Fuse Symbol
Definition: The Fuse symbol represents a single-use overcurrent protection device containing a fusible element that melts and permanently opens the circuit when current exceeds the rated value for a sufficient time, shown in electrical schematics as a two-terminal element between In and Out connection points, standardised under IEC 60617-08 (IEC symbol: a rectangle) and ANSI/IEEE 315-1975 (ANSI symbol: a curved line or S-shape inside a rectangle), with the reference designator F.
Also known as: overcurrent fuse, cartridge fuse, blade fuse, glass tube fuse, slow-blow fuse, fast-acting fuse, current limiter.
What the Fuse symbol means
The Fuse symbol denotes a passive overcurrent protection device containing a wire or strip of low-melting-point alloy (tin-lead, silver, zinc, or copper) sized to conduct normal operating current but to melt and interrupt the circuit when current exceeds the rated amperage multiplied by a time-current melting factor. In a schematic, the fuse symbol appears in series with the circuit it protects, between the supply source (In) and the protected load or wiring (Out).
Fuses are the simplest and most cost-effective form of overcurrent protection — no reset mechanism means a blown fuse provides a clear, permanent indication of a fault event. They protect wiring from overheating (fire risk), protect equipment from damage, and limit fault current in power systems. A fuse's characteristics are defined by its voltage rating, current rating (I_N), interrupting (breaking) capacity (I_K), and time-current (melting) characteristic (fast-acting, time-delay, or semiconductor protection).
How to identify the Fuse symbol
The IEC 60617 fuse symbol is a small filled rectangle (representing the fuse cartridge body) with a horizontal line or thin rectangle inside it, connected to two terminal leads. The ANSI/IEEE 315 fuse symbol is a curved S-shape or wavy line inside a thin rectangular outline, or sometimes just the wavy line between two terminals. Both symbols are placed in series with a circuit line. In diagrams using the industrial/indicator convention, the fuse may appear as a plain rectangle with In and Out terminals as shown in this catalogue.
Function in a circuit
A fuse conducts normal circuit current with negligible resistance and voltage drop. When a fault or overload causes current to rise above the rated value, Joule heating (P = I²R) in the fusible element raises its temperature to the melting point. Once melted, the element vaporises or retracts, creating an arc that is quenched by the fuse's arc-extinguishing medium (silica sand for HRC fuses, air for cartridge fuses). The circuit is permanently opened, removing the fault current. Unlike a circuit breaker, a fuse cannot be reset — the fuse element must be replaced after operation.
Standards: IEC vs ANSI
| IEC 60617 | IEC 60617-08 defines the fuse symbol as a rectangle with a line through it (qualifying the element). IEC 60269 series covers low-voltage fuses: IEC 60269-1 (general requirements), IEC 60269-2 (industrial applications), IEC 60269-3 (residential/household), and IEC 60269-4 (semiconductor protection). The designator is F. |
|---|---|
| ANSI/IEEE 315 | ANSI/IEEE 315-1975 represents the fuse as a curved or S-shaped element inside a rectangular outline. UL 248 series covers low-voltage fuses in North America (UL 248-1 general requirements, UL 248-4 class CC, UL 248-14 supplemental fuses). The reference designator is F. |
| Key difference | The IEC fuse symbol is a rectangle with a straight line element; the ANSI fuse symbol shows a curved or S-shaped element within the rectangle outline. Both standards use the designator F. Functionally the symbols are equivalent; the visual difference is purely a convention of the standard in use — IEC rectangle vs. ANSI curvilinear element. |
Terminals / pins
| Pin | Name |
|---|---|
| in | In |
| out | Out |
Typical values
Rated current I_N: 0.1 A to 6300 A (standard ranges). Rated voltage: 32 V DC to 1000 V AC. Breaking/interrupting capacity I_K: 1.5 kA (standard) to 100 kA (HRC industrial fuse). Time characteristics: gG (general purpose, slow for inrush), gM (motor protection), aM (motor branch backup), gR (semiconductor fast), aR (semiconductor ultra-fast). Common physical formats: cartridge (cylindrical), blade (ATO/ATC/mini/maxi for automotive), DIN-rail fuseholder.
Where the Fuse symbol is used
- Branch circuit protection in residential distribution boards: glass or ceramic cartridge fuse or blade fuse protects lighting and socket-outlet circuits from overloads and short-circuits
- Automotive electrical system protection: blade fuses (ATO/ATC 5–40 A, maxi 20–80 A) protect wiring harnesses feeding headlights, power windows, HVAC blower, and ECU circuits
- Industrial control panel branch protection: DIN-rail fuseholder with gG semiconductor fuse protects 24 V DC control circuits, PLC power supplies, and sensor wiring from short-circuits
- Electronic circuit protection: PCB-mount glass tube fuse (5×20 mm or 3AG) in series with the AC mains input of a power supply to interrupt fault current from internal failures
- Semiconductor and power electronics protection: ultra-fast aR or gR fuses protect thyristors (SCRs), diodes, and IGBTs in variable speed drives and converters from fault currents
- Battery system protection: fuse on the positive terminal of a battery bank (automotive, marine, solar) to protect wiring from catastrophic short-circuit current that could cause fire or explosion
Example
In a residential electrical wiring diagram, the fuse symbol appears in series between the distribution board busbar and a 16 A branch circuit feeding a ring of socket outlets. The In terminal of the fuse connects to the live busbar and the Out terminal connects to the live cable of the ring circuit. If a fault causes current to exceed the fuse's time-current curve, the fuse blows, opening the circuit and isolating the fault from the supply.
Key facts
- The Fuse symbol has two terminals: In (supply/line side) and Out (load/protected side), and is always placed in series with the circuit it protects; current designator is F per IEC 60617 and ANSI/IEEE 315.
- IEC 60617 draws the fuse as a rectangle with a straight internal line; ANSI/IEEE 315 draws it as a curved or S-shaped element within a rectangular outline — both symbols are equivalent in function.
- Fuse current ratings follow IEC 60269 standard series: common values are 1, 2, 4, 6, 10, 16, 20, 25, 32, 40, 50, 63, 80, 100 A; intermediate values exist for specialist applications.
- A fuse's time-current characteristic determines how quickly it operates: fast-acting (gR/aR) fuses for semiconductor protection, time-delay (gG) fuses for general purpose circuits tolerating motor start inrush.
- The breaking/interrupting capacity of a fuse is the maximum fault current it can safely interrupt without exploding or sustaining arc damage; HRC (high rupturing capacity) fuses can interrupt up to 100 kA.
- Unlike a circuit breaker, a blown fuse cannot be reset — the fuse element must be physically replaced, providing a clear indication that a fault event occurred.
- Fuse voltage ratings must equal or exceed the circuit voltage: using a fuse rated below the circuit voltage risks sustained arcing after element melting, failing to clear the fault.
- Automotive blade fuses (ATO/ATC format, standardised by SAE J1284) are colour-coded by amperage: orange 5 A, red 10 A, blue 15 A, yellow 20 A, clear 25 A, green 30 A.
Diagrams that use this symbol
- fuse box diagrams
- alternator connection diagram
- alternator diagram wiring
- wiring 3 phase
- alternator circuit diagram
- alternator diagram
- inverter connection diagram
- inverter wiring diagram
Frequently asked questions
What does the fuse symbol mean in a wiring diagram?
The fuse symbol represents a single-use overcurrent protection device that permanently opens the circuit when current exceeds its rated value. It is placed in series with the circuit between the supply (In terminal) and the load (Out terminal). When a fuse blows, the circuit is broken and the fuse element must be replaced.
What does the fuse symbol look like in a schematic?
The IEC 60617 fuse symbol is a small rectangle with a straight line element inside, placed in series with the circuit wire. The ANSI/IEEE 315 fuse symbol shows a curved or S-shaped element within a thin rectangular outline. Both symbols represent the same device; the visual style depends on which standard the schematic follows.
What is the designator letter for a fuse in a schematic?
The reference designator for a fuse is F in both IEC 60617 and ANSI/IEEE 315. Individual fuses in a circuit are numbered sequentially: F1, F2, F3. In automotive wiring diagrams, fuses are typically identified by circuit name or number rather than a letter designator.
What is the difference between IEC and ANSI fuse symbols?
The IEC 60617 fuse symbol is a filled rectangle with a straight line through it. The ANSI/IEEE 315 fuse symbol shows a curved or S-shaped element inside a rectangular border. Both symbols convey the same meaning — a series overcurrent protection device — but the visual appearance differs depending on the drawing standard used.
What standard covers fuse ratings and selection?
IEC 60269 series covers low-voltage fuse design, testing, and selection internationally: IEC 60269-1 (general), IEC 60269-2 (industrial), IEC 60269-3 (household). In North America, UL 248 series covers low-voltage fuses. Automotive fuses follow SAE J1284 (blade fuse colour coding) and ISO 8820.
What is the difference between a fast-acting and a time-delay fuse?
A fast-acting (quick-blow) fuse melts immediately when current exceeds its rating — used to protect sensitive electronics and semiconductors. A time-delay (slow-blow) fuse tolerates brief overcurrents such as motor start inrush (5–10× rated current for a few seconds) before melting — used to protect motor circuits and transformer primaries. IEC 60269 designates general-purpose time-delay fuses as gG and ultra-fast semiconductor fuses as gR or aR.
Why must a fuse voltage rating match the circuit voltage?
The fuse voltage rating is the maximum voltage across the fuse after the element melts. If the circuit voltage exceeds the fuse's rating, the arc that forms when the element melts may be sustained indefinitely, preventing the circuit from clearing the fault. A fuse must always be rated at or above the circuit voltage to ensure reliable arc extinction and safe fault clearing.
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