The standard fuse symbol in a circuit diagram is either a rectangle with a single horizontal line passing through it (IEC standard) or a squiggly, offset zigzag line (ANSI/IEEE standard), representing a sacrificial overcurrent protection device. Because electrical schematics vary wildly depending on the region, the era of the equipment, and the CAD software used, misidentifying a fuse can lead to dangerous troubleshooting errors or incorrect replacement parts.
Complete Fuse Symbol Reference Chart
The table below covers the most common fuse symbols you will encounter on modern and legacy schematics. Reference designators for fuses almost universally begin with the letter F (e.g., F1, F2, F101).
| Symbol Description | Standard | Component Type | Practical Meaning & Application |
|---|---|---|---|
| Rectangle with a solid horizontal line through the center | IEC 60617 | Standard Cartridge/Glass Fuse | The most common global symbol. Represents a standard one-time fast-acting or slow-blow fuse (e.g., 5x20mm glass or ceramic body). |
| Smooth squiggly line or offset continuous curve | ANSI/IEEE 315 | Standard Fuse | Common in North American schematics. Represents the same standard sacrificial overcurrent link as the IEC rectangle. |
| Rectangle with a line through it, plus a small hook or arrow pointing at the line | IEC 60617 | Thermal Fuse / Cutoff | Opens based on ambient or component temperature, not just current. Common in appliance heating elements and transformer windings. |
| Rectangle with a line, intersected by a diagonal switch blade | IEC 60617 | Switch-Disconnector with Fuse | A physical pull-out or rotary switch that houses the fuse. Common in industrial 3-phase motor control panels. |
| Squiggly line with a thickened center section or mechanical striker pin indicator | ANSI/IEEE 315 | High-Rupturing Capacity (HRC) / Power Fuse | Used for high-fault-current environments (e.g., service entrance mains). The physical fuse has a sand-filled ceramic body to quench arcs. |
| Two overlapping rectangles with a line, or a box with 'PTC' inside | IEC / General | Resettable Fuse (Polyfuse / PTC) | A positive temperature coefficient thermistor. Trips on overcurrent, resets when power is removed and it cools. Common on USB ports and low-voltage DC rails. |
Regional Standards: IEC vs. ANSI/IEEE vs. Legacy British
When reading a schematic, your first step is identifying which drafting standard the engineer used. This dictates how the symbols are drawn.
IEC 60617 (International / Europe / Modern UK / Asia)
The International Electrotechnical Commission (IEC) standard favors clean, orthogonal geometry. The fuse is a simple rectangle (representing the physical housing) with a straight line (representing the fusible link). If you are working on equipment manufactured in the EU, modern UK gear, or most Asian-market consumer electronics, you will use IEC symbols. According to the Electronics Tutorials standard symbol guide, IC and microcontroller datasheets almost exclusively use IEC-style symbols for their protection circuits.
ANSI/IEEE 315 (North America)
The American National Standards Institute and IEEE standard uses the squiggly line. This standard is dominant in US and Canadian industrial controls, HVAC schematics, and military/aviation documentation. The squiggle represents the actual physical wire element inside a glass fuse before it blows.
BS 3939 (Legacy British - Obsolete but Present)
If you are troubleshooting older machinery in the UK or Commonwealth nations, you may encounter BS 3939. This standard used a variety of complex graphical notations, sometimes depicting the fuse as a rectangle with diagonal hash marks or specific terminal dots. While officially superseded by IEC 60617, facility maintenance closets are still full of BS 3939 blueprints. If a symbol looks like a resistor but has terminal dots at both ends and a reference designator of 'F', treat it as a legacy fuse.
Symbols People Get Wrong (and Reading Faded Schematics)
Misreading a fuse symbol usually happens when the draftsperson uses non-standard CAD libraries, or when a physical paper schematic has been degraded by oil, heat, and time. Here are the most common traps and how to navigate them.
Confusing the IEC Fuse with the IEC Resistor
In IEC 60617, a resistor is drawn as an empty, unfilled rectangle. A fuse is a rectangle with a solid horizontal line through the middle. On a poorly printed schematic, or if the CAD line weight is too thin, that center line can disappear.
The Fix: Always check the reference designator. Resistors are labeled R1, R2, etc. Fuses are labeled F1, F2, etc. If the designator is smudged, trace the circuit: a component in series directly off the mains input or a DC power jack is almost certainly a fuse, while a component bridging a signal line to ground or setting a bias voltage is a resistor.
Confusing the ANSI Fuse with the ANSI Resistor
In ANSI/IEEE, a resistor is a sharp, jagged zigzag (like a heartbeat). A fuse is a smooth, continuous squiggle or an offset curve. In fast hand-drawns or low-res faxes, the smooth squiggle can look like a jagged zigzag.
The Fix: Look at the physical board. If the component is a small cylindrical glass or ceramic tube with metal end-caps, it is a fuse. If it is a dog-bone shaped component with color bands, it is a resistor.
Safe Interpretation of Faded or Missing Symbols
When markings are missing, rely on the standard reference designator conventions. If the PCB silkscreen is burned off, use a multimeter to trace the net. A component that bridges the live/hot input directly to the bridge rectifier or transformer primary, with no other connections, is functioning as the main input fuse.
Frequently Asked Questions
What does a fuse symbol with a solid dot in the middle mean?
A fuse symbol with a solid dot or a specific mechanical indicator attached to the center usually denotes a High-Rupturing Capacity (HRC) fuse or a fuse equipped with a striker pin. In industrial motor control, when an HRC fuse blows, the striker pin physically pops out to trigger a microswitch, which trips the main contactor and drops all three phases to prevent single-phasing the motor. In some older military schematics, a dot in the center of an ANSI squiggle simply indicated a soldered link rather than a replaceable cartridge.
How do I tell the difference between a fuse and a resistor symbol on a schematic?
Visually, look for the line through the IEC rectangle or the smooth curve of the ANSI squiggle. Contextually, look at the reference designator (F vs R). Electrically, a fuse is designed to have near-zero resistance (typically less than 1 ohm for standard glass fuses, and a fraction of an ohm for HRC types) and is placed in series with the power source. A resistor has a specific, intentional resistance value (from ohms to megaohms) and can be placed in series or parallel configurations to divide voltage or limit current.
What is the symbol for a thermal cutoff fuse in a circuit diagram?
Under the IEC standard, a thermal fuse is drawn as the standard rectangle with a line through it, but with a small 'hook' or a thermal arrow pointing directly at the fusible link line. This indicates that the device reacts to heat rather than purely magnetic or resistive overcurrent. On the physical board, these often look like small metallic cylinders or epoxy-coated teardrops pressed directly against a heat sink, transformer core, or motor winding.
How do I safely verify a physical fuse when the schematic symbol is unclear or missing?
First, completely de-energize the circuit and unplug the device. For mains equipment, verify the absence of voltage with a known-good CAT III or CAT IV multimeter. Once dead, remove the fuse from its holder or desolder one leg to isolate it from the rest of the circuit (testing in-circuit can yield false continuity readings through parallel transformer windings). Set your multimeter to continuity or the lowest ohms range. A good fuse will read less than 1 ohm and beep. A blown fuse will read 'OL' (Open Loop) or infinite resistance. If testing high-voltage HRC fuses, visually inspect the ceramic body for micro-cracks or use a dedicated high-voltage fuse tester, as a multimeter's 3V continuity test cannot detect internal partial arcing paths in sand-filled bodies.
