Fuse Selection Guide: Time-Current Curves and Breaking Capacity Explained

A fuse is the cheapest insurance policy on a PCB. A 20-cent glass cartridge fuse that opens correctly saves a 200-euro power supply, a 50-euro microcontroller board and the fire investigation report. Selecting the right fuse means understanding time-current curves, I²t, breaking capacity and temperature derating.

Fast-blow vs slow-blow: pick the right curve

Fast-blow (F) fuses open quickly at moderate overcurrent, typically within 0.1 seconds at 200% of rated current. They protect against hard faults: short circuits, semiconductor failures and wiring errors. Use them on the AC mains input of a power supply or on DC rails that feed sensitive electronics.

Slow-blow (T, time-delay) fuses tolerate short-duration surges. A power supply with a toroidal transformer pulls 10-20× rated current for a few milliseconds at turn-on as the bulk capacitors charge. A fast-blow fuse would nuisance-trip on every power cycle. A slow-blow fuse with an I²t rating above the inrush energy survives.

Rule of thumb: if your load has inrush current (motors, transformers, large capacitor banks, PFC front-ends), use slow-blow. If your circuit has no inrush and damage occurs quickly under fault, use fast-blow.

Reading a time-current curve

The TCC plots current (x-axis, logarithmic) against time (y-axis, logarithmic). The curve shows the minimum time the fuse takes to open at a given current. At 100% of rated current, the curve goes to infinity. At 200%, it shows perhaps 1 second for a fast-blow or 10-60 seconds for a slow-blow.

Two curves appear on most datasheets: the minimum melt time and the maximum clearing time. The band between them represents manufacturing tolerance. For coordination with upstream circuit breakers, use the maximum clearing time of the downstream fuse and the minimum trip time of the breaker to ensure selectivity.

I²t: the energy it takes to open

I²t (ampere-squared seconds) measures the thermal energy required to melt the fuse element. A 1 A fuse with an I²t of 0.5 A²s means that a 10 A pulse must last 0.5 / 100 = 5 ms to open the fuse. This is why a 10 A surge lasting 100 µs does not blow a 1 A fuse: the energy (10² × 0.0001 = 0.01 A²s) is far below the melting I²t.

When protecting semiconductors, the fuse I²t must be less than the semiconductor's withstand I²t. A MOSFET rated for 5 A²s needs a fuse with I²t below that number. If the fuse I²t is 15 A²s, the MOSFET fails before the fuse opens. This is why semiconductor fuses (ultra-fast, aR or gR class) exist: they have very low I²t, typically 1-2 orders of magnitude below general-purpose fuses.

Breaking capacity: voltage and current limits

Breaking capacity (interrupting rating) is the maximum fault current the fuse can safely interrupt without arcing or exploding. A typical 5 × 20 mm glass fuse might have a breaking capacity of 35 A at 250 VAC. That is fine for a 50 W power supply on a 230 V mains circuit with a 16 A breaker. It is inadequate on a 48 V battery bank capable of delivering 2000 A into a short circuit; in that case you need an HRC (high rupturing capacity) fuse rated for 50-100 kA.

In DC circuits, breaking capacity drops dramatically. A fuse rated for 1500 A at 250 VAC might handle only 100 A at 125 VDC. DC arcs do not self-extinguish at zero crossings. Always check the DC interrupting rating separately.

Temperature derating

Fuses are thermal devices. At 25°C ambient, a 2 A fuse opens at roughly 2 A. At 65°C inside a sealed enclosure, the same fuse might open at 1.7 A. Manufacturer datasheets provide derating curves; a typical rule is to derate by 0.5% per °C above 25°C.

A common practice: run fuses at no more than 75% of rated current at the highest expected ambient temperature. For a 2 A load at 65°C ambient with a derating factor of 0.8, spec a fuse rated for 2 / 0.75 / 0.8 = 3.33 A. The next standard value is 3.5 A or 4 A. This gives margin for nuisance trips without compromising protection.

PCB fuse selection: form factors

Through-hole cartridge fuses (5×20 mm): accessible, replaceable, cheap. Use a PCB-mount fuse holder (e.g., Littelfuse 01000056Z or Schurter 0031.8201). Ensure the holder is rated for the same voltage and current as the fuse.

SMD fuses: available in 1206, 0603 and smaller packages. A Bourns SF-1206S200-2 (2 A, 63 VDC, 1206) provides surface-mount circuit protection for DC rails. These are not user-replaceable. Design them for fault conditions only, not routine replacement.

PTC resettable fuses: polymer PTC devices increase resistance dramatically when heated by overcurrent and reset after cooling. A Littelfuse 1812L110/33MR (1.1 A hold, 2.2 A trip, 33 VDC) protects USB VBUS lines. PTCs are slower than conventional fuses and have higher resistance in normal operation (tens to hundreds of milliohms). Do not use them where voltage drop or precise current limiting is critical.

FAQ

Q: Can I replace a glass fuse with a ceramic one?

Ceramic (sand-filled) fuses have higher breaking capacity than glass. In a mains application where fault current might exceed 35 A, a ceramic fuse is safer. For low-energy DC circuits, they are interchangeable if the ratings match.

Q: What happens if I use a fuse with the wrong voltage rating?

Voltage rating is about arc-extinguishing capability, not operating voltage. A 32 V fuse in a 230 V circuit may melt correctly but fail to extinguish the arc, sustaining current flow through ionized gas. The resulting plasma can destroy the fuse holder and PCB. Always use a fuse rated for at least the maximum system voltage.

Q: How close should the fuse be to the power entry point?

As close as practically possible. The trace between the power connector and the fuse is unprotected: a short on that trace bypasses the fuse. Keep it under 10 mm on a PCB. For wired connections, place the fuse holder directly at the inlet.

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