Electrical Symbols for Fuses: IEC, ANSI, and Schematic Reference Guide

When reading a schematic or tracing a control panel, misidentifying an electrical symbols fuse designation can mean the difference between replacing a $2 glass cartridge and troubleshooting a blown VFD or fried control board. Schematic standards vary wildly by region, and substituting a fast-acting fuse for a time-delay fuse based on a misread symbol will cause immediate nuisance tripping on motor startup.

Below is the definitive reference for fuse symbols, mapped to modern global and North American standards, followed by practical bench advice for when the schematic is missing or the fuse markings are burned off.

The Master Electrical Symbols Fuse Reference Chart

The table below maps the most common fuse types to their IEC 60617 (Global/EU) and ANSI/IEEE 315 / NEMA (North America) schematic symbols. Because we cannot render raw vector graphics here, the 'Symbol Geometry' column describes the exact visual construction you will see on the page.

How to Read This Table in Practice

The most critical distinction in this chart is between the Standard and Time-Delay symbols. In modern CAD software (like AutoCAD Electrical or EPLAN), the filled half-rectangle (IEC) or the 'TD' annotation (ANSI) is your only visual cue that the designer intended to accommodate high inrush currents. If you replace a time-delay fuse with a standard fuse of the exact same amperage, the circuit will trip the moment the motor contactor pulls in.

Regional Standards: IEC vs. ANSI vs. Legacy UK

Fuse symbols are not universal. The standard you should follow depends entirely on the origin of the machinery or the region of installation.

• IEC 60617 (Europe, UK, Australia, Global Export): This is the modern global standard. It relies heavily on geometric shapes (rectangles) and fill patterns (the black half-box for time-delay). If you are working on modern Siemens, Schneider, or ABB equipment, you will use IEC symbols.
• ANSI/IEEE 315 & NEMA (USA, Canada): North American schematics often rely more on text annotations ('TD', 'FA') alongside the basic rectangle. NEMA standards also heavily feature the 'Fused Disconnect' symbol as a combined chassis unit, reflecting the physical reality of North American industrial control panels where the disconnect handle and fuse block are mechanically interlocked.
• Legacy UK BS 3939 (Pre-1990s UK Machinery): If you are maintaining older textile, printing, or manufacturing equipment in the UK, you may encounter the withdrawn BS 3939 standard. In this legacy system, a fuse was often depicted as a circle with a solid horizontal line through it, or a simple straight line with a thickened center node. Do not confuse the BS 3939 circle-and-line fuse with the IEC symbol for a generic terminal or junction point. Always verify the drawing's title block for the standard revision year.

Rows People Get Wrong & Interpreting Faded Markings

Even with a perfect schematic, physical reality often interferes. Fuses blow, arc, and burn, destroying the text printed on their bodies. Here is how to navigate the most common schematic confusions and physical identification failures.

Schematic Confusion: Thermal Fuse vs. PTC Resettable Fuse

A common and dangerous mistake is confusing a Thermal Cutoff (TCO) symbol with a PTC Resettable Fuse (Polyfuse).

A TCO (like the SEFUSE SF series) is a one-time, non-resettable device that melts a physical pellet when a specific temperature (e.g., 130°C) is reached. The symbol features a thermal curve overlay. A PTC resettable fuse (often used on USB ports or LiPo battery packs) increases its resistance when hot but resets when cooled. Its symbol usually features a small arrow pointing back at itself or a loop. Never replace a one-time TCO with a PTC; the TCO is there to prevent catastrophic thermal runaway, and a PTC will not open the circuit fast enough to stop a fire.

Safe Interpretation When Markings are Faded or Missing

When a fuse blows violently, the glass blackens or the ceramic body scorches, obliterating the amperage and speed ratings. If the schematic is missing, use these bench heuristics to identify the replacement:

1. The Flashlight Test (Glass Fuses Only): Hold an intact, identical spare fuse up to a bright LED. If you see a single, thin, straight wire element, it is a Fast-Acting (F) fuse. If you see a coiled spring, a thick mass of solder, or a dual-element design (a thin wire bridging a thick soldered joint), it is a Time-Delay (T or TT) fuse. The solder mass absorbs heat slowly, allowing temporary inrush spikes.
2. Circuit Context (Ceramic Fuses): Ceramic fuses (like 10x38mm or 5x20mm HRC types) are opaque. You must rely on the circuit. If the fuse protects a motor starter, a solenoid bank, or a transformer primary, it is almost certainly a Time-Delay fuse. If it protects a 24VDC control board, a PLC power supply, or solid-state relays, it is a Fast-Acting fuse.
3. Physical Dimensions Dictate the Standard: A 5x20mm fuse is an IEC standard size, ubiquitous in global electronics. A 1/4" x 1-1/4" (6.3x32mm) fuse is the ANSI/NEMA 3AG standard, common in US automotive and industrial panels. Do not force a 5x20mm fuse into a 3AG holder using foil shims; the contact resistance will cause the holder to melt at high loads.
4. The Multimeter Resistance Heuristic: If you have a known-good fuse of the same amperage but unknown speed, measure its DC resistance with a high-precision multimeter. A 1A Fast-Acting fuse typically reads a higher resistance (e.g., 0.4Ω) than a 1A Time-Delay fuse (e.g., 0.1Ω), because the time-delay requires a heavier, thicker internal mass to absorb thermal energy without clearing.