When reading a schematic or panel schedule, identifying the exact state of a disconnect is critical for safety and troubleshooting. The symbol of switch in off position varies depending on whether you are looking at a North American NEMA ladder diagram, an international IEC 60617 schematic, or a physical device label.
Master Reference Table: Switch 'Off' Symbols & States
The table below maps the most common switch types to their schematic representations and physical panel markings when in the 'off' (open circuit) state. Use this as your primary bench and jobsite reference.
| Switch Type | NEMA / ANSI (US) Schematic | IEC 60617 (Global) Schematic | Physical Panel Marking | 'Off' State Meaning in Practice |
|---|---|---|---|---|
| SPST (Normally Open) | Open gap with a hinged lever drawn away from the contact point. | Open gap with a straight diagonal line not touching the node. | 'O' or '0' (IEC 60417-5008) | Circuit is broken; no current flows to the load. |
| SPST (Normally Closed) | Closed gap with a slash through the lever, or lever resting on contact. | Diagonal line bridging the gap, often with a small perpendicular tick. | 'O' (when manually actuated to open) | Unactuated = current flows. Manually forcing it 'Off' opens the gap. |
| Multi-Position Selector (Rotary) | Wiper arm pointing to a blank terminal or a literal '0' node. | Wiper arm pointing to an unconnected node or '0'. | 'O', '0', or 'OFF' text | Wiper bridges no circuits; all control logic is de-energized. |
| Fused Disconnect (Knife Blade) | Open knife blade symbol drawn physically separated from the jaw. | Open rectangle with a clear gap and an 'O' indicator. | Red 'O', 'OFF', or green 'I' (region dependent) | Physical air gap established; mains power isolated from the panel. |
| Limit Switch (Mechanical) | NO contact drawn open; NC contact drawn closed (resting state). | Similar to SPST, but with a roller/lever actuator symbol attached. | N/A (Internal to machine) | Machine guard is closed or carriage is at rest; circuit state depends on NO/NC wiring. |
Regional Standards: Which Applies to Your Region?
The way a switch 'off' state is drawn on paper—and stamped on the physical bezel—depends heavily on the governing electrical standard in your region. Assuming a single standard is universal is a fast track to miswiring a control cabinet.
North America (NEMA, NFPA 79, ANSI Y32.2)
In the US and Canada, industrial schematics largely follow NFPA 79 and NEMA ladder logic conventions. The 'off' position for a standard switch is represented by an open gap in the line. For physical disconnects, you will often see the word 'OFF' spelled out, or a red 'O'. However, older US panels sometimes use a green indicator light to denote 'safe/off' and red for 'danger/on', which directly contradicts modern IEC color coding.
International / Europe (IEC 60617 & IEC 60417)
The global standard relies on binary logic symbols defined in IEC graphical symbol databases. The symbol for 'off' on a physical switch is a circle ('O'), representing binary 0 (IEC 60417-5008). The symbol for 'on' is a vertical line ('I'), representing binary 1 (IEC 60417-5007). On schematics, the 'off' state is simply the unactuated, open state of a Normally Open (NO) contact, drawn with a distinct gap.
Legacy UK (BS 3939)
If you are troubleshooting older machinery in the UK (pre-1990s), you may encounter BS 3939 symbols. While largely superseded by BS EN 60617, BS 3939 used distinct, sometimes cluttered graphical representations for switchgear. The 'off' position on rotary switches was often denoted by a thick black line pointing to a blank space, rather than the modern '0'. Always trace the wires physically if you suspect a legacy BS 3939 diagram has been partially updated over the decades.
Rows People Get Wrong & Interpreting Faded Markings
The 'Normally Closed' (NC) Confusion
The most frequently misinterpreted row in the table above is the SPST Normally Closed switch. Novices often assume 'NC' means the switch is in the 'Off' position. This is dangerously incorrect. 'Normally' refers to the unactuated, resting state of the switch. Therefore, a Normally Closed switch is actually ON (conducting current) when left alone. To put an NC switch into the 'off' position, you must physically actuate it (flip it, press it, or pull it). On a schematic, if you see an NC contact symbol, assume that wire is live until the switch is actively engaged.
The Multi-Position 'Zero' vs 'Letter O'
On rotary selector switches, people frequently confuse the number zero ('0') with the letter 'O'. In IEC standards, it is meant to be the letter 'O' (for Off/Open), but many manufacturers stamp a perfect zero. In practice, they mean the exact same thing: the wiper is bridging no contacts. However, if you see a '1', '2', or '3', those are 'On' positions routing power to specific circuits.
Safe Interpretation of Faded or Missing Markings
In harsh industrial environments, UV exposure, oil, and abrasion will obliterate the 'O' and 'I' stamps on a disconnect switch. When you cannot read the symbol of the switch in off position, follow this strict verification protocol:
- Lockout/Tagout (LOTO): Follow OSHA hazardous energy control procedures to secure the upstream breaker.
- Prove the Meter: Test your CAT III or CAT IV multimeter on a known live source to verify the leads and fuse are intact.
- Test Line-to-Line and Line-to-Ground: With the switch in the suspected 'off' position, measure across the load-side terminals. You must read < 1V AC/DC. Then measure from each load terminal to a known earth ground.
- Continuity Check (De-energized only): If the circuit is proven dead, use the multimeter's continuity setting across the switch terminals to physically verify the internal air gap is open.
Frequently Asked Questions
What does the circle symbol mean on a power switch?
The circle ('O') on a physical power switch is the international IEC 60417-5008 symbol for 'Off'. It represents the binary number 0, indicating an open circuit where no power is flowing to the device. It is universally paired with the vertical line ('I'), which represents binary 1 or 'On'.
How is a normally open switch drawn in the off position?
A Normally Open (NO) switch in its 'off' (unactuated) position is drawn with a visible gap between the stationary contact and the moving lever. In NEMA ladder diagrams, this looks like a gap with a hinged line pointing away. In IEC schematics, it is a diagonal line that does not touch the intersection node. Because it is 'Normally Open', its resting state is inherently 'Off'.
Is the symbol for a switch in the off position the same on a schematic and the physical device?
No. The physical device uses standardized binary stamps ('O' for off, 'I' for on) per IEC 60417. The schematic drawing uses geometric line-and-node representations (open gaps, wiper positions) per IEC 60617 or ANSI Y32.2. You will rarely see an 'O' stamped on a schematic diagram unless it is specifically labeling a rotary switch's terminal map.
What does a slashed circle or 'X' mean on an industrial disconnect?
If you see an 'X' or a slashed circle on a heavy industrial fused disconnect or motor starter, it typically indicates a tripped state or a fault condition, rather than a standard manual 'Off' position. However, on some legacy European rotary cam switches, an 'X' was used to denote a specific cross-connected routing position, not 'Off'. Always consult the specific manufacturer's wiring diagram (usually found on the inside of the panel door) to confirm the exact switch matrix.
