Overload Relay Explained: Types, Trip Classes, and Setting
The overload relay explained — thermal vs electronic, how it protects a motor, trip classes, how to set the FLA dial, and how electronic relays talk to a PLC.
Overload Relay Explained: Types, Trip Classes, and Setting
An overload relay is a protective device wired in series with a motor's contactor that monitors running current and opens the control circuit when the motor draws more current than it is rated to handle continuously. It is the last line of defense against winding burnout caused by mechanical overload, phase loss, or a jammed load — and it is a mandatory component in every properly engineered motor starter.
Understanding how an overload relay works, which type to choose, and how to set it correctly is a foundational skill for anyone working with motor start/stop ladder logic or commissioning a motor control center.
What an Overload Relay Is
An overload relay sits inside or immediately adjacent to the motor starter, in series between the contactor's load-side terminals and the motor terminals. When the motor draws current above the relay's trip threshold for long enough, the relay opens its normally-closed (NC) auxiliary contact, which drops out the contactor coil and disconnects the motor from the supply.
The key distinction from other protective devices is that an overload relay responds to sustained overcurrent — the kind caused by a mechanical problem or heavy load — rather than the instantaneous fault current that a fuse or breaker handles.
Three wires in, three wires out. The overload relay passes all three motor phases through current-sensing elements (heating elements or current transformers depending on type) and monitors the worst-case phase continuously.
For a broader look at how this fits into the full protection scheme, see motor protection explained.
Why It Is Not the Same as a Fuse or Circuit Breaker
This is the single most common point of confusion on the controls bench.
| Device | Protects against | Trip speed | Trip level |
|---|---|---|---|
| Fuse | Short circuit | Instantaneous (<1 cycle) | 10× FLA and above |
| Circuit breaker (MCB/MCCB) | Short circuit + severe overload | Fast for faults, slow for moderate overcurrent | Typically 1.25–10× rated current |
| Overload relay | Running overload | Seconds to minutes (I²t curve) | 1.05–1.25× FLA set point |
A fuse blows in microseconds on a dead short. An overload relay is intentionally slow — it must survive motor inrush current at startup (which can be 6–8× FLA) before deciding whether the motor is actually overloaded. If an overload relay tripped on inrush, the motor could never start.
The two devices work together: the fuse or breaker handles catastrophic short circuits, the overload relay handles the slow thermal death caused by a motor struggling against a heavy or jammed load.
Types of Overload Relay
Bimetallic Thermal Overload Relay
The bimetallic thermal overload relay is the most common type in legacy and low-cost installations. Current passes through heating elements wound around bimetallic strips. As the strip heats up, it bends. When the bending reaches the trip point, it snaps a mechanical latch that opens the NC contact.
How it mirrors the motor: Because the bimetal heats and cools at a rate roughly proportional to the motor winding, it provides a degree of thermal memory — if the motor has been running hot and then trips, the bimetal does not reset immediately. This mimics the actual thermal state of the winding.
Limitations:
- Ambient temperature affects trip point (hotter environment = lower effective trip current)
- No phase-loss detection without an additional feature called differential trip
- No communication — the only output is the mechanical contact state
- Setting resolution is coarse (a dial with a range, not a precise value)
Electronic (Solid-State) Overload Relay
The electronic overload relay uses current transformers on each phase to measure actual RMS current, then applies an I²t thermal model in firmware to simulate motor winding temperature. No moving parts, no bimetal, no mechanical wear.
Advantages over bimetallic:
- Ambient temperature compensation built in
- Phase-loss and phase-imbalance protection as standard (detects when one phase drops out — the leading cause of three-phase motor burnout)
- Ground fault detection on some models
- Adjustable trip class via a selector switch
- Communication ports — many models support I/O, Modbus RTU, EtherNet/IP, or PROFINET, enabling integration into a PLC system
- Digital display of actual current, thermal capacity used, and fault code
- Settable alarm thresholds separate from the trip threshold
Electronic relays cost more upfront but eliminate the ambient-temperature drift and phase-loss blindness that make bimetallic relays unreliable in demanding applications.
How It Works: Heating Elements and the I²t Curve
Whether bimetallic or electronic, both types implement the same underlying physics: motor winding temperature rises proportionally to I² (current squared) multiplied by time.
Double the current, quadruple the heating rate. A motor running at 110% FLA will take many minutes to accumulate enough thermal energy to trip. The same motor stalled at locked-rotor current (typically 6× FLA) will trip in seconds.
This relationship is captured in the trip curve published in the relay's datasheet:
- X-axis: Multiples of the current setting (1× = at FLA, 6× = locked rotor)
- Y-axis: Time to trip in seconds
The curve is not linear — it is an inverse-time curve. Light overloads take minutes; severe overloads take seconds. This shape is what allows the relay to ride through motor inrush without tripping while still protecting against genuine overloads.
Electronic relays compute this curve continuously in firmware. Bimetallic relays approximate it mechanically. The shape is what defines the trip class.
Trip Classes Explained
Trip class defines how quickly the overload relay will trip at 600% of the current setting (6× FLA — the standard test point representing a locked-rotor condition). The number is the maximum trip time in seconds at that multiple.
| Trip Class | Max trip time at 6× FLA | Typical application |
|---|---|---|
| Class 10 | 10 seconds | Pumps, fans, centrifugal loads — light-inertia loads that accelerate quickly |
| Class 20 | 20 seconds | General-purpose motors, conveyors, most industrial applications |
| Class 30 | 30 seconds | High-inertia loads — large fans, compressors, crushers, loads that take a long time to reach full speed |
Choosing the wrong trip class is a common commissioning mistake. A Class 10 relay on a large fan motor will nuisance-trip during every start because the motor spends more than 10 seconds at high multiples of FLA while it accelerates. A Class 30 relay on a pump gives the winding too long to cook before the relay acts.
The motor datasheet or nameplate will specify the minimum start time — use that to select trip class. When in doubt, Class 20 is the safe general-purpose default for most industrial applications. For a detailed walkthrough of reading motor nameplate data, see motor nameplate explained.
How to Set an Overload Relay
The primary setting on any overload relay is the FLA dial — the full-load ampere set point. This dial tells the relay what the motor's normal running current is so the relay can calculate multiples correctly.
Step 1: Find the Motor's Full-Load Ampere Rating
The FLA is printed on the motor nameplate. Use the FLA listed for the voltage and connection you are using (delta vs star, 230 V vs 400 V). Do not guess — nameplate data is the only authoritative source.
Step 2: Apply the Service Factor
If the motor has a service factor (SF) greater than 1.0 (common values are 1.15 or 1.25), the NEC and most standards allow setting the overload relay up to FLA × SF rather than bare FLA. This gives the motor room to run at its rated service factor loading without nuisance trips.
- Motor FLA = 10 A, SF = 1.15 → maximum set point = 11.5 A
- Motor FLA = 10 A, SF = 1.0 → set point = 10 A
Step 3: Turn the Dial
Align the dial arrow with the calculated set point. Most bimetallic relays have a range printed on the dial face (e.g., 7–10 A). The dial adjusts the gap between the bimetal and the trip latch, effectively shifting the trip curve.
On electronic relays, the FLA set point is entered via a rotary dial or keypad, and the relay displays the actual current so you can verify the setting is correct during commissioning.
Step 4: Select Trip Class (Electronic Relays)
On electronic relays with a trip class selector, choose the class appropriate for your load as described above.
Step 5: Verify Under Load
After setting, run the motor at full load and confirm the relay's displayed current (on electronic models) or ammeter reading matches the expected FLA. If the motor runs normally and the relay does not trip, the setting is correct.
Manual Reset vs Auto Reset
After a trip, the overload relay holds its NC contact open until someone investigates and resets it. There are two modes:
Manual reset: The operator must press the reset button on the relay to restore the NC contact. This is the required mode for most installations — it prevents the motor from restarting automatically after a thermal trip, which could be dangerous if the trip was caused by a jammed conveyor or seized pump.
Auto reset: The relay automatically resets after the bimetal cools or the electronic thermal model decays below the reset threshold. This mode is sometimes used in remote, unattended installations (e.g., a remote water pump) where an operator cannot be present to reset manually. Use with caution: auto reset can allow a motor to cycle on and off repeatedly while a fault persists, accelerating winding damage.
The reset mode is typically set by a small selector on the relay face. Default to manual unless the application explicitly requires auto reset and safety consequences have been evaluated.
Where the Overload Relay Sits in the Starter
A direct-on-line (DOL) motor starter consists of three components in series:
- Disconnect / isolation device (fused switch or circuit breaker) — upstream protection, isolation for maintenance
- Contactor — the high-current switching device controlled by the PLC or control circuit
- Overload relay — thermal protection, mounted on the contactor's load-side terminals
The overload relay's NC auxiliary contact is wired into the contactor coil control circuit. When the relay trips, this contact opens, the coil de-energizes, the contactor drops out, and the motor stops. Power to the motor is broken at the contactor, not at the overload relay itself — the relay only breaks the low-current control circuit.
This is a critical point for ladder logic: the overload relay contact appears as a normally-closed input in the motor start/stop ladder logic rung. If the relay trips, the input goes false, the rung goes false, and the contactor coil output turns off.
The Controls View: Electronic Overload Relays and PLCs
This is where modern electronic overload relays separate themselves from bimetallic devices. A smart relay does more than trip — it communicates.
Discrete I/O Integration
Most electronic relays provide two auxiliary contacts:
- Trip contact (NC): Opens on trip — wired into the control circuit as described above, also mapped to a PLC digital input for fault detection
- Alarm contact (NO or NC, configurable): Closes when current exceeds the alarm threshold (typically 90–110% of the trip threshold) — gives the PLC advance warning before a trip occurs
With just these two inputs, a PLC can distinguish between:
- Normal run (trip contact closed, alarm contact open)
- Impending overload (trip contact closed, alarm contact closed) → trigger alarm, alert operator
- Trip condition (trip contact open) → log fault, prevent restart until reset
Fieldbus Integration
Higher-specification electronic relays support fieldbus communication — typically Modbus RTU, EtherNet/IP, PROFINET, or DeviceNet. Over fieldbus, the PLC can read:
- Actual phase currents (all three phases, in amperes or % of FLA)
- Thermal capacity used (0–100%, where 100% = trip imminent)
- Trip cause code (overload, phase loss, phase imbalance, ground fault, thermistor)
- Number of trips and trip history
- Time to reset (seconds remaining before the thermal model allows restart)
And in some cases, write:
- Remote reset command (reset the relay from the PLC after the fault is cleared)
- Trip class and FLA set point (parameterization from the PLC or engineering tool)
This data feeds PLC troubleshooting workflows directly: instead of dispatching a technician to read a relay LED, the SCADA system displays the fault code, current at time of trip, and thermal history. Root cause analysis that previously required physical investigation can happen from the control room.
Alarm vs Trip Thresholds
A well-configured electronic relay uses two thresholds, not one:
| Threshold | Typical setting | PLC action |
|---|---|---|
| Alarm | 90–95% of thermal capacity | Trigger warning in SCADA, notify operator, log event |
| Trip | 100% of thermal capacity | Open NC contact, de-energize contactor, log fault with timestamp and cause code |
This two-stage approach prevents nuisance trips on transient loads while giving operators time to reduce load or investigate before the motor actually trips. It is standard practice in any installation connected to a VFD or PLC-controlled drive system where unplanned downtime has significant cost.
Frequently Asked Questions
What is an overload relay? An overload relay is a motor protection device that monitors running current and opens the motor control circuit when the motor draws more current than its rated full-load ampere value for a sustained period. It protects motor windings from thermal damage caused by mechanical overload, phase loss, or a stalled rotor.
How do you set an overload relay? Find the full-load ampere (FLA) rating on the motor nameplate. If the motor has a service factor above 1.0, multiply FLA by the service factor to get the maximum allowable set point. Turn the relay's FLA dial to that value. On electronic relays, also select the appropriate trip class for your load type (Class 10 for light-inertia loads, Class 20 for general use, Class 30 for high-inertia loads).
What is a trip class? Trip class defines how long an overload relay will wait before tripping when current reaches 600% of the FLA setting — the locked-rotor test point. Class 10 trips within 10 seconds, Class 20 within 20 seconds, Class 30 within 30 seconds. The correct class depends on how long the motor takes to accelerate to full speed under its actual load.
What is the difference between an overload relay and a circuit breaker? A circuit breaker protects against short circuits and severe faults — it trips in milliseconds on catastrophic overcurrent. An overload relay protects against running overloads — sustained overcurrent at 105–125% of FLA that would slowly cook the motor winding over minutes. Both are required in a properly designed motor starter; they protect against different failure modes at different timescales.
Summary
The overload relay is a simple device with a precise job: give the motor enough time to start, then trip if it runs too hot for too long. Bimetallic relays do this mechanically and cheaply. Electronic relays do it with greater accuracy, phase-loss protection, and the ability to report diagnostic data to a PLC over I/O or fieldbus.
Setting the relay correctly takes three inputs: the motor nameplate FLA, the service factor, and the load's acceleration time. Get those three right and the relay will protect the motor reliably for years. Get them wrong and you will either nuisance-trip on every start or — worse — let the motor cook slowly to failure.
For controls engineers, the shift to electronic overload relays with fieldbus integration is worth the additional cost on any motor that matters: you gain real-time current monitoring, early-warning alarms, and remote reset capability that bimetallic relays simply cannot provide.
