Temperature Switch Symbol
Definition: The Temperature Switch symbol represents a thermally-actuated switching device that opens or closes an electrical circuit when a preset temperature threshold is reached, using a bimetallic element, thermocouple, or thermistor as the sensing element; it is standardised in IEC 60617-07 (switching devices) with the addition of a temperature-indication qualifier, and designated TSW or TS in industrial schematic practice.
Also known as: thermal switch, thermostat switch, TSW, bimetallic switch, temperature-controlled switch, thermal cutout.
What the Temperature Switch symbol means
The Temperature Switch symbol denotes a switching element whose open/closed state is determined by the temperature of the monitored medium rather than by a manual or electrical control signal. When the sensed temperature crosses the setpoint, the switch contacts change state — either opening to cut power (for protective thermal cutouts) or closing to initiate a heating or cooling action (for control thermostats).
In circuit diagrams, the temperature switch symbol (labelled TSW or TS) appears in motor protection circuits, heating element control, HVAC safety cutoffs, battery charger thermal protection, and industrial process control. The symbol indicates that the circuit behaviour is temperature-dependent; the setpoint temperature, contact type (normally open or normally closed), and hysteresis are typically noted as annotations on the symbol.
How to identify the Temperature Switch symbol
The Temperature Switch symbol is drawn as a standard switch symbol (a movable contact line at a pivot) with a temperature-control qualifier — typically a small thermometer symbol, the letter 'θ' (theta), or the symbol for temperature variation drawn adjacent to or inside the switch body. A vertical line (In terminal) enters from the top and exits from the bottom (Out terminal). Some notations add 'NO' or 'NC' to indicate the normal state of the contacts. The IEC 60617 representation adds a qualifying box with a temperature indicator over the standard switch arc symbol.
Function in a circuit
A temperature switch continuously monitors a temperature via its sensing element (bimetallic strip, gas-filled actuator, NTC thermistor, or thermocouple). When temperature rises above (or falls below) the setpoint, the mechanism actuates the contacts — either snapping open (to disconnect power in a protective cutout) or closing (to energise a load in a control application). Many temperature switches have fixed setpoints; adjustable versions allow the user to set the trip temperature via a dial or screw.
Standards: IEC vs ANSI
| IEC 60617 | IEC 60617-07 defines switching device symbols; temperature-sensitive switches use the standard switch glyph with a thermometric qualifying symbol (a thermometer or θ indicator). IEC 60730-1 governs automatic electrical controls including temperature-actuated switches used in household and similar appliances. |
|---|---|
| ANSI/IEEE 315 | ANSI Y32.2 / IEEE 315 represents a temperature switch as a standard switch symbol with a thermal-actuator indicator (small θ or 'T' inside a circle) adjacent to the contact arc. IEC and ANSI symbols are functionally identical in construction; only the qualifier notation style may differ. |
| Key difference | IEC 60617-07 and ANSI Y32.2 both use the switch arc/contact symbol with a temperature-actuator qualifier. The IEC qualifier is typically a thermometer symbol or θ (theta); ANSI practice sometimes uses a small 'T' in a circle. In both cases, 'NO' (normally open at ambient) or 'NC' (normally closed at ambient) is annotated to clarify the default contact state. |
Terminals / pins
| Pin | Name |
|---|---|
| in | In |
| out | Out |
Typical values
Setpoint temperature range: −40 °C to +300 °C (product-dependent). Common fixed setpoints: 60 °C, 85 °C, 100 °C, 125 °C, 150 °C. Hysteresis: 5 °C to 30 °C (differential between trip and reset temperature). Contact rating: 1 A to 16 A at 250 V AC (typical). Sensing element types: bimetallic disc, capillary bulb, gas-actuated, NTC thermistor. Designator: TSW or TS.
Where the Temperature Switch symbol is used
- Motor winding protection — thermally-actuated cutout embedded in motor windings to trip if winding temperature exceeds rated limit
- Domestic appliance thermal cutouts in kettles, toasters, and hair dryers preventing overheating damage
- HVAC system high-limit switches shutting off the heat source if plenum temperature exceeds a safe limit
- Battery charger thermal protection preventing over-temperature charging of lithium-ion or lead-acid cells
- Electronic enclosure thermal fans switching cooling fans on when internal temperature rises above a setpoint
- Industrial furnace and oven control using temperature switches for upper and lower limit safety functions
- Transformer thermal protection sensing winding temperature and tripping the primary contactor at critical levels
Example
In a single-phase motor wiring diagram, a Temperature Switch (NC, setpoint 130 °C) symbol appears in series with the motor overload relay coil circuit. The switch's In terminal connects to the control circuit supply and the Out terminal connects to the relay coil. At normal operating temperature, the NC contacts keep the relay energised and the motor running; if motor winding temperature reaches 130 °C, the temperature switch opens, the relay de-energises, and the motor contactor opens to prevent thermal damage.
Key facts
- A Temperature Switch is a thermally-actuated device that opens or closes its contacts when a preset temperature threshold is crossed, using a bimetallic element, gas-actuated actuator, or electronic temperature sensor.
- The symbol is drawn as a standard switch arc/contact with a thermometer or theta (θ) qualifier symbol, per IEC 60617-07 and ANSI Y32.2 / IEEE 315; designator is TSW or TS.
- Temperature switches are either normally open (NO) at ambient — closing when temperature rises — or normally closed (NC) at ambient — opening when temperature rises (protective cutout).
- The two terminals are In (top) and Out (bottom), representing the series circuit path that is broken or made when the temperature threshold is crossed.
- Hysteresis (the difference between the trip temperature and the reset temperature) is an important parameter: a 10 °C hysteresis means a switch tripping at 100 °C will not reset until the temperature drops to 90 °C, preventing rapid cycling.
- IEC 60730-1 governs automatic electrical controls for household use including thermally-actuated switches; IEC 60947-5-1 covers industrial control switches including thermally-actuated types.
- Fixed-setpoint bimetallic disc temperature switches (klixon-type or snap-disc) are the most common form in appliance and motor protection applications; they are non-adjustable and must match the thermal class of the protected winding.
Frequently asked questions
What does the temperature switch symbol mean in a circuit diagram?
The Temperature Switch symbol represents a switch whose contacts open or close based on sensed temperature. When temperature reaches the preset setpoint, the switch actuates — either opening to cut power (protective cutout) or closing to start a heating or cooling device. It is drawn as a switch arc with a thermometer or theta (θ) qualifier symbol.
What does the temperature switch symbol look like?
The temperature switch symbol is a standard switch contact arc with a thermometer symbol or the Greek letter theta (θ) drawn adjacent to or inside the switch body, indicating thermal actuation. The symbol has two terminals (In at the top, Out at the bottom) and may be annotated 'NO' (normally open) or 'NC' (normally closed) to indicate the contact state at ambient temperature.
What is the difference between a temperature switch and a thermostat?
The terms are often used interchangeably, but a temperature switch typically refers to a fixed-setpoint or simple adjustable on/off switching device (like a bimetallic cutout or snap-disc), while a thermostat more commonly refers to a temperature-sensing and control device that maintains a set temperature range (with a feedback control function). In schematic symbols, both use the same temperature-switch symbol with thermal qualifier.
What standard defines the temperature switch symbol?
The temperature switch symbol follows IEC 60617-07 (switching devices with thermal qualifier) and ANSI Y32.2 / IEEE 315. Product safety standards for thermally-actuated switches in appliances are covered by IEC 60730-1 (automatic electrical controls) and IEC 60947-5-1 (industrial control devices).
What is NC and NO for a temperature switch?
NC (normally closed) means the switch contacts are closed at normal ambient temperature and open when the temperature rises to the trip setpoint — used for protective cutouts that disconnect power when overheating occurs. NO (normally open) means the contacts are open at ambient and close when temperature rises — used for control switches that activate a load (fan, heater, alarm) when a threshold is reached.
What is the designator letter for a temperature switch?
The designator for a temperature switch is TSW or TS in industrial schematic practice. In some conventions, it may be annotated as SW with a thermal qualifier, or as part of a protective relay designation. The setpoint temperature and contact type (NO/NC) are typically annotated next to the designator on the schematic.
What is hysteresis in a temperature switch?
Hysteresis is the difference between the trip temperature (where the switch opens or closes) and the reset temperature (where it returns to its normal state). For example, a switch with a 100 °C trip point and 10 °C hysteresis resets at 90 °C. Hysteresis prevents rapid on-off cycling when the temperature hovers near the setpoint, protecting both the switch contacts and the connected load from excessive switching.
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