The symbol of selector switch components on an electrical schematic represents a rotary actuator that mechanically opens and closes specific contact blocks based on its knob position. Unlike a simple pushbutton, the schematic symbol must convey both the rotary positions and the exact contact sequence (truth table) to ensure the control circuit operates safely across all modes. Below is the complete reference for reading these symbols, interpreting their truth tables, and identifying regional standard variations.
Selector Switch Symbol & Contact Sequence Reference
On a schematic, the graphical symbol for a selector switch typically features a circle or dial representing the rotary knob, with a dashed or solid line indicating the actuator shaft connecting to multiple contact points. However, the graphical symbol alone is rarely enough. Engineers rely on a Contact Sequence Table (often called a truth table) placed adjacent to the symbol to define exactly which terminals close in which positions.
The table below details the contact sequence for two of the most common industrial selector switches: a 3-position Hand-Off-Auto (H-O-A) switch and a 4-position 3-phase Voltmeter selector. These assume standard 10A, 600V pilot-duty contact blocks (such as the Eaton 10250T or Siemens 3SU1 series).
| Switch Type & Position | Knob Angle | Contacts 1-2 | Contacts 3-4 | Contacts 5-6 | Practical Application |
|---|---|---|---|---|---|
| H-O-A: Hand | Left (-45°) | X (Closed) | - (Open) | - (Open) | Manual motor starter override |
| H-O-A: Off | Center (0°) | - (Open) | - (Open) | - (Open) | Safe isolation / E-Stop reset |
| H-O-A: Auto | Right (+45°) | - (Open) | X (Closed) | - (Open) | PLC / Float switch control |
| Voltmeter: Off | 0° | - | - | - | Meter isolated from bus |
| Voltmeter: V12 | 90° | X | X | - | Reads Phase 1 to Phase 2 |
| Voltmeter: V23 | 180° | - | X | X | Reads Phase 2 to Phase 3 |
| Voltmeter: V31 | 270° | X | - | X | Reads Phase 3 to Phase 1 |
What Each Row Means in Practice
In the table above, an 'X' indicates that the specific contact block is closed (conducting), allowing current to flow between those terminal numbers. A '-' indicates the contact is open. For the H-O-A switch, the 'Hand' position closes contacts 1-2, which typically bypasses a PLC output to directly energize a motor starter coil. The 'Auto' position closes 3-4, routing power through a relay or sensor contact instead.
For the voltmeter selector, notice that in positions V12, V23, and V31, two contact blocks close simultaneously. This routes two different phase legs to the single-phase voltmeter. If the switch mechanism fails and crosses these contacts incorrectly, it will result in a dead-bolt phase-to-phase short circuit, destroying the switch and potentially causing an arc flash.
Regional Standards: IEC, NEMA, and Legacy Markings
The way a selector switch is drawn on a schematic depends entirely on the regional standard governing the facility. Misinterpreting an IEC symbol as a NEMA symbol can lead to reversed logic in critical safety interlocks.
| Standard | Region | Symbol Style | Contact Numbering | Common Hardware |
|---|---|---|---|---|
| IEC 60617 | Global, EU, Modern UK/AU | Abstract functional blocks; rotary dial with position lines. | Strict 2-digit system (e.g., 13-14 for NO, 21-22 for NC). | Siemens 3SU1, Schneider Harmony XB5 |
| ANSI/IEEE 315 (NEMA) | US, Canada | Physical representation; circle with radiating lines for contacts. | Sequential or functional (e.g., 1-2, 3-4) based on physical stacking. | Eaton 10250T, Allen-Bradley 800T |
| BS 3939 (Legacy) | Older UK installations | Graphical lines mimicking physical knife-switch mechanics. | Alphanumeric, often non-standardized across manufacturers. | Legacy MEM, older Crabtree panels |
Which Standard Applies to You?
If you are working on a panel built in North America, or dealing with UL-listed equipment from manufacturers like Eaton or Allen-Bradley, you will use the NEMA/ANSI standard. The contacts are numbered sequentially based on how the physical blocks are stacked on the back of the switch (e.g., the first block is 1-2, the second is 3-4).
If you are in Europe, or working with modern IEC-compliant machinery globally, you must follow IEC 60617. IEC uses a strict digit system where the first digit indicates the sequence number and the second digit indicates the function (e.g., 13 and 14 always denote a Normally Open contact; 21 and 22 denote Normally Closed). For more on general schematic symbols, refer to comprehensive guides like those on Electrical Engineering Portal.
The 'Rows People Get Wrong' & Faded Marking Protocols
When troubleshooting control panels, the physical markings on the selector switch cam or the schematic truth table are often misread. Here are the specific rows and notations that cause the most field errors.
1. Misinterpreting Spring-Return Arrows
Many selector switches are 'maintained' (they stay where you turn them). However, safety-critical switches often feature a spring-return mechanism from one specific position. On a schematic, this is denoted by a small arrow pointing toward the center position next to the specific angle row. The Mistake: Technicians often wire the 'Auto' circuit to a spring-return position, assuming it will maintain state. If the switch springs back to 'Off' the moment the operator lets go, the machine will immediately shut down. Always check the physical switch bezel for the small arrow symbol indicating spring-return.
2. Make-Before-Break vs. Break-Before-Make
Standard truth tables (like Table 1) show steady-state positions. They do not show the transition between positions. The Mistake: When replacing a voltmeter selector switch, using a 'Make-Before-Break' (shorting) type instead of a 'Break-Before-Make' (non-shorting) type. A shorting switch momentarily connects Phase 1, Phase 2, and Phase 3 together during rotation. While this prevents the voltmeter needle from dropping to zero during switching, it will trip upstream breakers or cause an arc flash if the switch contacts are rated below the available fault current. Always verify the datasheet for 'non-shorting' (break-before-make) cams when working on 3-phase busbars.
3. Safe Interpretation of Faded or Missing Markings
In older facilities, the physical position plate on a selector switch may be faded, painted over, or missing entirely. Guessing the position based on the knob angle is dangerous, as internal cams can be customized or replaced.
The Correct Tracing Protocol:
- Isolate and Verify: Shut off the main disconnect and test for absence of voltage at the switch terminals.
- Disconnect Wires: Remove at least one wire from each contact block you intend to test. If you leave wires connected, your multimeter will read 'closed' through the parallel paths of the PLC inputs or starter coils, giving you a false truth table.
- Map the Cam: Set your multimeter to continuity (audible beep). Place probes on terminals 1 and 2. Rotate the switch through all physical detents. Note every position where the meter beeps.
- Repeat and Document: Move to 3-4, then 5-6. Draw a fresh truth table on masking tape and apply it to the inside of the panel door for the next technician.
By treating the physical continuity test as the ultimate source of truth—rather than relying on faded bezel markings or assumed standard configurations—you eliminate the risk of cross-wiring critical interlocks. Whether you are reading an IEC 60617 schematic or tracing a legacy NEMA panel, the combination of the graphical symbol and a verified contact sequence table is the only safe way to interpret a selector switch.
