An overload relay is a protective device that guards a motor against sustained overcurrent, opening the motor circuit before the windings overheat and fail. It is not a switching device, and it is not interchangeable with a general-purpose control relay—which is a distinction worth getting right, as the two are easy to confuse on a spec. Installers often reach for an overload device when a standard relay with the correct amperage rating would handle the switching. A true overload relay is only needed where motor overload protection is the requirement.
Functional Devices, Inc. manufactures RIB® (Relay-In-a-Box®) relays, current sensors, and power supplies for building automation. Its relays are switching and control devices, not motor-protection devices. This guide covers what an overload relay does, how it works, the common types, and when a standard RIB® relay is the device the application calls for.
What Is an Overload Relay?
An overload relay monitors the current a motor draws and opens its circuit when that current stays above the motor's rated value long enough to risk overheating. It addresses overload: the sustained, moderate overcurrent that occurs when a motor is working too hard, a bearing is failing, or a load jams. That is a different fault from a short circuit or ground fault, which is a sudden, high-magnitude fault current handled by a circuit breaker or fuse.
An overload relay is typically one half of a motor starter. A contactor switches the motor on and off, and the overload relay sits in series with the motor to watch its current. The relay does not break the motor's power directly. Instead, its normally closed auxiliary contact is wired into the contactor's coil circuit; on a sustained overload, that contact opens, the contactor drops out, and the motor is disconnected.
How Overload Relays Work
An overload relay has to tell a genuine overload apart from the normal inrush a motor draws at startup, which is commonly six to ten times full-load current for a few seconds. It does this with a time-current characteristic: the higher the overcurrent, the faster it trips, but brief inrush passes without tripping. The relay is set to the motor's nameplate full-load amps (FLA), and trip classes (Class 10, 20, or 30) describe how many seconds the device tolerates a sustained overload before opening.
Thermal types generate heat in proportion to the motor current and trip on that heat; electronic types measure current directly and calculate the thermal effect. After a trip, the relay is reset, manually or automatically, once it has cooled. A repeated trip is a signal to find the cause, not to raise the setting.
Types of Overload Relays
- Bimetallic Thermal: Current heats a bimetallic strip made of two metals with different expansion rates; the strip bends as it heats and trips the mechanism. Common and cost-effective for general motor protection.
- Eutectic (Melting Alloy): Current heats a eutectic alloy that holds a ratchet in place; when the alloy melts, the ratchet releases and the contacts open. The device cannot reset until the alloy resolidifies.
- Electronic (Solid-State): Microprocessor-based current sensing, often with adjustable settings, faster trip on phase loss, and features such as ground-fault or stall protection. More precise than thermal types, with lower internal heat loss.
- Magnetic: Uses the magnetic force of the motor current through a coil to trip, suited to applications with high starting currents.
When a Standard RIB® Relay Is Sufficient
A RIB® relay is a switching and control device, not a protective one. It is rated by its contact amperage (10A, 20A, and 30A models); by a pilot duty rating for inductive loads such as contactor coils and solenoids; and, for conventional AC motors, by a horsepower rating, all listed on the product datasheet. It is the right device when the task is switching a load that falls within those ratings, and when overload protection is either not applicable or provided separately.
That covers a large share of building automation work: isolating a building automation system (BAS) controller output from a line-voltage load, fan coil control, lighting switching, and energizing a contactor or solenoid coil. The RIBU1C (10A SPDT) suits light-duty and pilot-duty switching; the RIB2401B (20A SPDT) handles most commercial switching points; and the RIB24P30 (30A DPDT) covers higher-current or dual-circuit loads. Each is UL Listed and factory-tested as a complete assembly.
What a RIB® relay does not do is protect a motor from overload. If a relay is switching a motor that requires overload protection under electrical code requirements, that protection still has to come from an overload device, a motor starter, or the motor's integral thermal protector. The relay handles the on and off; the overload device handles the protection. They are separate functions, and a motor circuit may need both. For loads beyond a relay's contact or horsepower rating, a contactor is the switching device, as covered in the relay vs. contactor selection guide.
Selecting the Right Device for Your Application
Work from the function the circuit needs.
- Protecting a Motor From Overload: Use an overload device sized to the motor nameplate FLA per electrical code, or rely on the motor's integral thermal protector. This is a protective function, not a switching one.
- Switching a Load Within a Relay's Ratings: A properly rated RIB® relay is sufficient. Confirm the general-use contact rating, the pilot duty rating, and the horsepower rating on the datasheet against the load before specifying.
- Switching a High-Current Motor or Compressor: Use a contactor, often paired with an overload relay to form a starter.
Because overload protection and switching are different functions, the question of relay or overload relay pertains to what the circuit needs to do at that point. Verify the load against the published contact, pilot duty, and horsepower ratings, and specify the device that matches the function.
Frequently Asked Questions
Does a RIB® relay provide motor overload protection?
No. A RIB® relay is a switching and control device. Where a motor requires overload protection under certain electrical code, that protection must come from an overload device, a motor starter, or the motor's integral thermal protector, separate from the relay that switches it.
What is the difference between an overload relay and a circuit breaker?
An overload relay protects a motor from sustained overcurrent. A circuit breaker or fuse protects against short-circuit and ground-fault currents. The breaker does not protect the motor against overload.
Does Functional Devices make overload relays?
Functional Devices manufactures RIB® switching and control relays, current sensors, and power supplies, not motor overload relays. For switching a load within a relay's rating, a RIB® relay is the appropriate device; for motor overload protection, use an overload device.
Overload Relay or RIB® Relay: Determining Which You Need
An overload relay protects a motor from sustained overcurrent; a RIB® relay switches a load. Those are different jobs, and a motor circuit may need both. Specify a true overload device where electrical code requires motor overload protection, and where the task is switching a load within a relay's ratings, a standard RIB® relay is the device.
To match a RIB® relay to a switching application, browse the RIB® relay lineup and check the contact, pilot duty, and horsepower ratings on the datasheet, or find an authorized Functional Devices distributor.