Overload relay


An overload relay is an electrical device used to protect motors and other electrical equipment from damage caused by excessive current. It is a type of protective relay that monitors the current flowing through a circuit and opens the circuit if the current exceeds a predetermined level.

When a motor or electrical equipment is in operation, the current flowing through the circuit may increase due to various reasons such as mechanical overload, electrical faults, or abnormal operating conditions. If the current exceeds the rated capacity of the motor or equipment for an extended period, it can cause overheating and damage to the motor windings or other components.

To prevent such damage, an overload relay is connected in series with the motor or electrical equipment. The overload relay typically consists of a current sensing device, such as a bimetallic strip or current transformer, and a set of contacts that open or close based on the current level.

When the current flowing through the circuit exceeds the predetermined threshold, the overload relay detects this and activates the contacts to open the circuit, thereby interrupting the power supply to the motor or equipment. This de-energizes the circuit and stops the operation of the motor, protecting it from damage.

Overload relays may have adjustable settings to accommodate different current levels and trip times. They are often used in combination with contactors or motor starters, which provide the means for controlling the operation of the motor.

What is Overload Protection?

Overload protection refers to the measures taken to prevent electrical equipment, such as motors, from being damaged due to excessive current or load. It is an essential aspect of electrical system design and operation to ensure the safety and longevity of equipment.

When electrical equipment is subjected to an overload condition, the current flowing through the circuit exceeds its rated capacity. This can occur due to factors such as mechanical overload, electrical faults, or abnormal operating conditions.

If the overload condition persists for an extended period, it can lead to overheating, insulation breakdown, and ultimately, equipment failure.

To protect against overloads, various protective devices and strategies are employed.

Here are some commonly used overload protection methods

Overload Relays: As mentioned earlier, overload relays are devices that monitor the current flowing through a circuit and open the circuit if the current exceeds a predetermined level. This interrupts the power supply and protects the equipment.

Circuit Breakers: Circuit breakers are protective devices that automatically open a circuit when a fault or overload is detected. They provide protection against short circuits, overloads, and other electrical faults by quickly interrupting the current flow.

Fuses: Fuses are another type of protective device that interrupt the current flow in case of excessive current. When the current exceeds the fuse’s rated capacity, the fuse element melts, opening the circuit and protecting the equipment.

Thermal Overload Protection: Many electrical devices, such as motors, have built-in thermal overload protection. These devices use temperature-sensitive elements, such as bimetallic strips or thermistors, to detect excessive heat caused by overloads. When the temperature reaches a critical point, the protection mechanism activates, either by opening the circuit or triggering an alarm.

Current Limiting: Current-limiting devices or strategies are employed to limit the current to a safe level. This can be achieved through the use of current-limiting resistors, inductors, or electronic current-limiting circuits.

The specific method of overload protection depends on the type of equipment and the application. It is important to select and implement the appropriate protection method based on the equipment’s specifications and the expected operating conditions. Regular inspection, maintenance, and testing of the protective devices are also crucial to ensure their effectiveness in preventing equipment damage due to overloads.

Principle of operation overload relay

The principle of operation of an overload relay involves monitoring the current flowing through an electrical circuit and providing protection against excessive currents. It typically consists of a current-sensing element, a trip mechanism, and a set of contacts.

Here is a simplified explanation of how an overload relay operates

Current Sensing: The overload relay is connected in series with the circuit it is protecting. It has a current-sensing element, such as a current transformer or a solid-state device, that measures the current flowing through the circuit.

Current Comparison: The current-sensing element compares the actual current with a predetermined setpoint or threshold value. This setpoint is typically adjustable based on the specific application and the rated current of the protected equipment.

Overload Detection: If the measured current exceeds the setpoint value, the overload relay detects it as an overload condition. The excessive current could be due to factors like motor overloading, short-term surge currents, or faults in the system.

Trip Mechanism Activation: Once an overload condition is detected, the trip mechanism of the overload relay is activated. The trip mechanism may use mechanical or electronic means to initiate the protective action.

Contact Operation: When the trip mechanism is activated, it causes a set of contacts within the overload relay to change their state. These contacts can be normally closed (NC) or normally open (NO) depending on the specific relay design.

Protective Action: The change in contact state can be utilized to stop or interrupt the power supply to the protected equipment. For example, the contacts may be connected in series with a control circuit, such as a motor starter, and when they open, they de-energize the circuit, shutting down the equipment.

Reset and Restart: After the overload condition is resolved, the overload relay can be manually or automatically reset to allow normal operation to resume.

How an Overload Relay Works?

An overload relay is a device that detects and protects electrical equipment, such as motors, from damage caused by excessive current or overloads. It works by monitoring the current flowing through a circuit and taking action when the current exceeds a predetermined threshold.

Here is a general overview of how an overload relay works

Current Sensing: The overload relay incorporates a current sensing device, such as a bimetallic strip or a current transformer, which is placed in series with the circuit carrying the current to be monitored. The sensing device detects the current flowing through the circuit.

Thermal Heating: When the current passes through the current sensing device, it generates heat due to the I^2R (current squared times resistance) effect. This heat causes the temperature of the sensing device, such as the bimetallic strip, to rise.

Bimetallic Strip Operation: In the case of a bimetallic strip, it consists of two different metals with different coefficients of thermal expansion bonded together. As the temperature rises, the metals expand at different rates, causing the bimetallic strip to bend or deform.

Contact Operation: The bending or deformation of the bimetallic strip activates a set of contacts within the overload relay. These contacts are normally closed, allowing the current to flow through the circuit uninterrupted. However, when the strip deforms due to excessive heat, the contacts open, interrupting the circuit.

Circuit Interruption: When the contacts open, the power supply to the motor or electrical equipment is disconnected, de-energizing the circuit. This prevents the excessive current from reaching the equipment, protecting it from damage.

Resetting: Once the overload condition is resolved and the current returns to a safe level, the bimetallic strip cools down, returning to its original position. This allows the contacts within the overload relay to close again, re-establishing the circuit.

It’s worth noting that modern overload relays may also incorporate electronic components and microprocessors for more precise monitoring and control. These advanced relays can offer additional features such as adjustable current settings, trip time delays, and communication capabilities for remote monitoring and diagnostics.

Types of Overload Relays

There are several types of overload relays available, each designed to provide protection against overload conditions in different types of electrical equipment.

Thermal Overload Relays

Thermal overload relays are the most widely used type of overload relays. They utilize a bimetallic strip or a heating element that expands and contracts with temperature changes. When the current exceeds the set threshold, the bimetallic strip heats up, causing it to bend and activate the relay contacts, opening the circuit.

Electronic Overload Relays

Electronic overload relays use solid-state electronic components, such as current sensors and microprocessors, to monitor and protect against overloads. These relays offer precise current sensing, adjustable trip settings, and advanced protection features. They may provide additional functionality like motor diagnostics, communication interfaces, and motor control integration.

Magnetic Overload Relays

Magnetic overload relays, also known as magnetic contactors, use an electromagnet to detect excessive current. The coil of the electromagnet is connected in series with the motor circuit. When the current exceeds the set threshold, the electromagnet creates a magnetic field that causes the relay contacts to open, breaking the circuit.

Motor Overload Relays

Motor overload relays are specifically designed for protecting electric motors. They typically integrate overload protection with other motor control functions, such as motor starters or contactors. These relays sense the current and provide motor-specific protection against overloads, phase imbalances, and other faults.

Solid-State Overload Relays

Solid-state overload relays utilize semiconductor devices, such as thyristors or transistors, for current sensing and protection. These relays offer fast response times, high accuracy, and adjustable trip settings. They are commonly used in applications where precise control and protection are required, such as in variable frequency drives (VFDs) or electronic motor controllers.

It’s important to consider the specific requirements of the equipment and the application when selecting the appropriate type of overload relay. Factors such as the type of load, current range, trip characteristics, and environmental conditions should be taken into account. Consulting with electrical engineers or referring to manufacturer specifications can help in choosing the right overload relay for a particular application.

Overload Relay Tripping

Overload relays are designed to trip or open the circuit when they detect an overload condition, thereby protecting electrical equipment from damage. When an overload relay trips, it means that the current flowing through the circuit has exceeded the set threshold, and the relay has activated its protective mechanism.

Some common reasons why an overload relay may trip

Excessive Current

The most common cause of an overload relay tripping is the presence of excessive current. This can occur due to mechanical overload on the equipment, such as a jammed motor or excessive load demand. It can also result from electrical faults, such as short circuits or ground faults, which cause a sudden surge in current.

Prolonged Overload

Overload relays are designed to tolerate a certain level of overload for a limited period. However, if the overload condition persists for an extended duration, the relay will trip to prevent overheating and damage to the equipment. This is a protective measure to ensure the longevity and safe operation of the electrical system.

Incorrect Relay Setting

Overload relays are typically adjustable, allowing for the setting of the current threshold or trip point. If the relay is not set correctly for the specific motor or equipment it is protecting, it may trip prematurely or fail to trip even in the presence of an overload. It’s important to ensure that the relay is properly set according to the manufacturer’s guidelines and the equipment’s specifications.

Faulty Relay or Components

Like any electrical device, overload relays can experience faults or failures. If the relay itself is faulty or if there are issues with its associated components, such as the current sensing device or the contacts, it may trip unexpectedly or fail to trip when necessary. Regular maintenance and inspection are crucial to identify and address any faulty components.

Ambient Temperature

Ambient temperature can affect the operation of overload relays, especially thermal overload relays that rely on temperature sensing elements. High ambient temperatures can cause the relay to trip at lower currents due to increased heat dissipation. On the other hand, extremely low temperatures can affect the performance of the relay. It’s important to consider the temperature conditions in the installation environment and select relays accordingly.

When an overload relay trips, it typically interrupts the power supply to the motor or equipment, causing it to stop or shut down. The cause of the trip should be investigated and resolved before resetting the relay and restoring power to the equipment. If recurrent or frequent tripping occurs, it’s important to assess the system’s capacity, load conditions, and the suitability of the overload relay for the application. Consulting with electrical professionals or experts can help in diagnosing and addressing the underlying issues.

Overload relay protect from short circuits?

OLR, or Overload Relay, is a protective device used in electrical systems to monitor and protect against excessive currents. While OLRs are primarily designed to protect electrical circuits and equipment from overcurrent conditions, they do not directly protect against short circuits.

A short circuit occurs when a low-resistance path is formed between two conductors, resulting in a sudden surge of current. The excessive current flow during a short circuit can cause significant damage to the electrical system, including overheating, equipment damage, and even fires.

To protect against short circuits, a different type of protective device called a circuit breaker or a fuse is used. Circuit breakers and fuses are designed to detect and interrupt the flow of current in the event of a short circuit. They provide a rapid response by quickly opening the circuit, thus preventing further damage.

However, it’s worth noting that some advanced motor protection relays, which may include overload protection features, can also detect and respond to short circuit conditions. These relays are typically used in motor control centers or industrial applications where motor protection is crucial. They employ additional protection functions such as short circuit protection, ground fault protection, and phase unbalance protection, along with overload protection.

In summary, while OLRs are not specifically designed to protect against short circuits, there are other types of protective devices like circuit breakers and fuses that are specifically engineered for that purpose.

Advantages and Disadvantages of Overload Relay

Overload relays offer several advantages in protecting electrical equipment from damage due to excessive current or overload conditions. However, they also have some limitations and potential disadvantages. Here are the advantages and disadvantages of overload relays:


Equipment Protection: The primary advantage of overload relays is their ability to protect electrical equipment, such as motors, from damage caused by overloads. By monitoring the current and tripping the circuit when an overload condition occurs, overload relays help prevent overheating and potential equipment failure.

Cost-Effective Protection: Overload relays are relatively cost-effective compared to other protective devices like circuit breakers or fuses. They provide a targeted protection mechanism specifically designed for overload conditions, without the need for frequent replacement of consumable components like fuses.

Adjustable Settings: Many overload relays have adjustable settings, allowing for customization based on the specific motor or equipment being protected. These settings can be adjusted to match the rated current, trip time, and other parameters, providing flexibility and precise protection tailored to the application.

Trip Delay Feature: Some overload relays offer a trip delay feature, which allows for a temporary overload condition to occur without immediately tripping the relay. This is useful in situations where equipment may experience short-duration spikes in current during startup or transient events.

Integration with Motor Control: Overload relays can be integrated with motor control devices such as contactors or motor starters, providing a comprehensive motor protection and control solution. This integration allows for seamless operation and coordination between the motor control and protection functions.


Limited Protection Scope: Overload relays primarily protect against excessive current or overload conditions. They may not provide protection against other electrical faults such as short circuits, ground faults, or phase imbalances. Additional protective devices like circuit breakers or fuses may be required to provide comprehensive protection.

Inherent Trip Time Delay: Overload relays have a built-in trip time delay to allow for temporary overloads. While this delay is beneficial in certain situations, it may not provide instantaneous protection against severe overloads. High-current conditions that persist for a very short duration may not be detected and tripped by the overload relay.

Sensitivity to Ambient Temperature: Thermal overload relays, in particular, can be sensitive to ambient temperature variations. High ambient temperatures can cause the relay to trip at lower currents, affecting the overall system’s performance. Extreme cold temperatures can also impact the operation of the relay.

Maintenance and Calibration: Overload relays require periodic maintenance and calibration to ensure their proper functioning. The contacts, sensing elements, and adjustment mechanisms should be inspected and serviced regularly to maintain reliable protection.

Compatibility Issues: Selecting the appropriate overload relay that is compatible with the specific motor or equipment is crucial. Incorrect selection or mismatched parameters may lead to unreliable protection or false tripping.

Overall, overload relays provide essential protection for electrical equipment against overload conditions. While they have limitations and considerations, proper selection, installation, and maintenance can ensure their effective operation in safeguarding electrical systems.

Application of Overload Relay

Overload relays find application in various industries and electrical systems where protection against overloads is necessary.

Motor Protection: One of the primary applications of overload relays is motor protection. Electric motors are used in a wide range of applications, such as pumps, compressors, conveyors, and industrial machinery. Overload relays are connected in series with the motor circuit to monitor the current and protect the motor from damage caused by overloads. They help prevent overheating, winding insulation failure, and motor burnout.

HVAC Systems: Overload relays are commonly used in heating, ventilation, and air conditioning (HVAC) systems. HVAC equipment, such as air conditioners, chillers, and fans, rely on motors for operation. Overload relays protect these motors from overload conditions, ensuring reliable and efficient performance of the HVAC system.

Industrial Machinery: Overload relays are employed in various types of industrial machinery, including pumps, compressors, conveyors, and production lines. These machines often have electric motors that require protection against overloads. Overload relays help prevent damage to the motors and ensure the smooth operation of the machinery.

Commercial Appliances: Overload relays are used in commercial appliances like refrigerators, freezers, dishwashers, and laundry machines. These appliances often have electric motors that may experience overload conditions due to mechanical issues or excessive load demand. Overload relays provide protection to prevent motor damage and extend the lifespan of the appliances.

Control Panels and Motor Starters: Overload relays are commonly integrated into control panels and motor starters, which are used to control the operation of motors. These relays provide essential motor protection within the control panel or motor starter enclosure, ensuring the safe and reliable functioning of the motor control system.


What is the overload relay and how does it work?
An overload relay, also known as a relay switch, is a device that opens the circuit in the event of an electrical, thermal, or power overload. When mounted with a contactor they create a motor starter. Overload relays are normally closed, meaning they only open if they experience an overload.

What is the formula for the overload relay?
Thermal overload relay setting = 120% x full load current (phase) max. Thermal Overload Relay Setting = 120% x 4 = 4 amps.

What is the range of the relay?
Although typical control relays operate in the 5 ms to 20 ms range, relays with switching speeds as fast as 100 μs are available. Reed relays that pick up at low currents and switch quickly are suitable for controlling small currents.

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By Aditya

Hi, I’m Aditya Sharma, a professional blogger from Gurgaon, India and I launched this blog called aadityacademy on July 2021. aadityacademy.com is a mechanical Project-oriented platform run by Aditya sharma and I got the motivation to start aadityacademy blog after seeing less technical education information available on google.

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