The symbol of an overload relay is never just a single shape; it is a dual-part schematic representation consisting of a thermal heater element (placed in series with the motor power lines) and an auxiliary trip contact (placed in the control circuit). In the IEC 60617 standard, the power element is drawn as a rectangle with a diagonal line or a heater loop, while the control contact is drawn as a standard normally closed (NC) switch marked with terminals 95 and 96. If you are reading a North American NEMA schematic, you will instead see a thermal element squiggle and a contact labeled "OL". Understanding both halves of this symbol is the difference between a motor that safely trips during a jam and a control circuit that fails to drop out the contactor.
The Overload Relay Symbol and Terminal Reference Table
Before wiring a motor starter, you must map the schematic symbol to the physical terminals on the device. The table below breaks down the exact symbols, terminal designations, and practical functions for both global and North American standards.
| Function | IEC 60617 Symbol / Terminals | NEMA / US Standard Symbol / Terminals | Practical Meaning & Wiring Target |
|---|---|---|---|
| Power Input | L1, L2, L3 (Lines from contactor) | 1, 3, 5 or L1, L2, L3 | Receives 3-phase power directly from the downstream side of the main contactor. |
| Power Output | T1, T2, T3 (Lines to motor) | 2, 4, 6 or T1, T2, T3 | Feeds the motor. The internal bimetallic strips or current transformers sit between L and T. |
| Thermal Heater Element | Rectangle with diagonal line or loop inside the power lines. | Squiggly line or resistor symbol in series with power lines. | Represents the physical sensing mechanism. It does not have separate control wiring; it senses the L-to-T current. |
| NC Trip Contact | 95, 96 (NC switch symbol) | OL1, OL2 or 95, 96 (NC switch labeled "OL") | Critical Safety Path. Wired in series with the main contactor coil. Opens to drop out the contactor during an overload. |
| NO Fault Indicator | 97, 98 (NO switch symbol) | NO1, NO2 or 97, 98 (NO switch labeled "OL") | Closes only when tripped. Wired to a PLC input, SCADA alarm, or red indicator light to signal a fault. |
Regional Standards: IEC vs NEMA vs Legacy BS
The physical device on your DIN rail might be identical, but the schematic symbol and terminal numbering change depending on the engineering standard governing your region.
- IEC 60617 (Global, EU, Australia, Modern US): Dominates modern industrial design. Uses strict two-digit terminal numbering. The NC trip contact is always 95 and 96. The NO fault contact is always 97 and 98. The power terminals are L1/L2/L3 and T1/T2/T3.
- NEMA (North America Legacy): Often relies on functional lettering rather than strict numbering. The NC trip contact is frequently labeled OL (Overload) or OL1/OL2. The symbol itself usually features a more pronounced "heater" squiggle to emphasize the thermal nature of older eutectic-alloy melting relays.
- Old BS 3939 (UK Legacy): You will still find these on schematics for machinery built in the UK before the late 1990s. The thermal element symbol is similar to IEC, but auxiliary contacts were often numbered sequentially based on the contactor block (e.g., 3 and 4 for NC) rather than the dedicated 95/96 fault designation used today.
The Rows and Terminals People Get Wrong
When reading the symbol of overload relay schematics, even experienced technicians make specific wiring errors that compromise motor protection.
Mistake 1: Wiring Control Logic Through Power Terminals
The thermal element symbol sits directly on the power lines (L1/T1). A common error on single-phase 120V/240V motor circuits is trying to route the control circuit power through the L and T terminals to "save wire." The power terminals are rated for motor full-load amps (FLA) and high inrush currents, not low-current control logic. Always keep the control circuit isolated and wired strictly to the 95/96 auxiliary contacts.
Mistake 2: Using 97/98 for the Safety Stop Circuit
Terminals 97 and 98 (Normally Open) close when the relay trips. Some builders mistakenly wire the main contactor coil drop-out circuit through 97/98, thinking "it completes the circuit when there is a fault." This is backwards. The contactor coil must be wired through 95/96 (Normally Closed). When an overload occurs, 95/96 opens, breaking the coil circuit and dropping out the motor. 97/98 is strictly for sending a "Hey, I tripped" signal to a PLC or indicator light.
Mistake 3: Ignoring Single-Phase Derating
If you are protecting a single-phase motor, the schematic will show you looping the power wire through all three phases (L1 to T1, T1 jumpered to L2, L2 to T2, etc.). If you only pass the single-phase wire through L1 and T1, the internal phase-loss detection (on electronic relays) or the mechanical differential trip bar (on bimetallic relays) will interpret the missing current on poles 2 and 3 as a phase loss and trip immediately.
Decision Path: Selecting the Right Overload Relay
Do not just buy the cheapest relay that fits the contactor. Use this decision matrix to select the exact part number based on your motor's operational profile.
| Application Scenario | Required Technology | Trip Class | Concrete Part Recommendation |
|---|---|---|---|
| Standard 3-phase induction motor, indoor, infrequent starts (pumps, fans). | Bimetallic Thermal | Class 10A or 10 | Schneider Electric TeSys LRD (e.g., LRD10 for 4-6A). Reliable, cost-effective (~$45-$60), direct-mounts to LC1D contactors. |
| High-inertia loads, long start times, or high ambient temperature fluctuations. | Electronic / Solid-State | Class 10, 20, or 30 (Selectable) | Allen-Bradley 193-EIB or ABB EF series. Immune to ambient temperature changes, prevents nuisance trips during long startups (~$150-$250). |
| Frequent jogging, reversing, or high-vibration environments (crushers, hoists). | Electronic with Phase Monitors | Class 10 or 20 | Eaton PKZM0 (Motor Protection Circuit Breaker combining breaker and overload) or solid-state relay with built-in phase imbalance detection. |
The Default Pick: If you are building a standard industrial control panel for a 3-phase conveyor or pump and have no extreme environmental constraints, default to the Schneider TeSys LRD series matched to your contactor frame. It is the global industry standard, widely available, and the IEC 95/96 terminal layout is universally understood by maintenance teams.
Safe Interpretation When Markings Are Faded or Missing
In harsh environments—like wastewater treatment plants or lumber mills—the printed schematic and terminal numbers on the side of an overload relay will eventually fade, melt, or get painted over. If you cannot read the symbol or the 95/96 designation, never guess. Wiring the contactor coil to the wrong auxiliary block will result in a motor that refuses to stop during a mechanical jam, leading to a fire.
Follow this bench procedure to map an unmarked overload relay:
- Locate the Reset Button: Find the physical reset button (usually blue or red) and the "Test" button (usually red or black). Ensure the reset button is fully depressed and the relay is in the "Ready" state.
- Test for the NC Trip Contact (95/96): Set your multimeter to continuity (the diode/beep setting). Place probes across the auxiliary terminals in pairs. The pair that beeps continuously in the Ready state is your NC trip contact (95/96). This is the pair you must wire in series with your contactor coil.
- Simulate a Trip: Press and hold the mechanical "Test" button on the relay face. This physically forces the internal trip mechanism to actuate without needing to push 100A through the power poles.
- Verify the State Change: While holding the Test button, the pair that previously beeped (95/96) should now read open (OL). The other auxiliary pair (97/98) should now beep, confirming it is the NO fault indicator contact.
- Label Immediately: Use a silver paint pen or industrial Brady labels to mark "95/96" and "97/98" directly on the plastic housing before wiring.
By treating the symbol of an overload relay not just as a schematic drawing, but as a direct map to physical terminal behavior, you ensure that your motor protection circuits will operate exactly as engineered when a mechanical fault occurs.
