When reading motor control schematics, the magnetic overload symbol represents the instantaneous short-circuit and high-current fault protection element within a motor starter assembly. Unlike thermal overloads that react to heat buildup over time (inverse-time tripping), magnetic overloads react instantly to massive current spikes, such as a dead short or a locked rotor. Recognizing this symbol—and distinguishing it from its thermal counterpart—is critical for safely troubleshooting and designing motor control circuits.

Because electrical drafting standards vary significantly by region, the exact geometry and designator letters for these symbols change depending on whether you are looking at a North American NEMA print or a global IEC diagram. Below is the definitive reference for identifying these components on the bench or in the field.

Magnetic Overload Symbol Reference Table (NEMA & IEC)

The table below breaks down the specific schematic symbols, standard designators, and terminal numbering conventions for magnetic overload components. Use this as your primary cheat sheet when tracing ladder logic or wiring diagrams.

Component / Function NEMA (US/Canada) Symbol & Designator IEC (Global) Symbol & Designator Practical Application & Trip Characteristic
Magnetic Sensing Coil
(Series with motor line)		 Rectangle or loop on the power line, labeled OL (often with a subscript like OL1). Rectangle or loop on the power line, labeled F (e.g., F1, F2 per IEC 60947). Senses line current. Triggers instantaneously when current exceeds the magnetic threshold (typically 8x-13x FLA).
Instantaneous Trip NC Contact
(Control circuit interlock)		 Normally Closed (NC) contact symbol, labeled OL. NC contact symbol, specifically numbered 95-96 (standard IEC fault circuit numbering). Wired in series with the contactor coil. Opens instantly on a magnetic fault to drop the contactor and halt the motor.
Trip Indication NO Contact
(Fault signaling)		 Normally Open (NO) contact symbol, labeled OL. NO contact symbol, specifically numbered 97-98. Closes upon tripping to illuminate a fault lamp or send a signal to a PLC/SCADA system.
Adjustable Magnetic Threshold
(Setting dial)		 Arrow pointing at the coil rectangle, marked with an Im or INST multiplier. Arrow pointing at the coil, marked with Im (e.g., 10-14 x In). Allows the technician to dial the instantaneous trip point to accommodate high-inertia starts without nuisance tripping.

Regional Standards: NEMA vs IEC vs Legacy UK

The way a magnetic overload symbol is drawn and labeled depends entirely on the standard governing the facility's electrical prints. Applying the wrong regional logic to a schematic is a primary cause of miswired control circuits.

NEMA Standards (North America)

Governed by NEMA ICS 2 and aligned with NEC practices, North American schematics use the blanket designator OL for all overload relays. In older NEMA drawings, the magnetic and thermal elements are often drawn as a single combined block. In modern, highly detailed NEMA ladder logic, the magnetic coil is drawn on the power lines (L1, L2, L3), while the associated NC contact is drawn in the control rung (typically between the stop button and the contactor coil 'M').

IEC Standards (Global / EU / Asia)

IEC 60947-4-1 is the governing standard for motor starters globally. IEC schematics are far more granular with terminal numbering. The overload relay is designated by the letter F (with K reserved for contactors). Crucially, IEC relies on strict two-digit terminal numbering rather than just component letters. If you see a contact labeled 95-96, it is universally the NC fault contact of the overload relay, regardless of the manufacturer (Schneider, Siemens, ABB, or Eaton).

Legacy BS 3939 (Old UK / Commonwealth)

If you are troubleshooting equipment in older UK or Commonwealth facilities, you may encounter the deprecated BS 3939 standard. In these prints, overload coils were often depicted as a simple semi-circle or a series of 'humps' on the line, designated by O/L or simply O. While largely replaced by IEC 60617, recognizing these legacy 'hump' symbols is essential when retrofitting old machine tools.

Safety Warning: Never assume a schematic is accurate to the physical panel. Previous technicians may have replaced an IEC-style motor protection circuit breaker (MPCB) with a NEMA-style bimetallic thermal relay without updating the prints. Always verify dead with a tested multimeter and trace the physical wiring before assuming the magnetic trip is functional.

The 'Rows People Get Wrong' and Faded Diagram Troubleshooting

Even experienced journeymen and bench technicians make specific errors when interpreting overload symbols. Here are the most common pitfalls and how to avoid them.

Mistake 1: Confusing Magnetic with Thermal Symbols

The most frequent error is treating the magnetic overload symbol as a thermal overload.

  • Thermal Symbol: Depicted with a curved 'heating element' line or a bimetallic strip graphic. It represents an inverse-time delay (Class 10, 20, or 30).
  • Magnetic Symbol: Depicted as a sharp rectangle, a distinct loop, or a solenoid coil. It represents an instantaneous trip with zero intentional delay.
In modern Motor Protection Circuit Breakers (MPCBs) like the Eaton PKZM0 or Schneider TeSys GV3, both mechanisms exist in one physical housing. However, on a detailed schematic, they will be drawn as two separate coils on the line: the thermal heater and the magnetic solenoid.

Mistake 2: The 95-96 vs 97-98 IEC Terminal Trap

When wiring an IEC overload relay, technicians often wire the PLC fault input to the 95-96 terminals, assuming it will go high on a fault. This is backwards. 95-96 is the Normally Closed contact that breaks the contactor coil circuit. The 97-98 terminals are the Normally Open contacts that close to signal the fault to a PLC or indicator light. Wiring a 24VDC PLC input to 95-96 will result in a dead short or a blown PLC fuse when the relay trips.

Safe Interpretation of Faded or Missing Markings

In harsh industrial environments, schematic prints fade, and physical component labels wear off. If you cannot read the magnetic overload symbol or the terminal markings on the physical relay, use this multimeter decision path:

  1. De-energize and Lockout/Tagout (LOTO): Ensure the motor starter is completely isolated from mains power.
  2. Identify the Line-Side Coils: Look for the three large lugs on the line side of the contactor. The magnetic sensing elements are in series directly below these lugs, feeding the T1, T2, and T3 load terminals.
  3. Locate the NC Control Contact: Set your multimeter to continuity or low-ohms. Probe the small auxiliary terminals on the side or front of the overload block. You are looking for a pair that reads < 1 ohm when the relay is in the 'RESET' state.
  4. Verify the Trip Mechanism: Press the manual 'TRIP' test button on the physical relay. The pair that previously read < 1 ohm should now read OL (Open Loop). This is your 95-96 (or NEMA OL NC) control circuit contact. The pair that flips from OL to < 1 ohm is your 97-98 fault indication contact.

Practical Application: Sizing the Magnetic Trip Threshold

Understanding the symbol is only half the battle; setting the physical device correctly is where proper motor protection is achieved. The magnetic trip (instantaneous element) is designed to catch dead shorts and locked rotors, but it must not trip during normal motor starting inrush.

When a 3-phase induction motor starts Direct-On-Line (DOL), it draws Locked Rotor Amps (LRA), which is typically 6 to 8 times the Full Load Amps (FLA) for a fraction of a second. If your magnetic overload symbol on the schematic indicates an adjustable threshold (Im), you must set it above the LRA to prevent nuisance tripping.

Rule of Thumb for Magnetic Settings: Set the instantaneous magnetic trip to 10x to 12x the motor's FLA. For a motor with a 10A FLA and a 70A LRA, set the magnetic dial to 120A. This allows the 70A inrush to pass through safely, while still providing instantaneous protection if a phase-to-phase short draws 500A+.

Always consult the specific manufacturer's trip curve documentation for the exact model you are installing. Solid-state electronic overloads (often represented by a diamond or microchip symbol inside the overload block on modern prints) calculate this mathematically via internal CTs, but the fundamental schematic representation of the magnetic trip logic remains identical to the electromechanical equivalents.