2. Fuse Basics & Ratings

A fuse is a one‑time (or resettable) over‑current protection device. When the current exceeds a safe limit for long enough, the fuse element heats and melts, opening the circuit.

Schematic Symbols of Fuse:

Simple FUSE series connection diagram:

Why do you always need a fuse

• Protects the circuit from overcurrent & short-circuits.
• Prevents thermal runaway, fire hazards, and PCB trace burnouts.
• Prevent wiring/PCB overheating and fire
• Protect ICs, batteries, and power stages
• Required by UL/IEC/CE in many products

Good vs blown fuse:

Where fuses are used (Practical use cases)

• AC/DC power entry (adapters, chargers, SMPS)
• Battery‑powered systems (packs, BMS, handhelds)
• Automotive 12/24 V harness protection
• Motors/solenoids (inrush, stall faults)
• I/O & ports (USB, auxiliary outputs with resettable PTC)

1. Based on Package / Form Factor

Types	Image	Description
Glass Cartridge Fuse		Visible element. Low cost. General use. Quick replacement.
Ceramic Cartridge Fuse		Higher breaking capacity, mains/industrial use. Quick replacement.
SMD Fuse		Used on compact circuit boards. And also used for secondary protection. Permanently soldered.  Preventing quick replacement.
Axial Lead		Through-hole. Used on small PCBs. Permanently soldered.  Preventing quick replacement.
Automotive Blade Fuse		Color-coded. Used in vehicle fuse box. Quick replacement.
Bolt-Down Fuses – Automotive		Safeguards high currents. Stops electrical arcing. Withstands harsh vibrations. Dissipates excess heat.
Indicator Fuses		Glows when circuit breaks. Accelerates fault finding time. Simplifies overall system maintenance. Protects standard electrical circuits.
Torpedo Fuse		Protects older vehicle circuits. Features exposed metal elements. Uses pointed contact ends. Often called ceramic fuses.
PICO Fuse		Protects printed circuit boards. Features compact, subminiature design. Looks like a resistor. Delivers fast-acting fault protection.
Strip Fuses		Simple flat metal strips. Bolts right on batteries. Easy to see breaks.

2. Based on Speed of Operation

• Fast-blow (F) – reacts instantly, protects sensitive semiconductors
• Slow-blow (T / Slo-Blo®) – tolerates inrush currents from motors/transformers

Typical time–current curve (Slo‑Blo® vs fast acting fuse):

3. Specialty Fuses

Types	Image	Description
High-voltage Fuses Overcurrent Protection, One-shot		Used for industrial, EV, medical, power conversion.
Thermal Fuses (TCO) Overtemperature Protection, One-shot		Open on excess temperature (motors, transformers).
Resettable Polyfuses (PTC) Overcurrent Protection, Resettable		Portable devices, USB, battery packs.
Special Fuse- Shunt (measure current)		Tracks the current flow. Just a precise resistor.  It never actually blows.

4. Based on Reset Capability

Types	Image	Description
One-time use Fuses		Must be replaced once blown.
Resettable Fuses		Self-reset after fault clears.

• Rated current (Ir): Maximum continuous current at 25 °C without opening.
• Rated voltage (Vr): Maximum system voltage at which the fuse can safely interrupt a fault.
• Breaking/interrupting capacity (Iᵦ): The maximum fault current that a fuse can safely interrupt at its rated voltage.
• Time–current characteristic: “How fast it blows” at multiples of Ir (Fast vs Slo‑Blo®/Time‑Lag).
• I²t (melting let‑through energy): I²t is the fuse’s “energy rating”- it tells how much fault current (amperes squared × time) the fuse can safely handle before it blows. Higher I²t means the fuse can withstand larger short bursts of current without opening; lower I²t means faster, more sensitive protection.
• Resistance / voltage drop / power dissipation: The fuse itself has some resistance, so when current flows, it causes a small voltage drop and heat loss. It is important to check in low-voltage circuits where even a small drop can affect performance.
• Temperature derating: Fuse opens sooner at high ambient; use manufacturer’s derating curves.

Example Littelfuse 217 Series Glass Cartridge Fuse Specifications:

Example Fuse Time-Current Characteristic (TCC) Curve:

Step 1 — Define the job

• Nominal load current (I_load, RMS/avg) and ambient temp range.
• Supply voltage (AC/DC) and fault current available (source + path).
• Expected inrush/starting profile (motors, caps, heaters).
• Safety/agency needs (UL/IEC/automotive ISO).

Step 2 — Pick the construction

• Mains / high interrupt: Ceramic time‑lag.
• General low‑energy: Glass or SMD thin‑film.
• Automotive harness: Blade (ATOF/MINI/MICRO2), MEGA for battery.
• Resettable needs: PTC 1812L (ports) or MHP (battery).

Step 3 — Size the current rating

• Start at 1.25× to 1.5× I_load (at 25 °C).
• Check temperature derating curve → reduce Ir if needed (hot environments).

Step 4 — Pass the inrush

• Use the time–current curve: ensure the fuse does not open at the inrush multiple for the duration. E.g., Inductive loads like Motors, solenoid valve (Select Slo‑Blo® for transformer/motor/capacitor inrush.)

Step 5 — Fault safety check

• Ensure Vr ≥ system voltage and Iᵦ ≥ available fault current (especially on AC mains → prefer ceramic high‑Iᵦ series like 215/514).

Step 6 — Verify I²t

• Compare fuse I²t (melting) with inrush pulse I²t (or downstream semiconductor limits). Favor a lower I²t if you must protect devices with a tight SOA.

Step 7 — Package & logistics

• PCB space (SMD vs THT), serviceability (cartridge in holder), cost, and availability.

Example A — 85–265 VAC 60 W SMPS

• AC mains, high surge + inrush → ceramic time‑lag
• Pick 5×20 mm 215 for input; NTC + MOV in front; check interrupt rating and surge withstand.

Example B — Automotive battery feed (100 A)

• Under‑hood battery distribution → MEGA® 32 V 100 A bolt‑down; confirm 2 kA interrupt @32 V and ambient derating.

• Choosing Vr too low → arc doesn’t extinguish on DC → fire risk. (Always check Vr.)
• Glass fuse where interrupt rating is insufficient→ use ceramic (215/514) on high‑energy mains.
• Ignoring ambient temp → nuisance blows in summer (use derating).
• Not accounting for inrush → fast‑acting fuse nuisance trips on power‑up (use Slo‑Blo® like 315/218/468).
• Using PTC where one‑time protection is required by safety→ select the proper cartridge fuse.

Mounting/torque errors on bolt‑down fuses → localized heating; follow torque specs (MEGA).