Diagram of a Switch
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A reference guide to switch diagrams covering SPST, SPDT, DPST, and DPDT configurations, circuit symbols, contact arrangements, and correct wiring methods for electrical and electronic applications.
A switch is a device that opens or closes an electrical circuit by making or breaking contact between conductors. Switch diagrams use standardised circuit symbols to represent the type of switching action, the number of poles, and the number of throw positions.
The two key parameters that define a switch are poles and throws. Poles refers to the number of separate circuits the switch controls simultaneously. Throws refers to the number of output positions each pole can connect to.
A single-pole single-throw (SPST) switch is the simplest type—one circuit, one on/off action. It appears in circuit diagrams as a single contact with an actuator arm. When closed, the circuit is complete; when open, the circuit is broken. Standard domestic light switches (one-way) are SPST.
A single-pole double-throw (SPDT) switch connects one common terminal to either of two output terminals. SPDT switches appear in circuit diagrams with a common (C or COM) terminal and two output positions (typically labelled NC for normally closed and NO for normally open). SPDT switches are used in changeover and selector circuits, including two-way light switching when wired in combination with a second SPDT.
A double-pole single-throw (DPST) switch operates two separate circuits simultaneously with one actuator. Mains isolators often use DPST to simultaneously break both the line and neutral conductors.
A double-pole double-throw (DPDT) switch is effectively two SPDT switches mechanically ganged. It can reverse motor direction, switch between two sources, or implement break-before-make sequencing.
Switch contact ratings specify maximum continuous current (A) and maximum voltage. Always verify that the switch contacts are rated for the actual load type—resistive, inductive, or motor loads—as inductive loads produce arc-inducing voltage spikes at switch-off that reduce contact life if the switch is under-rated.
A three-heat stove switch (also called a multi-heat or selector switch) controls the power delivered to a heating element by routing current through different combinations of two resistive coil sections wired in series, parallel, or independently, giving typically three distinct heat levels (low, medium, high). Understanding the circuit diagram of this switch is essential for replacing a faulty unit or diagnosing an element that operates at only one heat setting. The switch has four or more terminals, and the diagram shows which terminals are bridged at each rotary position. You can draw and annotate this type of rotary switch circuit free in the browser at Circuit Diagram Maker.
How to wire diagram of switch
- Identify the switch type required Determine the number of circuits to be controlled (poles) and the number of positions required (throws). A simple on/off single-circuit application needs SPST. Changeover or two-way control needs SPDT. Simultaneous dual-circuit switching needs DPST or DPDT.
- Check the switch rating against the load Identify the load voltage and current. For AC mains loads, confirm the switch is rated for AC at the mains voltage (e.g. 250 V AC). For inductive loads (motors, relays), ensure the switch has an appropriate AC1 or AC3 rating per IEC 60947-5-1. For DC switching, verify DC voltage rating separately—switch DC ratings are typically lower than AC ratings for the same contact.
- Identify switch terminals Examine the switch body for terminal markings. SPST: two terminals (input and output). SPDT: three terminals (C/COM, NC, NO). DPDT: six terminals (two sets of C, NC, NO). If unmarked, use a multimeter in continuity or resistance mode to map which terminals are connected in each switch position.
- Isolate the circuit before wiring Switch off and verify dead at the circuit protective device using an approved voltage indicator. Apply lockout/tagout if required. Do not rely on the switch itself to isolate the circuit—some circuit topologies leave conductors live even when the switch is open.
- Connect and secure conductors Route conductors to the correct terminals per the wiring diagram. Ensure correct conductor cross-section for the current rating. Secure all terminals to manufacturer's torque specification. No bare conductor should be exposed outside the terminal.
- Test operation before covering or closing enclosure Restore power carefully and test the switch operation through all positions. Verify correct function—circuit on and off at the correct switch positions—before fitting covers or completing installation.
Specifications
| Typical mains switch voltage rating | 250 V AC (single-phase) or 440 V AC (three-phase) |
|---|---|
| Typical domestic light switch current rating | 6 A or 10 A |
| Switch configurations | SPST, SPDT, DPST, DPDT (and multi-pole variants) |
| Contact types | Normally open (NO), normally closed (NC), changeover (C/O) |
| IEC load category (AC switching) | AC1 (resistive), AC2, AC3 (motor switching) per IEC 60947-5-1 |
| Mechanical endurance (typical quality switch) | 10 000–100 000 operating cycles minimum |
Safety warnings
- Never work on a switch circuit connected to mains voltage without first isolating the supply at the circuit breaker and verifying dead with an approved voltage indicator. The line conductor may remain live at switch terminals even when the switch is open.
- Switches used on AC mains circuits must be rated for the mains voltage and continuous load current. Do not substitute a low-voltage or low-current rated switch in a mains circuit—contact failure under load can cause arcing, fire, or electric shock.
- In mains wiring, the line (live) conductor should be switched, never the neutral only. Switching the neutral alone leaves the load terminals at mains potential and creates a serious shock hazard during lamp replacement or load maintenance.
- Comply with applicable wiring regulations (NEC/NFPA 70, BS 7671, AS/NZS 3000, or IEC 60364) for all fixed switch installations. Only competent, licensed persons should carry out fixed electrical installation work.
Tools needed
- Digital multimeter (continuity and voltage functions)
- Approved voltage indicator (two-pole tester)
- Insulated screwdrivers (slotted and Philips)
- Wire strippers and cutters
- Cable fish tape or draw wire (for in-wall installations)
Common mistakes
- Wiring the switch in the neutral conductor instead of the line conductor in a mains circuit, leaving the load terminals live when the switch is off and creating a shock hazard.
- Using a switch rated only for DC in an AC mains circuit, where AC arc interruption requirements are fundamentally different and a DC-only rated switch may fail to break the arc.
- Confusing NC and NO terminals on an SPDT switch, resulting in inverted logic (circuit on when switch is off, and vice versa).
- Over-tightening screw terminals, crushing or shearing conductor strands and reducing current-carrying capacity.
- Failing to check the switch rating against an inductive load—a motor or relay coil can generate transient voltages several times the supply voltage at switch-off, which will pit and erode contacts rated only for resistive loads.
Troubleshooting
- Switch operates but circuit does not respond
- Cause: Wiring is connected to incorrect terminals (e.g. NC instead of NO, or input and output transposed), or there is a broken conductor in the circuit. Fix: Isolate the circuit. Use a multimeter to trace continuity from power source through the switch terminals to the load. Verify that the correct pair of terminals changes from open to closed when the switch is actuated.
- Switch sparks or arcs visibly when operated
- Cause: The switch contact rating is too low for the load current or the load is inductive, generating high-voltage transients at switch-off that exceed the contact's arc-interruption capability. Fix: Replace with a switch rated for the actual load type and current. For inductive loads, add a snubber circuit (RC network across the load) to reduce transient voltages. For DC inductive loads, a freewheeling diode across the load is effective.
- Switch feels hot to the touch during operation
- Cause: Contact resistance is too high due to worn, pitted, or oxidised contacts, or the switch is carrying a current close to or above its rated maximum, causing resistive heating. Fix: Measure voltage drop across the switch contacts while carrying load current. A voltage drop above a few tens of millivolts indicates high contact resistance. If contacts are worn, replace the switch. If the switch is undersized for the load, replace with an appropriately rated unit.
Frequently asked questions
What is the difference between SPST, SPDT, DPST, and DPDT switches?
SPST (single-pole single-throw): one circuit, on/off only. SPDT (single-pole double-throw): one circuit, selects between two output positions. DPST (double-pole single-throw): two separate circuits switched simultaneously, both on or both off. DPDT (double-pole double-throw): two circuits each with two throw positions, switched simultaneously.
What do NC and NO mean on a switch diagram?
NC stands for Normally Closed—the contact is closed (circuit complete) when the switch is in its rest or un-actuated state. NO stands for Normally Open—the contact is open (circuit broken) in the rest state. These terms apply to SPDT, DPDT, and relay contacts and define the default state before actuation.
How is a two-way light switch wired using SPDT switches?
Two SPDT switches are connected with their common terminals going to the line supply and the load respectively. The two output terminals of the first switch (the strappers) connect to the two output terminals of the second switch. Toggling either switch changes the circuit state, allowing the light to be controlled from two locations independently.
What is a momentary switch versus a latching switch?
A momentary switch only maintains its actuated state while physically held down (like a doorbell button). It returns to its default position when released. A latching (or maintained) switch holds its position after actuation until deliberately operated again, like a standard light switch or a key switch.
How do I read a switch symbol on a circuit diagram?
The standard switch symbol shows a contact (a small circle at each terminal end) with an angled actuator line between them. An open switch has the actuator line at an angle, leaving a gap. A closed switch shows the line bridging both contacts. For SPDT, the symbol shows one common contact with the actuator pointing toward one of two possible output contacts.
What does the circuit diagram of a three-heat stove switch look like?
A three-heat stove switch typically has four terminals: a common supply terminal and three output terminals connected to different tap points on a two-part heating element. At the low-heat position the two element sections are wired in series, giving the highest total resistance and lowest power; at medium heat only one section is energised; at high heat both sections are wired in parallel, giving maximum power. The rotary switch mechanism bridges the appropriate terminals at each click position, and the circuit diagram shows this as a set of contact paths radiating from the common terminal to the element taps. Some designs use five terminals to accommodate both an indicator lamp and the element.
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