The overload relay symbol on an electrical schematic tells you exactly how the motor protection device interrupts the control circuit during a phase loss or overcurrent event. Whether you are reading a NEMA (North American) or IEC (International) ladder diagram, misinterpreting these symbols leads to tripped breakers, contactor welding, or unprotected motors. Below is the definitive reference for decoding these symbols and mapping them to physical terminals.
Overload Relay Symbol Reference Chart (IEC vs NEMA)
Read this table by matching the schematic symbol you see on your print to the physical terminal numbers on your device. IEC standards (IEC 60617 / IEC 81346) dominate global and modern US panels, while NEMA (NEMA ICS 19) persists in older North American industrial installations.
| Function / Component | IEC Symbol Description | NEMA Symbol Description | Standard Terminal Pins | Physical Equivalent Example |
|---|---|---|---|---|
| Thermal Heater Element (Power Circuit) | Rectangle with a diagonal line through it, or a bimetallic squiggle. Often grouped in threes for L1, L2, L3. | Rectangle with a horizontal line through it, or a simple zig-zag resistor symbol labeled "OL". | L1-T1, L2-T2, L3-T3 (Power path) | Schneider TeSys LRD, Eaton PKZM0 |
| Magnetic / Solid-State Trip (Power Circuit) | Rectangle with "μP" (microprocessor) or a solenoid coil symbol with a dashpot indicator. | Box labeled "SSOL" or a coil symbol with an instantaneous trip hash mark. | L1-T1, L2-T2, L3-T3 (Power path) | Allen-Bradley 193-EIO, ABB AF |
| NC Trip Contact (Control Circuit) | Standard NC contact symbol with a dashed mechanical link line pointing back to the heater elements. | Standard NC switch symbol, often drawn inline with the contactor coil, labeled "OL". | 95 - 96 (Breaks coil circuit) | Universal across IEC/NEMA direct-on-line starters |
| NO Fault Contact (Control Circuit) | Standard NO contact symbol with a dashed mechanical link line pointing back to the heater elements. | Standard NO switch symbol, labeled "OL", routing to a pilot light or PLC. | 97 - 98 (Closes on trip) | Universal across IEC/NEMA direct-on-line starters |
| Manual/Auto Reset | Not typically drawn in the schematic; represented by a physical switch symbol on the device face in P&IDs. | Rarely shown on ladder diagrams; assumed manual unless a solenoid reset coil is drawn. | N/A (Physical dial/button) | Blue (Reset) / Red (Test) buttons |
Rows People Get Wrong (And How to Avoid Miswiring)
The most common bench and jobsite mistake is confusing the auxiliary contacts of the contactor with the auxiliary contacts of the overload relay.
Under the IEC 60445 terminal marking standard, the 9x decade (95-98) is reserved exclusively for overload relays. The contactor’s auxiliary contacts use the 1x and 2x decades (e.g., 13-14 for NO, 21-22 for NC). If you wire a PLC fault input to terminal 13-14 thinking it’s the overload fault, your PLC will read the contactor’s running state, not the overload’s tripped state. Always verify you are landing your wires on the 95-96 (Normally Closed, wired in series with the contactor coil) and 97-98 (Normally Open, wired to the fault indicator or PLC input) terminals.
Another frequent error involves the mechanical link symbol (the dashed line in IEC schematics). Junior techs often assume this dashed line means the contacts are electrically tied to the power phase. They are not. The dashed line purely indicates a mechanical relationship: when the bimetallic strip inside the heater element bends from excess heat, it physically pushes a plunger that opens the 95-96 contact. The power circuit (millivolts to 600V AC) and the control circuit (24V DC to 240V AC) remain galvanically isolated.
Reading Faded or Missing Nameplate Symbols Safely
UV exposure, heat, and oil mist destroy the schematic stickers on overload relays over a 10-year lifespan. If you are troubleshooting a legacy panel with a faded Square D, Siemens, or generic import relay and cannot read the symbol or pinout, follow this bench procedure:
- Drop the Relay: Remove the overload relay from the contactor. If you try to beep out the 95-96 terminals while the relay is mated to the contactor, your multimeter will read the low DC resistance of the contactor’s A1-A2 coil, giving you a false reading.
- Set to Continuity: Put your multimeter in continuity/diode mode.
- Find the NC (95-96): Probe the bottom auxiliary terminals. You are looking for the pair that reads < 1 ohm (beeps) while the relay is in the untripped, reset state. This is your 95-96. Press the red "Test" button with a small screwdriver; the continuity should break (OL on the meter). This confirms it is the NC trip contact.
- Find the NO (97-98): Probe the remaining auxiliary terminals. They should read open (OL) in the normal state, and beep (continuity) only when you press and hold the red "Test" button. This is your 97-98 fault contact.
- Check the Power Path: Probe L1 to T1, L2 to T2, and L3 to T3. They should read near 0 ohms. If any phase reads open while the reset button is engaged, the internal heater element is burned open and the relay must be replaced.
Frequently Asked Questions
What is the difference between a thermal overload relay symbol and a magnetic one?
A thermal overload relay symbol represents a bimetallic strip or eutectic melting alloy that reacts to heat over time. The symbol typically shows a heater element with a curved mechanical link, indicating an inverse-time trip curve (Class 10, 20, or 30). A magnetic overload relay symbol represents a solenoid or current-sensing coil that trips instantaneously based purely on magnetic flux, ignoring thermal mass. In modern schematics, you will frequently see solid-state or microprocessor-based symbols (a box labeled "μP" or "SSOL"), which combine both thermal memory and instantaneous magnetic trip logic into a single digital package, like the Allen-Bradley 193-EIO series.
How do I identify the NC (95-96) and NO (97-98) contacts on an overload relay symbol?
Look at the two-digit terminal numbers printed next to the contact symbols on the schematic or the physical device. The IEC standard strictly reserves the number 9 as the first digit for overload relay auxiliary contacts. The second digit dictates the function: 5 and 6 indicate a Normally Closed (NC) break contact, while 7 and 8 indicate a Normally Open (NO) make contact. Therefore, 95-96 is always the NC contact used to drop out the motor starter, and 97-98 is always the NO contact used to trigger an alarm or PLC fault input. For a deeper dive into terminal marking standards, refer to the IEC and NEMA motor starter comparison guide.
Why does my motor starter diagram show two different overload relay symbols?
This is a point of confusion for those new to reading ladder diagrams. A complete direct-on-line (DOL) starter schematic splits the circuit into two sections: the power circuit and the control circuit. The first symbol you see (the three heater elements in series with L1, L2, and L3) represents the power circuit path where the actual motor current flows. The second symbol (the 95-96 NC contact in series with the contactor coil) represents the control circuit. They are drawn separately because they operate at different voltages and serve different logical functions, but the dashed mechanical link line drawn between them on the schematic indicates they are housed in the same physical device. For specific wiring configurations and trip class selections, consult the manufacturer's technical data, such as the Rockwell Automation overload relay documentation.
