Thermal Overload Relay Symbol
Definition: The Thermal Overload Relay symbol represents a protective device—drawn as a set of heating elements (three rectangles or zigzag strips in series with the line conductors) paired with a normally-closed contact symbol—that trips and disconnects a motor circuit when sustained overcurrent causes excessive heat, as defined under IEC 60617 and ANSI/IEEE 315.
Also known as: thermal overload, overload relay, OL relay, motor overload, heater relay, bimetallic overload.
What the Thermal Overload Relay symbol means
The Thermal Overload Relay symbol denotes a current-sensing, thermally-actuated protective device inserted in series with a motor's supply leads. In schematic diagrams, the heater element portion appears in the power circuit while the associated normally-closed (NC) auxiliary contact appears in the control circuit; when the heater trips, the NC contact opens and de-energises the motor contactor coil.
In motor-control schematics, the thermal overload relay (designator OL or F, depending on regional convention) is the primary means of protecting three-phase motors against sustained overloads, phase-loss, and locked-rotor conditions. The device uses bimetallic strips or melting-alloy elements that respond to I²t energy, making it sensitive to the cumulative thermal effect of overcurrent rather than just its instantaneous magnitude.
How to identify the Thermal Overload Relay symbol
In IEC-style schematics the thermal overload relay heater is drawn as a small rectangle (or three small rectangles for three-phase, one per phase) placed in series on each line. The associated trip contact is shown as a normally-closed contact symbol—two vertical lines with a diagonal slash through the junction—in the control rung. In ANSI/IEEE schematics, the heater element may be drawn as a resistor zigzag symbol in the power path, also labelled OL, with the NC auxiliary contact in the control ladder.
Function in a circuit
The thermal overload relay monitors the RMS current through a motor's supply conductors. When current exceeds the relay's trip-current setting for a sustained period, the bimetallic strip bends (or the alloy melts) and mechanically actuates the trip mechanism, opening the NC auxiliary contact in the contactor control circuit and shutting down the motor. After cooling (manual or automatic reset), the contact resets, allowing restart. The device also provides phase-loss protection because loss of one phase causes asymmetric heating that trips the remaining elements.
Standards: IEC vs ANSI
| IEC 60617 | IEC 60617 (database symbol reference 06-06-07 and 06-06-08) shows heater elements as small filled rectangles in the line conductors; the associated NC contact follows IEC 60617-07 contact conventions. The relay as a device is further specified in IEC 60947-4-1 (low-voltage contactors and motor starters). |
|---|---|
| ANSI/IEEE 315 | ANSI/IEEE 315-1975 (IEEE Std 315) shows the thermal overload heater as a resistor-zigzag element in the power line, labelled OL. The trip contact follows the standard NC contact symbol. NEMA ICS standards govern the physical trip-class ratings (Class 10, 20, 30). |
| Key difference | IEC uses small rectangles for heater elements; ANSI uses the zigzag resistor form. Functionally identical. IEC labels the device F (Fehlerschutz / protective device) in some conventions; ANSI/NEMA labels it OL. |
Terminals / pins
| Pin | Name |
|---|---|
| in | In |
| out | Out |
Typical values
Trip-current set to 100–125 % of motor full-load current (FLC). Trip classes: Class 10 (trips in ≤10 s at 6× FLC), Class 20 (≤20 s), Class 30 (≤30 s). Reset modes: manual or automatic. Ambient compensation range typically −5 °C to +40 °C.
Where the Thermal Overload Relay symbol is used
- Three-phase induction motor direct-on-line (DOL) starter circuits to prevent overload burnout
- Star-delta motor starters where OL relays protect during both starting and running phases
- Pump motor control panels to guard against locked-rotor and dry-run conditions
- Conveyor and compressor motor control centres (MCCs) as the primary overload protection device
- HVAC fan and chiller motor circuits in commercial building automation panels
- IEC motor-starter combinations (contactor + OL relay) in industrial automation cabinets
Example
In a standard DOL motor-starter circuit, three thermal overload relay heater elements (one per phase) appear in series between the main contactor output terminals and the motor terminals T1, T2, T3; the normally-closed OL contact is wired in the control rung between the stop button and the contactor coil A1, so any thermal trip immediately de-energises the contactor and stops the motor.
Key facts
- The Thermal Overload Relay symbol consists of heater element symbols (IEC: rectangles; ANSI: zigzags) in the power circuit plus a normally-closed auxiliary contact in the control circuit.
- Standard designator for the thermal overload relay is OL (ANSI/NEMA) or F (IEC protective function); the heater elements in three-phase circuits are labelled OL1, OL2, OL3.
- IEC 60617 governs the schematic symbol; IEC 60947-4-1 governs the device's construction and trip-class performance requirements.
- Trip classes define the maximum trip time at 600 % FLC: Class 10 ≤10 s, Class 20 ≤20 s, Class 30 ≤30 s — selected to match motor starting characteristics.
- The thermal overload relay uses a bimetallic strip or melting-alloy (eutectic) element that responds to I²t (current squared × time), providing cumulative thermal protection rather than instantaneous overcurrent protection.
- Phase-loss protection is inherent: loss of one phase causes unbalanced heating in the remaining two heaters, tripping the relay before the motor overheats from single-phasing.
- Thermal overload relays must be reset (manually or automatically) after a trip; automatic-reset types can cause repeated restarts and motor damage if the root cause is not corrected.
- The OL relay is distinct from a fuse or circuit breaker — it protects the motor from sustained overloads, not short circuits; short-circuit protection is provided by a separate upstream fuse or breaker.
Diagrams that use this symbol
- dol starter diagram
- delta star connection diagram
- star delta circuit diagram
- star delta starter diagram
- star delta wiring diagram
- star delta control circuit diagram
- star delta control diagram
- star delta control wiring diagram
Frequently asked questions
What does the thermal overload relay symbol look like in a schematic?
The thermal overload relay symbol consists of two parts: heater elements in the power (main) circuit and a normally-closed contact in the control circuit. In IEC schematics, the heaters are small rectangles in series on each supply line; in ANSI schematics they appear as zigzag resistor symbols. The NC control contact is drawn as two parallel lines with a diagonal slash, labelled OL.
What does the thermal overload relay symbol mean?
The symbol means the circuit contains a thermally-actuated protective device that will disconnect the motor if sustained overcurrent causes the heater elements to overheat. It represents both the current-sensing heaters (power circuit) and the trip contact (control circuit) that opens to de-energise the motor contactor.
What is the difference between IEC and ANSI thermal overload relay symbols?
In IEC 60617 schematics, the heater elements are drawn as small filled rectangles in the line conductors. In ANSI/IEEE 315 schematics, the heater element is drawn as a zigzag (resistor-style) symbol. Both use the same normally-closed contact symbol for the trip contact in the control circuit. Functionally the symbols represent identical devices.
What designator letter is used for a thermal overload relay?
In ANSI/NEMA practice the designator is OL (overload relay). In IEC practice the protective device is often assigned the letter F, or OL is retained for clarity in motor-control schematics. Individual heater elements in three-phase circuits are labelled OL1, OL2, and OL3.
What is the difference between a thermal overload relay and a circuit breaker?
A thermal overload relay protects the motor from sustained overcurrents (overloads) by responding to cumulative I²t heating; it does not interrupt short circuits. A circuit breaker provides short-circuit and sometimes overload protection by magnetically tripping on fault currents. In a DOL starter, both devices are used: the circuit breaker upstream for fault protection and the thermal overload relay for motor overload protection.
What trip class should I select for a thermal overload relay?
Trip class is selected to match the motor's starting characteristics. Class 10 (trips in ≤10 s at 600 % FLC) suits normal loads with short start times. Class 20 suits motors with moderate starting loads. Class 30 suits high-inertia loads such as large fans and centrifuges that need longer start times. The motor's service factor and starting current duration determine the correct class.
How many terminals does a thermal overload relay have?
A three-phase thermal overload relay has six main power terminals (three-in, three-out, labelled 1/L1, 3/L2, 5/L3 in and 2/T1, 4/T2, 6/T3 out) plus auxiliary contact terminals for the NC trip contact (95–96) and optionally an NO alarm contact (97–98), following IEC terminal numbering conventions.
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