The standard temperature switch symbol depicts a set of electrical contacts actuated by a thermal element, drawn as a semicircle with a straight line (thermometer bulb) or a bimetallic curve intersecting the mechanical linkage. Whether you are troubleshooting an HVAC control board, repairing a commercial oven, or designing a motor protection circuit, correctly identifying the specific type of thermal switch on your schematic is critical for both functionality and fire safety.
Temperature Switch Symbol Reference Table
Below is the complete reference matrix for thermal actuation symbols found on modern electrical schematics. The geometry of the actuator line and the contact state at room temperature dictate how the device behaves in practice.
| Symbol Name | IEC 60617 Graphic Geometry | Typical Application | State at Room Temp (20°C) | Reset Type |
|---|---|---|---|---|
| Thermostat (NO) | Normally Open (NO) contacts with a thermometer bulb (circle and line) crossing the actuator dash. | Cooling fan triggers, heater enable circuits. | Open (No continuity) | Automatic |
| Thermostat (NC) | Normally Closed (NC) contacts with a thermometer bulb crossing the actuator dash. | High-limit safety cutoffs, boiler over-temp protection. | Closed (Continuity) | Automatic |
| Thermal Cutoff (One-Shot) | Rectangle with a diagonal line and a fuse-like link, often annotated with a specific trip temp (e.g., 125°C). | Transformer fire protection, appliance internal fusing. | Closed (Continuity) | None (Must replace) |
| Capillary Tube Switch | Standard switch symbol, but the thermal bulb is connected via a dashed or wavy line to the main switch body. | Oven thermostats, water heater remote sensing. | Varies (NO or NC) | Automatic |
| Bimetallic Overload Relay | Heater element (zigzag or box) mechanically linked via a dashed line to an NC contact. | Motor starter overload protection (e.g., NEMA size starters). | Closed (Continuity) | Manual or Auto |
Rows People Get Wrong
- Confusing the Thermal Cutoff with an Auto-Reset Thermostat: The one-shot thermal cutoff symbol looks deceptively similar to a standard fuse or an NC thermostat. If you replace a blown one-shot cutoff (which physically melts an internal pellet or link) with an auto-resetting bimetallic switch, you remove the ultimate fire-safety fail-safe. Always check the annotation for a specific fixed temperature rating (like 133°C) and a lack of a reset mechanism.
- Misreading the Capillary Dashed Line: The dashed line connecting a remote bulb to a capillary switch is frequently mistaken for a pneumatic control line, a ground wire, or a shielded signal cable. In reality, it represents the physical copper tubing filled with volatile fluid that expands to push the mechanical contacts.
- Overload Heater vs. Thermal Switch: The bimetallic overload relay symbol includes a heater element in series with the motor load. The thermal switch does not carry the main load current; it only monitors ambient or surface heat. Mixing these up leads to incorrect wiring of motor control centers.
Regional Standards & Faded Marking Protocols
Schematic symbols are not universally identical. The governing standard depends on the origin and age of the equipment you are servicing. According to the IEC Systems Management Bureau and North American standard bodies, here is how regional variants differ.
- IEC 60617 (Global, EU, Modern UK): Uses the distinct thermometer bulb graphic (a small circle with a straight line bisecting it) placed against the mechanical latch. This is the most common standard on modern imported appliances and industrial PLC schematics.
- ANSI/IEEE 315 & NEMA (North America): Often uses a simpler semicircle to denote thermal actuation, or relies heavily on device function numbers. For example, a temperature switch might be labeled 26TS (Apparatus Thermal Device) or 49TS (Machine Thermal Device) rather than drawing the full graphic. You will see this on older US-made HVAC and industrial panels.
- Old UK (BS 3939): Largely obsolete, but you will encounter it in 1970s and 1980s British machinery. It used a distinct cross-hatched thermal element graphic. If you are retrofitting legacy UK gear, treat these cross-hatched symbols as standard thermostats but verify the physical device, as BS 3939 did not always clearly differentiate between auto-reset and one-shot in the basic symbol.
Safe Interpretation When Markings Are Faded or Missing
On jobsites, schematics are often degraded by heat, oil, or UV exposure. If the symbol is unreadable, you must identify the switch physically and electrically before applying power.
- Count the Terminals: Two terminals usually indicate a simple NO/NC switch or a one-shot fuse. Three terminals indicate a changeover switch (Common, NO, NC).
- Look for the Reset Button: A physical reset button (often red or blue) confirms an auto-resetting bimetallic or manual-reset thermal overload. A smooth, sealed cylindrical body with fiberglass sleeving is almost certainly a one-shot thermal cutoff.
- Perform a Cold Continuity Test: With the circuit de-energized and isolated, use a multimeter to check continuity across the terminals at room temperature. If it beeps, it is Normally Closed (NC). If it reads open (OL), it is Normally Open (NO).
- Trace the Capillary: If you see a copper tube leading to the switch body, it is a capillary thermostat. Do not kink or cut this tube, or the volatile charge will escape, ruining the switch.
Never bypass a blown thermal cutoff with a jumper wire to 'test' if the rest of the circuit works. Thermal cutoffs are installed inside transformer windings, motor housings, and appliance heating chambers specifically to prevent catastrophic fires when cooling fails. If a thermal cutoff is open, replace it with an identical part number matching both the temperature rating (e.g., 125°C) and the current/voltage rating (e.g., 15A / 250V). Always de-energize, lock out/tag out, and verify dead with a tested meter before probing thermal devices in mains-powered equipment. Consult the NFPA 70 (NEC) and local AHJ requirements for permanent thermal protection modifications.
Frequently Asked Questions
How do I wire a 3-terminal temperature switch symbol on a schematic?
A 3-terminal temperature switch is a Single Pole Double Throw (SPDT) changeover device. The terminals are typically labeled C (Common), NO (Normally Open), and NC (Normally Closed). To wire it, your incoming control voltage or signal connects to the C terminal. If you want the circuit to activate when the temperature rises above the setpoint (e.g., turning on a cooling fan), wire the load to the NO terminal. If you want the circuit to shut down when the temperature rises (e.g., a safety interlock for a heater), wire the load to the NC terminal. Never wire power to both NO and NC simultaneously, and never use the NO and NC terminals as a simple pass-through without utilizing the Common terminal.
What is the difference between a temperature switch and a temperature sensor symbol?
This is a critical distinction in control schematics. A temperature switch (thermostat) is a binary device; its symbol shows physical electrical contacts that snap open or closed at a specific threshold. It directly switches a load or a control relay. A temperature sensor (like an RTD, thermocouple, or NTC thermistor) is an analog device. Its symbol is drawn as a resistor with a temperature coefficient arrow or a specific probe graphic, and it does not have moving contacts. Sensors output a variable resistance or millivolt signal that must be read by a PLC, microcontroller, or analog transmitter. If your schematic shows a resistor symbol connected to an analog input card, you are dealing with a sensor, not a switch.
Can I replace a normally closed thermal cutoff with a standard bimetallic thermostat?
No. While both may be Normally Closed (NC) at room temperature and open when hot, their failure modes and design intents are entirely different. A standard bimetallic thermostat is designed to cycle thousands of times and will automatically reset when the temperature drops. A thermal cutoff is a sacrificial, one-shot safety device designed to permanently sever the circuit if a primary cooling system fails or a contactor welds shut. If you replace a one-shot cutoff with an auto-resetting bimetallic switch, the equipment will repeatedly overheat, cool down, and restart (short-cycling), which can lead to insulation breakdown, melted wiring, and eventual electrical fire. Always replace a thermal cutoff with the exact OEM-specified one-shot fuse.
