The standard symbol of fuse in modern international schematics (IEC 60617) is a simple rectangle with a horizontal line passing through its center, while the North American ANSI/IEEE standard typically uses a zigzag line or a rectangle with offset terminals. Knowing which standard your blueprint uses prevents catastrophic miswiring and ensures you select the correct interrupting rating for the circuit. Below is the complete reference table to identify fuse symbols across global standards.
The Complete Fuse Symbol Reference Chart
Use this table to match the schematic symbol on your drawing to the physical component and its operational behavior. Note that visual descriptions are provided since schematic rendering varies by CAD software (AutoCAD Electrical, EPLAN, KiCad).
| Standard / Type | Visual Symbol Description | Physical Component Match | Schematic Meaning & Application |
|---|---|---|---|
| IEC Standard Fuse | Rectangle with a single horizontal line bisecting it end-to-end. | Standard cartridge (glass/ceramic), NH blade, or cylindrical ferrule. | General overcurrent protection. Opens when current exceeds rating for a specified time. |
| ANSI/IEEE Standard Fuse | Zigzag line (similar to a resistor) or a rectangle with offset diagonal terminals. | Class J, Class RK1/RK5, or Class CC North American industrial fuses. | North American industrial and commercial panel overcurrent protection. |
| Slow-Blow (Time-Delay) | IEC rectangle with a curved, wave-like line inside, or a thick solid block in the center. | MDL (slow-blow 3AG), T-rated glass fuses, or dual-element time-delay industrial fuses. | Allows temporary inrush currents (e.g., motor starting, transformer energization) without opening. |
| Fast-Acting (Quick-Break) | IEC rectangle with a straight, thick vertical line segment in the center. | AGC (fast-acting 3AG), F-rated glass fuses, semiconductor protection fuses. | Protects sensitive solid-state electronics; opens almost instantly on slight overcurrent. |
| Automotive Blade Fuse | Rectangle with a solid block in the middle and two distinct prongs extending downward. | ATO/ATC (standard), Mini, Maxi, or Micro2/Micro3 automotive blade fuses. | 12V/24V DC vehicle circuits. Color-coded housing indicates amperage (e.g., Yellow = 20A). |
| Switch-Fuse (Disconnect) | Standard fuse symbol intersected by a switch lever line (often with a manual handle indicator). | Fused disconnect switches (e.g., 3-pole 600V Class J disconnects). | Provides both overcurrent protection and a visible mechanical disconnect for LOTO procedures. |
Rows People Get Wrong: Common Symbol Confusions
- Slow-Blow vs. Standard IEC: Hobbyists frequently miss the subtle curved line inside the IEC rectangle, treating a slow-blow symbol as a standard fast fuse. If you install a fast-acting fuse in a circuit designed for a slow-blow (like a switching power supply input), it will nuisance-blow every time the device is turned on due to capacitor inrush current.
- ANSI Fuse vs. ANSI Resistor: The older ANSI zigzag fuse symbol looks nearly identical to the ANSI resistor symbol. The key differentiator is the connection points: a fuse symbol usually has the line breaking outside the zigzag or includes a small box around it, whereas a resistor is strictly the zigzag between two nodes.
- Fuse vs. Circuit Breaker: A circuit breaker symbol (IEC) is a rectangle with a line that has a distinct 'hump' or arc over it, representing the mechanical trip mechanism. Do not substitute a breaker where a high-interrupting-capacity (HRC) fuse symbol is specified for short-circuit protection.
Regional Standards: IEC vs. ANSI/IEEE vs. Legacy UK
When reading schematics, your geographic location and the origin of the equipment dictate which symbol library the engineer used. Mixing these up during panel retrofits leads to incorrect component ordering.
IEC 60617 (International / Europe / Asia):
The International Electrotechnical Commission standard is the dominant global language for modern electronics and industrial controls. If you are working on equipment manufactured in the EU, Asia, or modern global platforms (like Siemens or ABB PLCs), you will see the 'rectangle with a center line' symbol. IEC symbols prioritize geometric simplicity and are easily drawn by hand on the bench.
ANSI/IEEE 315 & NEMA (North America):
In the US and Canada, older industrial prints and NEMA-compliant motor control centers rely on ANSI/IEEE standards. Here, the symbol of fuse often appears as a zigzag or a rectangle with specific terminal offsets. If you are troubleshooting a legacy US manufacturing panel (e.g., Allen-Bradley MCC buckets from the 1990s), expect ANSI symbols. Modern US CAD software often defaults to IEC, but 'ANSI mode' is still heavily used in utility and heavy industry prints.
Legacy UK (BS 3939):
Before harmonizing with IEC standards, British Standard 3939 used distinct graphical symbols. You will still encounter these when servicing older UK-installed machinery or marine switchgear. The legacy BS fuse symbol often featured a circle with a horizontal line and specific cross-hatching to denote high-rupturing capacity (HRC). Always verify the drawing's title block for the standard revision before ordering replacement Littelfuse or Bussmann components.
Safe Interpretation When Markings Are Faded or Missing
On the bench or in a dusty industrial panel, you will frequently encounter fuses where the ceramic stamping is worn off, the glass is darkened by arc flash residue, or the automotive blade housing has melted. Guessing the rating based on wire size is a fire hazard. Follow this physical identification protocol to safely interpret unmarked fuses.
1. Measure Physical Dimensions (The Form Factor Check)
Fuses are strictly standardized by physical size to prevent inserting a high-amp fuse into a low-amp holder. Use digital calipers to measure the body (excluding end caps).
- 5 x 20 mm: Standard IEC PCB and panel mount (typically 250V, 0.1A to 20A).
- 1/4' x 1-1/4' (3AG / AGC / MDL): North American standard automotive and electronics (typically 250V or 32V DC).
- 10 x 38 mm: European industrial / solar PV (often rated up to 1000V DC for string protection).
- Class CC (13/32' x 1-1/2'): North American industrial control circuits (600V AC).
2. Inspect End Cap Materials and Internal Elements
If the glass is clear but the text is faded, look at the internal wire and the metal end caps. According to All About Circuits reference guides, fast-acting fuses typically feature a thin, straight wire element, often with silver or nickel-plated brass caps. Slow-blow fuses feature a coiled spring-like element or a wire with a deliberate 'solder joint' blob in the center designed to melt thermally over time. If the end caps are copper-colored rather than silver, it is often an indicator of a specific time-delay series in legacy Bussmann lines.
3. The Safe Continuity Test
Never test a fuse for continuity while it is seated in a live or even de-energized circuit, as parallel paths (like transformer windings or bleeder resistors) will give a false 'good' reading. Remove the fuse entirely. Set your multimeter to continuity or low-ohms. A good fuse reads < 1.0 ohm. An open fuse reads OL (Over Limit). If a glass fuse is physically blackened inside, it has cleared a high-energy short circuit; replace it with the exact OEM specified rating, never a higher one.
Frequently Asked Questions About Fuse Symbols
What does a solid line through the rectangle fuse symbol mean?
A solid horizontal line passing completely through the center of a rectangle is the standard IEC 60617 symbol of fuse. It represents a general-purpose, non-specified overcurrent protection device. If the line has a curve or a thick block added to it, it modifies the symbol to indicate a time-delay (slow-blow) or fast-acting characteristic, respectively.
How is the symbol of fuse different from a circuit breaker symbol?
While both provide overcurrent protection, their schematic symbols reflect their physical operation. The IEC symbol for a circuit breaker includes a mechanical trip indicator—usually a line that arcs or 'humps' over the main conductor path, sometimes with a small box representing the thermal/magnetic trip unit. A fuse symbol is strictly a passive element (a line through a box or a zigzag) with no mechanical linkage drawn, indicating it destroys itself to break the circuit.
What does a curved line inside the fuse symbol indicate?
A curved, wave-like, or coiled line drawn inside the standard rectangular fuse symbol designates a 'slow-blow' or time-delay fuse. This tells the technician that the circuit experiences high inrush currents (such as a motor starting or a large capacitor bank charging) and requires a fuse that can tolerate brief overloads without opening.
Is there a specific symbol of fuse for automotive DC circuits?
Yes. In automotive wiring diagrams (often following SAE or ISO 8888 standards), the symbol of fuse is frequently depicted as a rectangle with a solid block in the middle and two distinct prongs extending downward, representing the physical blades of an ATO/ATC or Mini blade fuse. In simpler 12V DC schematics, technicians may just use the standard IEC rectangle, but the BOM will specify the DC voltage and blade form factor.
