The magnetic overload relay symbol differs strictly by standard: NEMA (North America) uses a coil with a semicircle or dashed line to denote instantaneous magnetic tripping, while IEC 60617 (Global) uses a rectangle with a diagonal line or a specific 'F' designator combined with a magnetic release indicator. Unlike thermal overloads that rely on heat buildup, magnetic overloads trip based on the electromagnetic field generated by fault current, making them critical for high-inertia loads or instantaneous short-circuit protection.
The Complete Magnetic Overload Relay Symbol Reference
Before wiring or troubleshooting a motor control center (MCC), you must correctly identify the schematic symbols. The table below maps the exact graphical representations across the three major standards you will encounter in the field.
| Standard | Symbol Graphic Description | Standard Designator / Label | What It Means in Practice |
|---|---|---|---|
| NEMA (ICS 2) | Straight line with a semicircle on top (Instantaneous) OR Circle with 'M' inside | OL (with magnetic trip note) or MCP | The main power passes through a magnetic coil. If current spikes past the adjustable threshold, the magnetic field pulls a plunger to trip the mechanism instantly. |
| IEC 60617 | Rectangle with a diagonal line through it, OR Rectangle with a semicircle attached | 'F' (Protection) + Magnetic release indicator | Denotes a magnetic release unit within a motor protective circuit breaker. The diagonal line specifically separates it from thermal (bimetallic) protection symbols. |
| Old UK (BS 3939) | Circle with a horizontal line striking through the center | O/L (Magnetic) | Legacy British standard. Functions identically to the NEMA coil, but the horizontal strike-through was used to differentiate from standard contactor coils. |
| Dashpot Magnetic (NEMA) | Magnetic coil symbol with a small adjacent rectangle (dashpot) | OL (Dashpot) | Uses a fluid-filled tube to delay the magnetic trip. Allows for long motor acceleration times without nuisance tripping, while still protecting against locked rotors. |
| Auxiliary Trip Contact | Normally Closed (NC) contact symbol with a dashed mechanical linkage line to the main coil | 95-96 (IEC) or NC (NEMA) | The control circuit interlock. When the magnetic overload trips, this contact opens to drop power to the main contactor coil, shutting down the motor. |
Source references: Symbol definitions align with the IEC 60617 Graphical Symbols database and the NEMA ICS 2 standard for industrial control systems.
Rows People Get Wrong: Critical Symbol Distinctions
Misinterpreting a single line on a schematic can result in a dead short or a motor that refuses to start. Here are the most common schematic errors made by junior technicians and hobbyists:
The magnetic overload element (the semicircle or rectangle on the main power lines) must be wired in series with the motor phases (L1/T1, L2/T2, L3/T3). Beginners often see the coil symbol and assume it belongs in the 120V/24V control circuit in parallel with the contactor coil. Doing this will instantly blow the control fuses and fail to protect the motor.
The symbol for the magnetic trip mechanism (in the power circuit) is physically linked via a dashed line to the NC auxiliary contact (in the control circuit). People frequently wire the control circuit through the main power terminals of the overload relay because they don't follow the dashed mechanical linkage line to the 95-96 (NC) control terminals.
If you see the small rectangle (dashpot) attached to the NEMA magnetic coil symbol, you cannot swap it for a standard instantaneous magnetic trip. The dashpot provides a crucial time-delay for high-inertia loads (like large centrifugal fans). Swapping to an instantaneous trip will cause nuisance tripping every time the motor starts.
Regional Standards: Which One Applies to Your Panel?
Knowing which standard governs your equipment dictates how you read the prints and source replacement parts.
- NEMA (North America): Dominates the US and Canada. If you are working on legacy US manufacturing equipment, Allen-Bradley (Rockwell Automation) MCCs, or standard NEMA-rated motor starters, you will use the NEMA semicircle and circle symbols. NEMA devices are generally physically larger and built for harsh, high-fault environments.
- IEC 60617 (Global/EU/Modern US): The international standard. If you are working on modern European imports, Schneider Electric TeSys systems, or newer compact control panels, you will see the IEC rectangular symbols. IEC components are more compact and modular.
- Old UK (BS 3939): You will only encounter this in legacy British infrastructure built before the widespread adoption of IEC standards in the UK. If you see the circle-with-a-strike-through, treat it functionally identical to NEMA, but expect metric wire sizing and older BS color codes (e.g., red/yellow/blue phases).
Faded or Missing Markings: Safe Interpretation Protocol
In older panels, schematic labels fade, or previous technicians swap components without updating the prints. When you cannot rely on the printed magnetic overload relay symbol or terminal markings, use this multimeter protocol to safely map the device.
Prerequisite: De-energize the panel. Lock out and tag out (LOTO) the main disconnect. Verify zero voltage with a tested CAT III/IV multimeter (like a Fluke 87V) across all phases and to ground.
- Identify the Main Power Pass-Through: Set your multimeter to continuity/resistance (Ω). Probe the top and bottom terminals of the suspected overload relay on each phase (e.g., L1 to T1). A healthy magnetic overload element will read < 0.5 ohms. If it reads OL (open), the internal magnetic element is blown or the device is tripped and requires a manual reset.
- Identify the Control Interlock (NC Contact): Look for a smaller set of terminals, usually on the side or front of the device. Probe these terminals. With the relay in the 'RESET' state, you should read < 1 ohm (continuity).
- Verify the Trip Linkage: Press the manual 'TRIP' button on the relay face. While holding it, probe the same small control terminals. The reading must immediately jump to OL (open circuit). If it stays at < 1 ohm, the internal mechanical linkage is broken, and the relay will not drop the contactor coil during a fault. Replace it immediately.
- Check for Ground Faults: Probe from the main power terminals to the metal mounting din-rail or ground lug. It must read strictly OL. Any resistance reading here indicates internal insulation breakdown.
Decision Path: Selecting the Right Magnetic Overload Replacement
When a magnetic overload relay fails, or when you are designing a new high-inertia motor circuit, use this decision matrix to select the exact replacement architecture. Do not default to a standard thermal bimetallic relay if the schematic calls for a magnetic trip.
| Application Scenario | Required Trip Characteristic | Device Architecture to Select |
|---|---|---|
| Standard conveyor or pump (normal start time < 10s) | Time-delayed thermal trip | Standard Thermal Overload Relay (Not magnetic) |
| High-inertia load (centrifugal fan, crusher) with long start times (30s+) | Delayed magnetic or specialized thermal | Dashpot Magnetic Overload OR Class 20/30 Thermal |
| Instantaneous short-circuit protection required without thermal memory | Instantaneous magnetic trip only | Magnetic Motor Circuit Protector (MCP) |
| Need both thermal overload and instantaneous short-circuit in one compact DIN-rail footprint | Thermal-Magnetic combined | Motor Protective Circuit Breaker (MPCB) |
If your schematic specifically demands a pure, adjustable instantaneous magnetic overload relay symbol (typically for a Motor Circuit Protector application to pair with a separate contactor), the industry-standard concrete pick is the Allen-Bradley 140M-C2E-B25 (Bulletin 140M Magnetic Motor Circuit Protector).
Why this part: It provides an adjustable magnetic trip range from 10A to 25A, features a high short-circuit breaking capacity (up to 100kA at 400V), and perfectly matches the NEMA/IEC instantaneous magnetic trip symbol requirements without introducing unwanted thermal memory. Set the front dial to exactly 125% of your motor's Full Load Amps (FLA) for precise lockup protection.
