Fuse Symbol
Definition: The Fuse symbol represents a protective two-terminal device that interrupts a circuit by melting its conductive element when current exceeds a specified rating, drawn as a short horizontal line inside a small rectangle (IEC 60617-06) or as a small curved loop (S-shape) between two lead lines (ANSI Y32.2 / IEEE 315-1975), with current rating measured in amperes (A); it is assigned the schematic designator F.
Also known as: fuse symbol, overcurrent protection symbol, F symbol schematic, blow fuse symbol, circuit fuse symbol, electrical fuse symbol meaning.
What the Fuse symbol means
The fuse symbol denotes a sacrificial protective element that is deliberately designed to fail — by melting — before excessive current can damage more expensive or safety-critical components downstream. When current exceeds the fuse's rated value for long enough, the fusible element heats to its melting point, the circuit opens, and all current ceases. This irreversible action protects the circuit and the wiring from fire or thermal damage caused by overload or short-circuit faults.
In a circuit diagram, the fuse symbol is placed in series with the supply rail or the load it protects — typically immediately after the power inlet or battery terminal. The two terminals (pin id 'a' and pin id 'b') are functionally equivalent: a fuse is non-polar and can be connected in either orientation. The current rating annotated beside the symbol (e.g. '5 A', '250 mA') specifies the value at which the fuse will eventually open under sustained overcurrent conditions; a voltage rating (e.g. '250 VAC') specifies the maximum circuit voltage across which the open fuse can safely extinguish the arc.
How to identify the Fuse symbol
The IEC 60617-06 fuse symbol is drawn as a short horizontal line inside a small narrow rectangle (a box), with lead lines extending from both short ends of the rectangle — the rectangle encloses the fusible element, and the internal line represents the wire or strip that melts. This form is used in European and international schematics and is easy to recognise: a small box with a line through it. The ANSI Y32.2 / IEEE 315-1975 fuse symbol is drawn as a small S-shaped or curved loop (resembling a stretched or curly Z or a single-turn coil) between two lead lines, sometimes described as a 'squiggly line in a lens'. Some older ANSI schematics show a thin slanted line inside a diamond outline. Both symbols occupy a short horizontal span and are oriented in series with the circuit conductor they protect.
Function and usage boundary
A fuse provides overcurrent protection by incorporating a thin conductive element — typically a fine wire or strip of metal alloy (tin, lead, silver, copper, or zinc-silver alloy) — that has a precise melting point calibrated to the fuse's current rating. During normal operation, the element carries the rated current without significant heating. When a fault causes sustained overcurrent (overload) or a sudden short-circuit, the I²R heating in the element exceeds its thermal capacity, the metal melts, and the circuit is permanently opened. The interrupting capacity (breaking capacity, kA) specifies the maximum fault current the fuse can safely interrupt without arc-over or rupture. A blown fuse must be replaced after the fault is identified and corrected.
Standards: IEC vs ANSI
| IEC 60617 | IEC 60617-06 defines the fuse symbol as a short line inside a small rectangle (box) with lead connections at each end. IEC 60269 is the governing standard for low-voltage fuses (types gG general-purpose, aM motor circuit, etc.), defining rated current, voltage, breaking capacity, and time-current characteristics. IEC 60127 covers miniature fuses used in electronic equipment. |
|---|---|
| ANSI/IEEE 315 | ANSI Y32.2-1975 / IEEE 315-1975 defines the fuse symbol as a small S-curve or loop between two lead lines. UL 248 (USA) and CSA C22.2 No. 248 (Canada) govern the physical performance of fuses in North American markets. The designator F is assigned by IEEE 315-1975. |
| Key difference | The IEC 60617 fuse symbol is a line inside a rectangle (box); the ANSI Y32.2 / IEEE 315 fuse symbol is an S-shaped or curved loop between leads. The two glyphs are visually distinct and immediately signal which standard the schematic follows. Both represent an identical component function — overcurrent interruption. |
The displayed SVG is the live editor implementation, not a licensed reproduction or certification of an IEC or IEEE database glyph. Verify the exact official entry and project edition before issuing work.
Logical terminals
| Logical ID | Editor terminal name |
|---|---|
| a | A |
| b | B |
Where the Fuse symbol is used
- Primary power inlet protection in mains-powered equipment — a fuse immediately after the live terminal of the power connector, per IEC 60364 and NEC requirements
- PCB-mount polyfuses (resettable PTC fuses) on USB port protection circuits to limit current from a host port if a downstream device draws excessive current
- Automotive wiring fuse boxes where individual blade fuses (5 A–40 A, ISO 8820) protect separate circuits: lighting, ignition, audio, HVAC, and accessories
- Battery pack protection in lithium-ion battery assemblies, where a fuse opens on short circuit before thermal runaway can develop
- Switch-mode power supply (SMPS) primary-side fuses rated for the maximum input fault current with appropriate breaking capacity
- Three-phase motor control panels where HRC fuses provide short-circuit protection upstream of the motor starter contactor
Key facts
- The fuse symbol in IEC 60617-06 is a horizontal line inside a small rectangle; the ANSI Y32.2 / IEEE 315-1975 fuse symbol is an S-shaped or curved loop between two leads — these are visually distinct glyphs for the same protective component.
- The schematic designator for a fuse is F, per IEEE 315-1975; fuses are labelled F1, F2, F3 in sequence on a schematic.
- A fuse is non-polar and has two interchangeable terminals (pin id 'a' and pin id 'b'); it can be connected in either orientation without affecting its protective function.
- Fuse current ratings are in amperes (A) or milliamperes (mA), per IEC 60127 (miniature fuses) and IEC 60269 (low-voltage power fuses); the rated current is the maximum sustainable current below which the fuse will not blow.
- A blown fuse permanently opens the circuit and must be replaced; it is a sacrificial, one-shot protective device. Resettable fuses (polyfuses, PTC devices) are an exception — they recover after the fault is removed and the device cools.
- Fuse time-current characteristics are classified as FF (very fast), F (fast), M (medium), T (time-lag/slow-blow), and TT (very slow); slow-blow fuses are used where loads have high inrush currents (transformers, motors) to prevent nuisance blowing.
- The breaking (interrupting) capacity of a fuse — rated in amperes (A) or kiloamperes (kA) — is the maximum fault current the fuse can safely interrupt without arc-over or fragmentation; selecting a fuse with insufficient breaking capacity is a serious safety error.
- A voltage rating on a fuse (e.g. 250 VAC, 600 VDC) specifies the maximum circuit voltage across which the open fuse element can safely extinguish the fault arc; using a 250 VAC fuse in a 400 VAC circuit may result in sustained arcing after the element melts.
Diagrams that use this symbol
- delta star connection diagram
- star delta circuit diagram
- star delta starter diagram
- star delta wiring diagram
- star delta control circuit diagram
- star delta control diagram
- star delta control wiring diagram
- solar panel wiring diagram
Frequently asked questions
What does the fuse symbol look like in a circuit diagram?
In IEC 60617-06 (used in European and international schematics), the fuse symbol looks like a short horizontal line inside a small narrow rectangle (box) with lead lines at each end. In ANSI Y32.2 / IEEE 315-1975 (used in North American schematics), the fuse symbol is an S-shaped or curved loop between two lead lines. Both symbols occupy a small horizontal span in series with the circuit conductor.
What does the fuse symbol mean in a circuit?
The fuse symbol means that a sacrificial overcurrent protection device is placed at that point in the circuit. If current exceeds the fuse's rated value for long enough, the fusible element melts and permanently opens the circuit, protecting downstream components and wiring from damage or fire. The rating annotated beside the symbol (e.g. '5 A') is the maximum current the fuse carries without blowing.
What is the difference between the IEC and ANSI fuse symbols?
The IEC 60617 fuse symbol is a line inside a small rectangle (box); the ANSI Y32.2 / IEEE 315-1975 fuse symbol is an S-shaped or curved loop. These are visually distinct glyphs. Both represent the same functional component — a fusible overcurrent protection element. Mixed-standard schematics should label which convention is used.
What letter designates a fuse on a schematic?
The designator letter for a fuse is F, as defined in IEEE 315-1975. Individual fuses are labelled F1, F2, F3 in sequence on a schematic. The designator appears adjacent to the fuse symbol and in the bill of materials.
What is the difference between a fast-blow and slow-blow fuse symbol?
The standard fuse symbol is the same for both fast-blow and slow-blow types — the time-current characteristic (FF, F, T, TT) is specified in the annotation beside the symbol, not by a different glyph. A slow-blow (time-lag, type T) fuse annotation beside the symbol indicates the fuse is rated to withstand brief inrush currents without blowing, while still protecting against sustained overloads.
What standard defines the fuse symbol?
The fuse symbol is defined in IEC 60617-06 (protective devices, International Electrotechnical Commission) for the box/rectangle form, and in ANSI Y32.2-1975 / IEEE 315-1975 for the S-curve form. IEC 60269 governs low-voltage fuse performance; IEC 60127 governs miniature fuses; UL 248 applies in the United States.
Is the fuse symbol polarised — does connection direction matter?
No. The fuse symbol is non-polar; it has two equivalent terminals (pin id 'a' and pin id 'b') and the fuse operates identically in either orientation. However, in AC mains wiring, the fuse is always placed in the live (L) conductor — not the neutral — to ensure the circuit is de-energised when the fuse blows, which is a safety wiring requirement, not a component polarity requirement.
Related symbols
- AFCI Breaker symbol
- Circuit Breaker symbol
- Fuse Terminal Block symbol
- GFCI Breaker symbol
- Fuse symbol
- Surge Protector symbol
Physical breadboard, PCB, and panel mapping
No universal physical profile is published for this symbol yet. Create a versioned custom part from the exact manufacturer dimensions and terminal map before fabrication.
Place the Fuse symbol directly on a wiring diagram or schematic in the free online editor — no download required.