Short Circuit Basics
What happens in a short circuit — I²t, interrupting ratings, and protection.
Reference
Fault current
- I_fault
- = V_source / R_loop
- Loop impedance
- Source impedance + wiring + contact — usually milliohms to ohms
- Prospective
- Worst-case current assuming zero-impedance fault
- Let-through
- Current actually passed before a fuse opens (reduced by I²t limiting)
Energy and heat
- I²t
- Thermal stress on wires and devices — integral of I² over duration
- Wire damage
- Occurs quickly — typical limits: copper ≈ 80 A²s / mm²
- Arc flash
- Fault at higher voltage generates plasma; separate hazard — requires IE analysis
Protective devices
| Device | Typical clearing time | Notes |
|---|---|---|
| Fast-blow fuse | < 10 ms | Let-through I²t limited |
| Slow-blow fuse | 100 ms to seconds | Tolerates inrush |
| Circuit breaker | < 40 ms (instantaneous trip) | Resettable |
| eFuse (IC) | < 10 µs | Electronic, very fast; low current |
| PTC (resettable) | seconds | Self-resets when cool |
| Crowbar SCR | < 1 ms | OVP — shorts rail to trip upstream fuse |
| Foldback current limit | continuous | Limits into short, may not clear |
Selection checklist
- Rating > prospective fault current: interrupt rating must exceed the maximum fault at the device location.
- Coordination: downstream devices should clear first so upstream stays energized.
- Wire size: selected so short-circuit current doesn't exceed the wire's I²t withstand.
- Battery systems: huge prospective current (kA range) — use DC-rated fuses with proper interrupt rating.
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