When you trace a fault on a control panel, the schematic is your map, and the protective devices are drawn with symbols that depend on where the drawing comes from. A European machine is usually documented with IEC 60617 symbols; a North American panel shop often uses symbols in the style of ANSI/IEEE 315 and the ladder-diagram conventions found in NFPA 79 drawings. This article describes the symbols for circuit breakers, disconnectors and fuses in both styles, and the reference letters that usually go with them.
Why the symbol matters
A circuit breaker, a disconnector and a switch can all be drawn as "a contact that opens", but they do very different jobs. A breaker trips automatically on overcurrent; a disconnector only isolates a circuit, usually with no load; a fuse protects once and must be replaced. Reading the symbol correctly tells you what you are looking for in the panel, which device to check after a trip, and what part number to look up when ordering a replacement. Always start with the drawing legend: many manufacturers and panel builders add their own variants.
IEC 60617 style
IEC drawings build symbols from a basic contact plus small qualifying marks placed at the fixed-contact end. The basic contact is a short line (the moving contact) drawn at an angle across a gap in the conductor.
- Circuit breaker: a normally open power contact with a small cross (x) at the fixed-contact end. The cross is the "circuit-breaker function" mark.
- Disconnector (isolator): the same contact with a short bar drawn across the line at the fixed-contact end, the "disconnector function" mark.
- Switch-disconnector: a contact that combines the disconnector mark with a mark for load-switching capability. Check the drawing legend for the exact form used.
- Contactor main contact: a contact with a small semicircle at the fixed-contact end.
- Fuse: a narrow rectangle with the conductor line running through it lengthwise.
- Protective releases: the trip functions of a breaker are often shown as small boxes linked to the contact by a dashed line (the mechanical link). An overcurrent release is commonly labeled with "I>", and a thermal element is drawn with a stepped or zig-zag mark representing the heating effect.
A three-pole breaker is drawn as three of these contacts side by side, joined by a dashed line to show they operate together. A motor-protective circuit breaker (manual motor starter) is often shown as a breaker contact combined with both a thermal and a magnetic release symbol.
ANSI/IEEE 315 and North American ladder style
North American schematics tend to use simpler pictorial symbols, drawn between two small terminal circles.
- Circuit breaker: two terminals bridged by a curved arc, like a shallow bow above the gap. A multi-pole breaker shows several arcs joined by a dashed line. A thermal-magnetic breaker may add a small symbol for the trip elements next to the arc.
- Disconnect switch: two terminals with a straight blade drawn at an angle, open, between them. Multi-pole switches show several blades joined by a dashed line to the operating handle.
- Fused disconnect: the disconnect blade drawn in series with a fuse symbol on each pole.
- Fuse: commonly a small rectangle in the conductor, or a sinuous S-shaped line between two terminals in older drawings.
- Overload relay heater: in motor ladder diagrams, the heater element in the power circuit is often drawn as a small loop or zig-zag, with the overload contact (OL) shown in the control circuit.
Side-by-side summary
| Device | IEC 60617 style | ANSI/IEEE 315 style | Common reference letters |
|---|---|---|---|
| Circuit breaker | Contact with a cross (x) at the fixed end | Curved arc between two terminals | Q (IEC), CB (North America) |
| Disconnector / isolator | Contact with a short bar at the fixed end | Angled open blade between two terminals | Q (IEC), DISC or DS (North America) |
| Switch-disconnector / fusible switch | Disconnector mark combined with a load-switching mark, or a disconnector with fuses | Blade switch in series with fuses | Q (IEC), DISC or FDS (North America) |
| Fuse | Rectangle with a line through it | Rectangle, or S-shaped line in older drawings | F (IEC), FU (North America) |
| Contactor main contact | Contact with a semicircle at the fixed end | Pair of parallel lines (normally open contact) tagged with the starter number | Q or K (IEC), M or C (North America) |
| Overload relay | Box with a thermal mark, linked to its contact | Heater loop in the power circuit, OL contact in the control circuit | F or B (IEC), OL (North America) |
For guidance only: symbol shapes and letter codes vary with the edition of the standard and the drawing office. Always check the legend of the drawing you are working from.
IEC and ANSI symbols for common protective and switching devices.
Reference letters (device tags)
Next to each symbol you will find a tag that identifies the device in the panel. In IEC documentation, letter codes come from IEC 81346-2 (or the older IEC 60750 system on legacy drawings), so the same letter can mean slightly different things on an old drawing and on a recent one. In North America, tags such as CB, FU, DISC, M, CR and OL are common on machine drawings. The tag is usually followed by a number and sometimes by a location code, for example a cabinet and rail position, which helps you find the device quickly.
Tips for reading protective devices on a schematic
- Find the legend first. It shows the exact symbols and tags used on that drawing set.
- Follow the dashed lines. They show which contacts and releases belong to the same device.
- Check the device list. Many drawing sets include a bill of materials that links each tag to a manufacturer and catalog number. That catalog number, not the symbol, is what you need to order a replacement.
- Look for ratings next to the symbol. Drawings often note the current rating, curve, fuse class or setting beside the device.
- Compare with the panel. Labels on the panel should match the tags on the drawing; if they do not, the drawing may be out of date.
Work on electrical equipment must be done by qualified personnel following the applicable electrical code and the manufacturer's instructions.
To learn more about the devices behind these symbols, read Circuit Breakers Explained: MCB vs MCCB vs ICCB/ACB and Disconnect Switches: UL 98 vs UL 508.
Key takeaways
- IEC 60617 symbols use qualifying marks at the fixed contact: a cross for a circuit breaker, a bar for a disconnector, a semicircle for a contactor.
- North American drawings often show a breaker as a curved arc and a disconnect as an angled blade.
- Fuses are commonly drawn as a rectangle with a line through it.
- Reference letters (Q, F, CB, FU, DISC...) depend on the standard and the age of the drawing.
- The legend and the bill of materials, not the symbol alone, give you the part to order.
Related parts at 3JIndustry
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