The thermal overload switch symbol represents a protective device that opens a control circuit when heat from excessive current exceeds a calibrated threshold. In North American ANSI/IEEE 315 schematics, it typically appears as a box containing a heater element (zig-zag line) mechanically linked to a set of normally closed (NC) contacts. In international IEC 60617 drawings, it is denoted by the letter code 'F' (or 'FR') alongside a thermal relay graphic. Understanding these symbols is critical because confusing the power-circuit heater element with the control-circuit switch contact is one of the most common wiring errors in motor control and HVAC installations.
Thermal Overload Symbol Reference Table
The table below maps the physical device to its schematic representation across the major global standards. Use this as your primary lookup when reading a new print or designing a control panel.
| Device Type | IEC 60617 Symbol / Letter Code | ANSI/IEEE 315 Symbol | NEMA Designation | Practical Application & Reset Type |
|---|---|---|---|---|
| Thermal Overload Relay | Letter 'F' or 'FR'. Graphic: Rectangle with thermal wave and NC contact. | Heater element (zig-zag) mechanically linked (dashed line) to an NC switch. | OL | 3-phase motor protection. Manual or auto-reset via dial on physical unit (e.g., Schneider TeSys LRD). |
| Bimetallic Thermal Switch | Letter 'S' or 'B'. Graphic: Switch with a bimetallic strip indicator. | Switch symbol with a 'U' shaped bimetallic element and temperature setting arrow. | TS | HVAC compressors, appliance heating elements. Auto-reset when temperature drops. |
| Thermal Fuse (One-Shot) | Letter 'F'. Graphic: Standard fuse symbol with a solid dot or thermal wave overlay. | Standard fuse graphic with a thermal dot and a line striking through the reset capability. | TF | Coffee makers, space heaters, transformer secondaries. Non-resettable; must be replaced. |
| Solid-State Overload | Letter 'F'. Graphic: Rectangle with semiconductor symbols and thermal wave. | Box with internal logic block symbols linked to an output relay contact. | SSOL | Precision motor protection, VFD bypass lines. Features phase-loss and jam protection. |
Regional Standards and Schematic Variants
Schematic symbols are not universal. The standard your diagram follows dictates how the thermal overload switch symbol is drawn and, more importantly, how the terminals are numbered on the physical component.
IEC 60617 (International / Europe / Modern Global)
IEC schematics rely heavily on alphanumeric designations rather than purely descriptive graphics. A thermal overload relay is designated with the letter F (for protective devices). The physical terminals follow a strict numbering convention: power in is L1/L2/L3, power out is T1/T2/T3. The control contacts are universally 95 and 96 for the Normally Closed (NC) overload trip contact, and 97 and 98 for the Normally Open (NO) trip indicator contact. If you see '95/96' on a faded print, you are looking at the thermal overload switch.
ANSI/IEEE 315 & NEMA (North America)
North American prints, governed by IEEE Std 315 and NEMA ICS standards, favor descriptive graphics. The thermal overload switch symbol explicitly draws the heater element (the part that carries the motor's full load current) and uses a dashed mechanical line to show it actuating the NC switch (the part that breaks the contactor coil circuit). NEMA physical devices, like the Eaton C306 series, often use terminal numbers 1 and 2 for the NC contact, rather than the IEC 95/96 standard.
Legacy UK (BS 3939)
On older British schematics (pre-IEC harmonization), thermal overloads were sometimes denoted by the letter 'O/L' inside a circle, with graphical elements that look like a hybrid of ANSI and modern IEC. If you are retrofitting a panel in an older UK facility, expect to see non-standard terminal numbering and verify every connection with a multimeter before energizing.
Rows People Get Wrong & Faded Print Troubleshooting
Even experienced technicians misinterpret specific aspects of the thermal overload switch symbol. Here are the most common pitfalls and how to resolve them on the bench or jobsite.
Mistake 1: Wiring the Control Contact into the Power Circuit
In the ANSI/IEEE symbol, the heater element and the NC switch are drawn right next to each other. The heater element (zig-zag) is rated for the motor's full load amperage (FLA) and must be wired in series with the motor phases. The NC switch contact is only rated for low control current (usually 10A max at 120VAC) and must be wired in series with the contactor coil. Wiring the NC control contact into the 480V motor power circuit will result in an immediate, violent arc flash and destroyed contacts when the motor starts.
Mistake 2: Confusing Thermal Switches with Thermal Fuses
Look closely at the graphic overlay. A bimetallic thermal switch symbol implies a resetting mechanism—the contacts will close again once the metal cools. A thermal fuse symbol indicates a one-shot meltable link. If you replace a blown thermal fuse with an auto-resetting bimetallic switch, you risk a fire hazard, as the switch will continuously cycle and eventually weld its contacts closed during a sustained fault.
Interpreting Faded or Missing Schematics
When the print on an HVAC control board or an old lathe is sun-faded or covered in oil, you cannot rely on the printed symbol. Use this physical verification sequence:
- Locate the physical component: Look for a block mounted directly below the motor contactor (IEC style, like a Schneider TeSys block) or a standalone ceramic disc (appliance style).
- Check the terminal stamps: If the control terminals are stamped '95' and '96', it is an IEC thermal overload relay. If it is a small disc with two spade connectors and a temperature stamped on it (e.g., '130°C'), it is a bimetallic thermal switch.
- Verify with a multimeter: De-energize the panel. Set your meter to continuity. Place probes across the suspected NC control terminals. You should read less than 1 ohm. If the motor has tripped the overload, you will read open (OL). Press the physical 'Reset' button on the device; the meter should beep, confirming you have identified the correct thermal overload switch.
Frequently Asked Questions
What is the difference between a thermal overload relay symbol and a circuit breaker symbol?
A circuit breaker symbol (usually designated 'Q' or 'CB') represents a device designed to protect the wiring from short circuits and massive overloads using both thermal and magnetic trip mechanisms. It trips almost instantly on a short circuit. A thermal overload relay symbol ('F' or 'OL') represents a device designed exclusively to protect the motor windings from sustained, moderate overcurrent (running overload). It has no magnetic trip and is intentionally slow-acting to allow for the high inrush current when a motor starts.
How do I identify a thermal overload switch on a faded HVAC wiring diagram?
On HVAC schematics, thermal overload switches (often called internal motor protectors or line-break overloads) are usually drawn inside the motor compressor symbol itself, or immediately adjacent to it. Look for a small switch symbol with a 'C' (Common) and 'S' (Start) or 'R' (Run) terminal designation nearby. Physically, these are often small, sealed metal discs inserted into the compressor terminal box. If the diagram is faded, trace the wire from the contactor's T1 terminal; the first protective device in that line is your thermal overload.
Why does my schematic show two thermal overload symbols for a single three-phase motor?
Seeing multiple thermal overload switch symbols for one motor usually indicates one of two scenarios. First, in older ANSI drawings, the draftsman may have drawn all three individual heater elements (L1, L2, L3) as separate symbols, all mechanically ganged to a single NC contact. Second, the motor may be a dual-voltage (e.g., 230V/460V) or part-winding start motor, which requires two separate overload blocks to protect the different winding configurations depending on the starting sequence. Always check the motor nameplate and the starter wiring diagram to confirm the configuration.
Can I bypass a thermal overload switch if the symbol indicates it is normally closed (NC)?
Never bypass or jumper a thermal overload NC contact. The 'Normally Closed' designation simply means the circuit is complete during normal operation, allowing the contactor coil to energize. If the motor overheats, the switch opens to drop out the contactor and save the motor from catching fire. Bypassing this switch defeats the primary protection mechanism. If the overload is tripping repeatedly, the root cause is either a mechanical bind in the driven load, a failing motor bearing, incorrect overload dial settings (check the FLA on the motor nameplate against the dial on the overload relay), or single-phasing on the supply line.
