If you are asking what is the symbol for a fuse, the direct answer depends on the drafting standard used in your region. Under the international IEC 60617 standard, a standard fuse is drawn as a rectangle with a single straight line passing through its center. Under the US IEEE/ANSI (NEC-style) standard, it is historically drawn as a zigzag line (similar to a resistor) or a rectangle with offset connection terminals. Modern US schematics increasingly adopt the IEC rectangle to align with global CAD libraries.

The Complete Fuse Symbol Reference Table

Use this spec-sheet-table to decode the exact type of protection a schematic is calling for. Never substitute a fast-acting fuse for a time-delay variant just because the base symbol looks similar.

Component / Variant IEC 60617 Symbol Description IEEE/ANSI (US) Symbol Description Practical Application & Meaning
General / Fast-Acting Fuse Rectangle with a straight line through the center. Zigzag line, or rectangle with a straight line. Protects resistive loads (heaters, lighting). Blows instantly on overcurrent.
Slow-Blow (Time-Delay) Fuse IEC rectangle with a thick/blackened center line or a small 'box' in the middle of the line. Zigzag or rectangle with a curved line or 'S' indicator. Protects inductive loads (motors, transformers). Withstands brief inrush currents without opening.
Thermal Fuse (Cutoff) Standard fuse symbol enclosed in a diamond, or with a 'heat wave' arc above it. Rectangle with a thermal switch symbol (half-circle with an upward arrow) overlaid. Opens based on ambient or component temperature, not just current. Common in coffee makers and hair dryers.
Fuse-Disconnector (Switch-Fuse) Fuse symbol with a hinged lever line extending from one terminal, ending in an open gap. Similar hinged lever extending from the zigzag or rectangle. Allows manual physical disconnection of the circuit for lockout/tagout (LOTO) safety while providing overcurrent protection.
Striker / Indicator Fuse Standard fuse symbol with a small 'X' or a dot connected to a secondary pin. Rectangle with a secondary parallel line and an 'X'. Features a mechanical pin that pops out when blown to trigger a microswitch or provide a visual blown-fuse indication on a panel.

Regional Standards: Which Symbol Applies to Your Bench?

Before tracing a fault, you must identify which standard the original equipment manufacturer (OEM) used. Misinterpreting a regional variant can lead to reading a fuse symbol as a resistor or an inductor.

Bench Tip: Look at the schematic title block in the bottom right corner. If it cites IEC 60617 or EN 60617, use the international column. If it cites IEEE 315 or NEMA, use the US column.
  • IEC (International Electrotechnical Commission): The dominant standard in Europe, Asia, and modern global equipment. The rectangle-and-line is clean, easy to draw in CAD, and avoids confusion with the US resistor symbol. Most modern equipment imported into the US uses IEC symbols on internal schematics.
  • IEEE/ANSI / NEMA (North America): Governed by NFPA 70 (NEC) guidelines and IEEE 315 drafting standards. Older US industrial panels and legacy military gear heavily feature the zigzag fuse. Note that modern US CAD packages (like AutoCAD Electrical) often default to IEC rectangles even for domestic projects to maintain library consistency.
  • Old UK (BS 3939): Superseded by IEC, but you will still encounter this in legacy British wiring diagrams (pre-2000). It used a rectangle with a line, but often included a specific dot notation to indicate the fuse rating tier. Treat these as IEC equivalents but verify the physical component.

Rows People Get Wrong (and Costly Mistakes)

When reading schematics or inspecting physical boards, these specific symbol and marking confusions cause the most blown boards and repeated fuse failures.

1. Confusing the US Resistor with the US Fuse

In legacy ANSI/IEEE schematics, both the resistor and the fuse use a zigzag line. The critical differentiator is that the fuse symbol usually has a straight line drawn directly through the center of the zigzag, or the zigzag is broken in the middle with a gap. If you misread a 5-ohm power resistor as a fuse and attempt to bypass it with a wire, you will destroy the downstream power supply.

2. Ignoring the 'F', 'T', and 'M' End-Cap Markings

The schematic might just show a generic fuse symbol, but the physical component dictates the speed. According to standard reference texts, you must read the microscopic letters stamped on the metal end caps:

  • F (Fast-Acting): Opens immediately. Used for sensitive electronics.
  • T (Time-Lag / Slow-Blow): Withstands inrush. Never replace a 'T' with an 'F'; the fast fuse will blow every time the device powers on.
  • M (Medium-Acting): A compromise between F and T, common in specific European appliance standards.

3. Missing the Breaking Capacity Indicator (L vs. H)

A schematic might call for a 250V fuse, but fail to specify breaking capacity. Physical fuses are stamped L (Low Breaking Capacity, typically 35A) or H (High Breaking Capacity, typically 1500A). Putting a glass 'L' fuse in a high-fault-current industrial circuit can cause the glass tube to explode under a dead short. Use ceramic 'H' fuses for high-energy mains panels.

Safe Interpretation When Schematics or Markings Are Faded

Safety Warning: When a fuse fails catastrophically, the inside of the glass tube often turns completely black or opaque, and the end-cap text is vaporized. Never guess the rating based on the wire thickness inside a blackened fuse. Always de-energize, lock out the panel, and verify dead with a multimeter before extracting the fuse for inspection.

When the schematic is missing and the fuse cap is burnt, you must rely on physical dimensions and holder geometry to safely interpret the required replacement:

  1. Measure the Diameter and Length: The two most common glass cartridge sizes are 5x20mm (Metric, IEC standard) and 1/4" x 1-1/4" (Imperial, US AGC/MDL standard). They look nearly identical to the naked eye. A 1/4" fuse is 6.35mm in diameter. Forcing a 6.35mm US fuse into a 5mm metric holder creates a high-resistance friction fit that will overheat and melt the panel mount.
  2. Inspect the Holder Clips: Panel-mount holders often have the voltage and current limits molded directly into the plastic bezel (e.g., '250V 10A'). This tells you the maximum circuit rating, but not necessarily the specific fuse value required. You must trace the downstream load to calculate the correct amperage.
  3. Use a Magnifying Loupe on the Caps: Even if the glass is black, the metal end caps usually survive. Look for stamps like F5AL250V (Fast, 5 Amp, Low breaking capacity, 250 Volts).

Decision Path: Selecting the Right Replacement Fuse

Do not leave your bench trip to the parts bin to chance. Use this decision tree to terminate on the exact physical part number you need to order or stock.

Condition / Question If YES / Condition Met If NO / Condition Not Met
Is the load inductive (motors, large transformers, switching power supplies with high inrush)? Require Slow-Blow (Time-Delay). Proceed to size check. Require Fast-Acting. Proceed to size check.
Is the physical holder metric (measures exactly 20mm long)? Require 5x20mm form factor. Require 1/4" x 1-1/4" (AGC/MDL) form factor.
Is the circuit voltage above 125VAC or any high-energy DC bus? Require Ceramic body (High Breaking Capacity, 'H'). Glass body (Low Breaking Capacity, 'L') is acceptable.
Is the application a DC circuit (e.g., 12V/24V solar or automotive)? Require a DC-rated fuse (DC arcs do not self-extinguish at zero-crossing; AC fuses are unsafe for high-voltage DC). Standard AC-rated fuse is acceptable.

The Concrete Default Recommendation

For 90% of hobbyist, bench equipment, and consumer electronics repairs requiring a standard panel-mount glass fuse, stock these two specific part families from Littelfuse or Eaton Bussmann:

  • For Fast-Acting Needs: Buy the Littelfuse 0218 Series (or Bussmann GDC). These are 5x20mm, 250VAC, fast-acting glass fuses. They are the industry standard for protecting sensitive PCB traces and semiconductor loads.
  • For Slow-Blow Needs: Buy the Littelfuse 0215 Series (or Bussmann MDL). These are 5x20mm, 250VAC, time-delay fuses. They are the exact replacement for switching power supply inputs and motor circuits where nuisance tripping from inrush current is the primary failure mode.

By matching the schematic symbol to the correct IEC/IEEE standard, verifying the physical dimensions with calipers, and selecting the correct time-delay characteristic, you will eliminate repeated fuse failures and protect your downstream circuitry from catastrophic fault currents.