The standard power switch symbol on electrical schematics is an open gap with a hinged lever (for maintained switches) or a pushbutton actuator (for momentary switches). On physical equipment panels, the universal IEC 60417 standard dictates using "I" for closed (ON) and "O" for open (OFF). Below is the complete reference for both schematic drafting and physical panel identification.
The Master Power Switch Symbol Reference Table
This table bridges the gap between the schematic symbols you see in wiring diagrams and the physical markings you interact with on the jobsite or workbench. Ampacity figures assume standard 60°C copper column ratings for residential/light commercial branch circuits.
| Switch Type | Schematic Symbol Description | IEC Physical Marking | NEC/NEMA Panel Marking | Contact State at Rest | Typical Max Ampacity |
|---|---|---|---|---|---|
| SPST (Single Pole, Single Throw) | Single line broken by a hinged lever angled upward. | I / O | ON / OFF | Open (OFF) | 15A - 20A |
| SPDT (Single Pole, Double Throw) | Hinged lever pivoting between two separate contact points (Common, NO, NC). | I / O / II (or 1/0/2) | ON1 / OFF / ON2 | Position dependent | 15A - 20A |
| DPST (Double Pole, Single Throw) | Two SPST symbols linked by a dashed mechanical line. | I / O | ON / OFF | Open (OFF) | 30A (240V) |
| DPDT (Double Pole, Double Throw) | Two SPDT symbols linked by a dashed mechanical line. | I / O / II | ON1 / OFF / ON2 | Position dependent | 30A (240V) |
| Momentary NO (Normally Open) | Open gap with a pushbutton actuator and a return spring symbol. | I (push) / O (rest) | START / OFF | Open (OFF) | 10A (Control) |
| Momentary NC (Normally Closed) | Closed gap with a pushbutton actuator and a return spring symbol. | O (push) / I (rest) | STOP / ON | Closed (ON) | 10A (Control) |
| Disconnect / Isolator | Hinged lever with a horizontal bar across the blade (indicating lockout capability). | I / O | ON / OFF | Open (OFF) | 60A - 400A+ |
Regional Standards: IEC vs. NEC/NEMA vs. Old UK
While the schematic symbols remain relatively consistent globally (governed largely by IEC 60617 and IEEE 315), the physical markings on switch housings and panel faces vary by region and standard body. Misinterpreting these can lead to dangerous operational errors.
IEC 60417 (International / EU / Modern Global)
The International Electrotechnical Commission standardizes the binary-derived "I" and "O" markings. The "I" represents a binary 1 (circuit closed, power flowing), and the "O" represents a binary 0 (circuit open, power stopped). This is the most common marking on modern consumer electronics, DIN-rail components, and international industrial machinery. As documented in the ISO 7000 / IEC 60417 graphical symbols database, these symbols are designed to transcend language barriers.
NEC / NEMA (North America)
In the US and Canada, the National Electrical Code (NEC) and NEMA standards often require explicit text or highly visible color coding for industrial disconnects. While "I" and "O" are accepted, NEMA-heavy industrial panels frequently use spelled-out "ON" and "OFF" or "START" and "STOP" to eliminate any ambiguity for operators. For standard residential wall switches, the NEC (NFPA 70) dictates that the switch must be installed so that the "up" position is ON and the "down" position is OFF, relying on physical orientation rather than printed symbols. Refer to the NFPA 70 NEC guidelines for specific mounting and identification requirements.
Old UK / BS Standards (Legacy)
Older British Standard (BS) installations and legacy UK equipment sometimes used a solid line (|) for ON and a circle (O) for OFF, or simply the numbers "1" and "0". If you are retrofitting a panel in an older UK facility, you may also encounter red/green indicator lights where, counterintuitively to modern traffic-light logic, Red sometimes meant ON (energized/danger) and Green meant OFF (safe/de-energized). Always verify legacy panels with a meter before assuming the color logic.
Never guess the state of a switch if the physical "I/O" or "ON/OFF" markings are faded, painted over, or missing. Arc flash and electrocution risks are severe. Treat the switch as energized. De-energize the upstream breaker, apply Lockout/Tagout (LOTO), and use a Category III or IV multimeter to verify the absence of voltage on the load side before touching any terminals.
Rows People Get Wrong (And How to Fix Them)
When reading schematics or wiring physical switches, hobbyists and junior technicians frequently make a few specific errors. Here is how to avoid them.
Confusing SPDT with DPST
This is the most common schematic reading error. An SPDT (Single Pole, Double Throw) switch has three terminals: Common, Normally Open, and Normally Closed. It routes one incoming line to one of two outgoing paths. A DPST (Double Pole, Single Throw) switch has four terminals and acts on two completely separate circuits simultaneously (like breaking both the Line and Neutral of a 240V circuit, or both poles of a split-phase system).
The Fix: Look for the dashed mechanical linkage line in the schematic. If the dashed line connects two separate hinge points to a single actuator, it is a multi-pole switch (DPST). If there is only one hinge point throwing between two contacts, it is an SPDT. I once saw a junior tech wire a 240V DPST disconnect as if it were an SPDT transfer switch because they misread the linkage line; the result was a dead short across phases.
Misinterpreting the "O" as the Letter O
People unfamiliar with the IEC 60417 standard sometimes read the "O" as the letter "O" for "On". This is backwards. The "O" is the number zero, meaning the circuit is broken (zero continuity). The "I" is the number one, meaning the circuit is complete.
Momentary vs. Maintained (Latching) Pushbuttons
In control circuits, confusing a momentary pushbutton with a maintained switch will cause your circuit to fail or behave erratically. A momentary switch (like a doorbell or a motor START button) only closes the circuit while physical pressure is applied. A maintained switch stays in its last position until manually toggled again.
The Fix: On a schematic, look for the return spring symbol (a small zigzag line adjacent to the actuator). If the spring is present, it is momentary. If you are wiring a physical panel and cannot read the schematic, use your multimeter's continuity mode. Press the button: if it beeps only while held down, it is momentary. If it beeps and continues to beep after you release it, it is maintained.
Safe Interpretation When Markings Are Faded
When physical panel markings are illegible, you must rely on empirical measurement rather than visual assumptions. Follow this exact verification sequence:
- De-energize: Turn off the upstream breaker. If the switch is a main disconnect, coordinate with the utility or upstream feed.
- Test the Meter: Verify your multimeter works on a known live source or continuity loop before testing the target circuit.
- Measure Continuity: With power OFF, place probes across the line and load terminals of the switch. Toggle the switch. The position that reads < 1 ohm is the closed (ON / "I") state. The position that reads > 20 MΩ (megohms) or "OL" (Over Limit) is the open (OFF / "O") state.
- Relabel: Once verified, use an industrial label maker or paint pen to permanently re-mark the panel with IEC or NEMA standard symbols before re-energizing.
