A thermal switch symbol represents a temperature-actuated contact that opens or closes a circuit based on ambient heat or load current. Because thermal protection is critical in motor controls, HVAC, and power supplies, misreading these symbols can lead to unprotected circuits or nuisance tripping. Below is the definitive reference for identifying these components across major global standards.

The Complete Thermal Switch & Overload Symbol Reference

Use this table to match the symbol on your schematic to the physical component in your panel. Note that the exact graphical representation varies slightly by CAD software (AutoCAD Electrical vs. EPLAN), but the functional identifiers remain constant.

Component Type IEC 60617 Symbol / Identifier ANSI/IEEE Y32.2 & NFPA 79 Symbol Practical Application & Typical Part
Thermostat (Temp-Actuated Switch) Standard switch contact with a bimetallic hook or the letters 't°' or 'θ' (theta) adjacent. Switch contact with a semi-circle loop or 't°' indicator. HVAC limit switches, appliance temp control. Example: KSD301 bimetallic disc thermostat.
Thermal Overload Relay (Motor) Heater element (rectangle or zig-zag) mechanically linked (dashed line) to a trip contact. Heater element (zig-zag) with a mechanical linkage line to the normally closed (NC) overload contact. 3-phase motor protection. Example: Allen-Bradley 193-E series or Schneider TeSys LRD.
Thermal Cutoff (One-Time Fuse) Standard fuse symbol (rectangle with internal line) marked with 't°'. Fuse symbol with a thermal loop or 't°' adjacent to the element. Non-resettable protection in power supplies or coffee makers. Example: Littlefuse SMD thermal fuses.
Self-Resetting Thermal Protector Switch contact with 't°' and a manual/automatic reset indicator (often a small 'R' or spring symbol). Switch contact with thermal loop and a designated reset coil or bimetallic return arrow. Compressor motor windings, transformer secondary protection. Example: Texas Instruments 2MP series.

Regional Variants: IEC vs. ANSI vs. Old UK

When troubleshooting legacy panels or importing machinery, you will encounter regional drafting differences. Knowing which standard applies to your region prevents dangerous misinterpretations.

  • IEC 60617 (International / Europe / Modern Global): The dominant modern standard. IEC relies heavily on functional identifiers rather than drawing the physical mechanism. You will almost always see the Greek letter theta (θ) or t° next to a standard switch symbol. The physical mechanism (bimetallic strip vs. liquid expansion) is rarely drawn in IEC schematics; the focus is purely on the logical function.
  • ANSI/IEEE Y32.2 & NFPA 79 (US / North America): US standards, heavily referenced in NFPA 79 (Electrical Standard for Industrial Machinery), prefer showing the physical actuation method. You will see the "heater element" (drawn as a resistor zig-zag) physically linked via a dashed line to the switch contact it trips. This is crucial for US motor control circuits where the heater is in the power circuit, but the tripped contact is in the 120V control circuit.
  • Old UK (BS 3939): Largely superseded by IEC 60617, but you will still find BS 3939 symbols in British industrial plants built before the 1990s. Thermal switches were often depicted as a standard switch with a capital 'T' enclosed in a circle, or a bimetallic strip drawn as a straight line with a distinct kink in the middle.
SAFETY WARNING: Never assume a thermal switch is functioning correctly based solely on the schematic symbol. Always de-energize the panel, apply lockout/tagout (LOTO), and verify dead with a CAT III or CAT IV multimeter before testing continuity across thermal contacts. Local electrical codes may require a licensed electrician for motor control panel modifications.

Rows People Get Wrong (and How to Avoid Miswiring)

Even experienced bench technicians and electricians trip up on specific thermal symbol nuances. Here are the most common schematic-to-reality translation errors.

1. The "One-Shot" vs. "Resettable" Trap

The most dangerous mistake is confusing a thermal cutoff (fuse) with a thermostat (switch). On a schematic, both may feature the 't°' identifier. However, the thermal cutoff symbol will always incorporate the standard fuse box (a rectangle with a solid line through it). If you replace a blown thermal cutoff with a resettable KSD301 bimetallic switch because you misread the symbol, the equipment will lose its ultimate fail-safe, creating a severe fire hazard. Always check the BOM (Bill of Materials) for part numbers like SEFUSE (cutoff) vs. KSD (switch).

2. The Heater Element vs. The Actuated Contact

In ANSI/NEMA motor control schematics, the thermal overload relay is split into two symbols: the heater (in the 3-phase power lines) and the NC trip contact (in the control circuit). Beginners often read the heater symbol as a standard power resistor and try to wire it into the control logic. The heater is a low-resistance, high-current element (often < 1 ohm) meant to carry full motor FLA (Full Load Amps). The dashed mechanical linkage line on the drawing is your clue that these two symbols represent a single physical component.

3. Safe Interpretation When Markings are Faded

Physical components degrade. If the silkscreen on a motor overload or the stamping on a thermostat is rubbed off, do not guess based on the schematic alone.

  • Bimetallic Switches (Resettable): Usually feature a visible metal can (like the KSD301) with a distinct bulge on one side where the disc lives, and two spade terminals.
  • Thermal Cutoffs (One-time): Typically encased in a smooth, cylindrical resin or plastic body with axial wire leads. They lack the mechanical "snap" feel of a bimetallic disc when pressed.
If in doubt, isolate the component and apply controlled heat (using a heat gun at a safe distance) while monitoring continuity. A resettable switch will click open and close as it cools; a cutoff will open permanently.

Frequently Asked Questions

What does the thermal switch symbol look like on a multimeter or schematic?

On a schematic, the symbol dictates the logical state. A "normally closed" (NC) thermal switch (common in motor overloads) is drawn with the contact touching, meaning it passes current under normal temperatures. When testing with a multimeter in continuity or resistance mode, a healthy NC thermal switch at room temperature should read < 1 ohm across its terminals. If the schematic shows a "normally open" (NO) symbol (common in cooling fan thermostats), the multimeter should read OL (Open Loop) until the component reaches its trip threshold.

How do I test a thermal overload switch if the symbol is faded on the casing?

If the physical markings are gone, you must verify the trip curve electrically. First, isolate the component. Use your multimeter to confirm it is currently closed (continuity). Next, use a thermocouple and a heat gun. Slowly apply heat to the component body. For a standard 85°C KSD301, the multimeter should snap to OL right around 85°C (± 5°C). When you remove the heat and the body cools to roughly 70°C, you should hear an audible mechanical "click" and the multimeter should return to < 1 ohm. If it does not reset, you are dealing with a one-time thermal cutoff that has already blown internally, or a failed bimetallic disc.

Is a thermal cutoff symbol the same as a thermal switch symbol?

No, and treating them as identical is a critical safety failure. While both react to heat, a thermal switch (thermostat) is designed to cycle thousands of times, opening and closing as temperatures fluctuate. Its symbol features standard switch contacts. A thermal cutoff (often called a thermal fuse) contains a specialized alloy pellet that melts at a precise temperature, severing the connection permanently to prevent catastrophic thermal runaway. Its symbol always integrates the standard IEC/ANSI fuse rectangle. Never bridge a blown thermal cutoff with a wire, and never substitute it with a resettable switch without explicit manufacturer authorization and a complete review of the protection scheme.