The thermal line break overload symbol represents a protective sensing element wired in series with a motor's power line, designed to open the control circuit when excess heat from overcurrent is detected. In North American NEMA schematics, it is typically drawn as a standard switch contact linked to a separate thermal heater coil in the power circuit. In global IEC schematics, it appears as an integrated rectangular block with a bimetallic curve indicator. Understanding these symbols is critical for troubleshooting motor starters, HVAC compressors, and industrial control panels without miswiring the control logic.
Standard Thermal Line Break & Overload Symbol Reference Table
Before wiring or troubleshooting a motor control circuit, identify which thermal protection variant you are looking at. The table below maps the most common thermal line break and overload symbols to their NEMA and IEC designations, detailing their contact types and typical applications.
| Symbol / Device Name | NEMA Designation (NFPA 79) | IEC Designation (IEC 60617) | Contact Type & Terminals | Typical Application |
|---|---|---|---|---|
| Thermal Overload Relay (Bimetallic) | Heater coil in power line; separate NC auxiliary contact in control line. | Rectangle with a bimetallic strip curve; integrated trip block. | NC (Normally Closed). IEC: 95-96. NEMA: OL or 95-96. | Standard 3-phase induction motors, conveyor drives, pumps. |
| Thermal Overload Relay (Eutectic Alloy) | Heater coil melting a solder pot; mechanical ratchet releases NC contact. | Rarely used in modern IEC; represented similarly to bimetallic with a melting drop symbol. | NC. Requires manual reset after cooling. Terminals vary by OEM. | High-inertia loads, heavy compressors where nuisance tripping must be avoided. |
| Line Break Thermal Cutout (Auto-Reset) | Single-pole switch with a thermal element box; no separate control contact. | Switch symbol with a thermal loop and a dashed auto-return line. | NC (in-line power break). Opens the main power line directly. | Single-phase fractional HP motors, HVAC blower motors, sump pumps. |
| Line Break Thermal Cutout (Manual-Reset) | Single-pole switch with thermal element and a manual reset lever indicator. | Switch symbol with thermal loop and a solid manual-reset mechanical line. | NC (in-line power break). Requires physical button press to close. | Garbage disposals, table saws, appliances where auto-restart is a safety hazard. |
| Solid-State / Electronic Overload | Current transformers (CTs) feeding an electronic block; outputs to NC relay. | Rectangle with a microchip/diode symbol inside; connected to CTs. | NC (95-96) and NO (97-98) programmable outputs. | VFD-fed motors, precision servo drives, critical process pumps. |
Regional Variants: NEMA vs. IEC vs. Legacy Standards
The way a thermal line break overload symbol is drawn depends entirely on the regional standard governing the schematic. Applying the wrong mental model to a drawing will lead to severe wiring errors, particularly regarding where the protective contact sits in the control circuit.
NEMA (North America / NFPA 79)
NEMA standards, heavily influenced by NFPA 79 for industrial machinery, treat the thermal overload as two distinct physical and symbolic components. The heater element (a zigzag or rectangular coil symbol) is drawn in the main power circuit (L1, L2, L3). The trip contact (a standard NC switch symbol) is drawn in the low-voltage control circuit. This reflects the physical reality of legacy NEMA motor starters (like the Eaton C306 series), where the heat from the power line physically actuates a separate mechanical switch in the control block.
IEC (Global / IEC 60617)
IEC schematics favor functional representation over physical layout. The modern IEC overload relay (such as the Schneider TeSys LRD series) is drawn as a single integrated block. The thermal sensing and the contact trip mechanism are combined into one symbol, usually placed directly below the main contactor coil. IEC also standardizes terminal numbering: the NC thermal trip contact is almost universally designated as 95 and 96, while the NO (Normally Open) fault indication contact is 97 and 98.
Legacy UK (BS 3939)
If you are troubleshooting equipment in older British facilities, you may encounter BS 3939 symbols. These are largely deprecated in favor of IEC 60617 but feature a distinct "crossed circle" or specific hatched thermal box that differs from both modern NEMA and IEC. Treat these as historical artifacts and trace the physical wires rather than trusting the faded ink.
The "Rows People Get Wrong" & Faded Marking Protocols
When reading schematics or looking at physical terminal blocks, misinterpreting the thermal line break symbol is the leading cause of contactor chatter and unprotected motors.
Mistake 1: Confusing the Heater Element with the Trip Contact
Beginners often see the NEMA heater coil symbol and assume it acts as a physical switch that breaks the main power line. It does not. The heater element only generates heat. The actual line-breaking action for 3-phase motors happens in the control circuit via the NC auxiliary contact (which drops power to the contactor coil). If you wire the heater in series with the control circuit, you will instantly vaporize the control wiring when the motor starts.
Mistake 2: Assuming Auto-Reset vs. Manual Reset
Look closely at the mechanical linkage line on the symbol. A dashed line intersecting the switch indicates an auto-reset (the bimetallic strip cools and snaps back). A solid line with a small perpendicular tick indicates a manual reset (a physical button must be pressed). Wiring a manual-reset relay into an automated PLC restart sequence will cause the system to hang indefinitely after a single thermal fault.
Safe Interpretation When Markings are Faded or Missing
On 20-year-old machine tools, the terminal stamps (95, 96, 97, 98) are often worn away or covered in grease. Do not guess. Use this bench protocol:
- Identify the Power Pass-Through: The top three terminals (usually L1, L2, L3 or 1, 3, 5) connect directly to the bottom three (T1, T2, T3 or 2, 4, 6). Set your DMM to resistance (Ω). You should read < 0.5 Ω across these pairs. These are the heater elements or CT pass-throughs.
- Locate the NC Control Contact: Switch the DMM to continuity/diode mode. Probe the smaller auxiliary terminals in pairs. The pair that beeps (reads near 0 Ω) is your NC thermal trip contact (95-96).
- Verify the Trip Mechanism: Use a small flathead screwdriver to press the mechanical "Trip" or "Test" button on the face of the relay. The continuity on the NC pair should immediately break (read OL / Open). If it doesn't, the internal linkage is fused or broken, and the relay must be replaced.
Sizing, Trip Classes, and Real-World Bench Verification
Recognizing the symbol is only half the battle; selecting the correct thermal element and trip class ensures the motor survives startup without nuisance tripping. Thermal overloads are categorized by their Trip Class, which defines how many seconds the relay will hold at 600% (6x) of the motor's Full Load Amps (FLA) before opening.
- Class 10 (Fast Trip): Trips within 10 seconds at 6x FLA. Used for submersible pumps, compressors, and motors that cannot tolerate long acceleration times without overheating.
- Class 20 (Standard): Trips within 20 seconds at 6x FLA. The default for 90% of general-purpose industrial motors (fans, blowers, standard conveyors).
- Class 30 (Slow Trip): Trips within 30 seconds at 6x FLA. Mandatory for high-inertia loads like large centrifuges, rock crushers, or heavy flywheels that require extended acceleration periods.
Bench Verification Example: Suppose you are commissioning a 5 HP, 230V 3-phase motor with an FLA of 15.2A. You select a Schneider TeSys LRD21 (Class 10A/20 selectable, range 12-18A). You set the dial to 15.2A. During startup, you use a clamp meter to measure the inrush current. If the motor draws 90A (approx. 6x FLA) for 4 seconds and settles to 14A, a Class 20 relay will hold perfectly. If you accidentally installed a Class 10 relay on a high-inertia load that takes 12 seconds to reach full speed, the thermal line break will trip prematurely. In that scenario, you must swap the physical overload block or adjust the solid-state relay's digital trip class parameter via its front-panel HMI.
