts.