Fuse Symbols Explained: Electrical, Thermal, and Circuit Protection Symbols

A fuse is an overcurrent-protection device, often designed for one-time use. When a fault causes enough current to flow for long enough, its internal element opens, disconnecting the circuit. In a schematic, a fuse symbol marks both the protective component and the point where that circuit can be interrupted.

Fuse symbols vary between standards and schematic libraries. The quickest way to identify one is to look for a small symbol inserted in series with a power path, then check the associated reference designator and part details. A thermal fuse or resettable positive temperature coefficient thermistor (PTC) may appear nearby, but those devices respond differently and should not be treated as interchangeable.

What Does a Fuse Symbol Look Like?

The general fuse symbol is a small, often rectangular shape in the middle of a line. In some standards, the fuse can look like a squiggle. In the IEC 60617 symbol system, the general fuse symbol is a narrow rectangular body centered on the conductor. North American drawings and EDA libraries may use other fuse-body shapes or internal markings. The label and component record help resolve any ambiguity.

A fuse commonly uses the reference-designator prefix F. F1 identifies one fuse instance, F2 another. The text beside F1 might give a current rating, but that alone is not a complete specification. The bill of materials (BOM) should identify the selected part, including its voltage rating, response characteristics, and package or holder.

A fuse symbol differs from a resistor symbol. Both are two-terminal components, but a resistor is selected to provide a defined resistance in normal operation. In contrast, a conventional fuse is selected to remain intact in normal operation and open under specified fault conditions (e.g., overcurrent). Both resistor symbols and fuse symbols can correspond to a rectangle, so make sure to look carefully — the fuse symbol will have a line through the rectangle, or two vertical lines on the ends.

Common Types of Fuse Symbols

Cartridge fuses are replaceable fuses often used with holders. A schematic may show the general fuse symbol. Then, the cartridge dimensions, response type, and holder will appear in the component or BOM details. The drawing does not need a different symbol for every cartridge size.

Thermal fuses, also called thermal cutoffs, open when the device reaches a specified temperature. Some libraries provide a dedicated thermal-fuse symbol and others use a general protective-device symbol and a clear label. A thermal fuse protects against a temperature condition, which is different from that of a current fuse. Check the temperature rating, placement, and manufacturer instructions.

Resettable fuses are often polymeric positive-temperature-coefficient devices, or PPTCs. Although “resettable fuse” is a common name, a PPTC does not behave like a conventional fuse that permanently opens. During a fault, it heats and moves to a much higher-resistance state, limiting current. Then, the PPTC can recover after the fault is removed and the device cools. Its hold and trip behavior depends on temperature, so engineers should identify it explicitly as a PTC rather than assuming that an ordinary F1 symbol communicates its behavior. Littelfuse’s PPTC guidance emphasizes checking hold current at the expected ambient temperature.

Automotive fuses may be blade, cartridge, or other vehicle-specific types. A circuit diagram can still use a general fuse symbol. The selected component and its location in the vehicle’s power distribution system matter more than drawing the physical blade shape.

Semiconductor fuses are chosen for applications where the protected devices and fault conditions require a particular clearing performance. They may also use the general fuse symbol, with the precise fuse series and ratings specified in the design records.

PCB-mounted fuses may be surface-mount or through-hole components soldered to a board. The schematic symbol expresses their electrical role; the footprint and part number determine how they appear in the PCB layout. In every case, let the component data—not a decorative symbol variation—carry the exact specification.

Fuse vs Circuit Breaker and Other Protection Symbols

Protection components address different faults. Their symbols should help a reviewer see what each device is intended to do.

A circuit breaker is not simply a fuse that happens to be reusable: it has a different mechanism and its own ratings. Likewise, a PTC may continue to pass some current in its tripped, high-resistance state. A varistor or TVS diode principally addresses voltage transients, not the same sustained overcurrent condition a fuse is selected to interrupt. Some designs use these components together. For example, a fuse in series with an input and a surge suppressor connected across the appropriate nodes can provide different layers of protection—but the complete circuit must be evaluated for its actual fault conditions.

Where Should a Fuse Be Placed in a Circuit?

In a basic DC design, a fuse should be placed close to the power entry, in series with the power supply feeding the circuitry it is meant to protect:

This placement aims to limit how much downstream wiring and circuitry are unprotected before the fuse. A fuse connected in parallel with the supply would be ineffective. Further, placing a fuse somewhere in the schematic in series with a given branch will only make sure the given branch is protected.

For an AC-powered product, decisions about protecting the live conductor, both current-carrying conductors, or individual branches depend on the supply, wiring setup, grounding, and applicable safety requirements. Do not infer a universal mains-fusing rule from a simplified diagram. Importantly, mains and other hazardous-energy designs should be reviewed against the relevant standards by a qualified engineer.

Fuse selection also requires more than just matching its printed ampere rating to normal load current. Check the maximum operating voltage, expected steady-state current, ambient temperature, startup or inrush pulses, time-current behavior, and the interrupting rating against the maximum prospective fault current in your circuit. A fuse that carries the load but cannot safely interrupt the available fault current should not be used. Littelfuse’s selection guide covers these factors and recommends testing against the actual application.

Finally, a fuse does not replace thermal protection, surge suppression, correct wire and trace sizing, or safe enclosure design. It is one part of a protection strategy.

Adding a Fuse to a Schematic in Flux

In Flux, open the Parts Library and search for “fuse,” a fuse type, or a manufacturer part number. The library documentation describes keyword and part-number search as well as filters for component information. Choose a component whose schematic symbol, available footprint, and data fit the intended design stage.

Place the fuse near the power supply on the schematic canvas. Connect one terminal to the node with the power supply and the other to the node you want to protect. Then trace the net to confirm the loads you intend to protect are downstream of the fuse. Give it a readable designator such as F1.

Document the selected part’s current and voltage ratings, response type, interrupting capability, and relevant package or holder. If the first revision of the schematic uses a generic fuse, mark its final selection as unresolved rather than letting an arbitrary placeholder enter the BOM. Before doing layout, check that the footprint and pad numbering match the manufacturer’s data.

Review the finished protection circuit with the power source, wiring, fault paths, and any TVS diode, MOV, or thermal device visible. Flux’s PCB design software can keep the schematic and board work in one project, but the engineering judgment about ratings and safety remains essential.

FAQs

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