Overload Current Relay Selection Guide

In today’s increasingly complex electrical systems, protecting equipment from overcurrent conditions is crucial. Overload current relays play a vital role in safeguarding electrical circuits, preventing damage from excessive current flow. These relays are essential components in industrial, commercial, and residential settings, offering an automatic response to abnormal current levels by disconnecting the power supply.

Choosing the right overload current relay for your application can be a daunting task, especially with the wide variety of options available on the market. In this guide, we will cover everything you need to know about overload current relays, including key features, types, and specifications to help you make an informed decision.

What is an Overload Current Relay?

An overload current relay is a protective device designed to monitor the current flowing through an electrical circuit. If the current exceeds a preset limit for a specified duration, the relay will trip, signaling the system to disconnect power to prevent damage to the connected equipment or wiring. Overload relays are commonly used in motor protection, industrial equipment, and transformers.

While they are primarily designed to handle overloads—conditions where current exceeds the normal operating range but does not necessarily result in immediate short circuits—these relays also contribute to long-term system reliability. They prevent overheating, insulation degradation, and excessive wear on electrical components.

Key Features of Overload Current Relays

When selecting an overload current relay, it is important to consider several key features to ensure optimal performance and protection. Here are the most critical features to evaluate:

• Current Rating

The most fundamental characteristic of an overload relay is its current rating. The current rating defines the maximum current the relay can handle without tripping. Overload current relays typically allow some level of overcurrent (within limits) for short periods, but they will trip if this threshold is exceeded for an extended duration.

It is essential to select a relay with a current rating suitable for the application. For example, if you are protecting a motor, the relay’s rating should align with the motor’s full load current.

• Trip Class

The trip class of an overload current relay refers to the time it takes for the relay to trip under overload conditions. Different applications require different trip classes based on the type of load being protected. Trip classes are generally categorized as follows:

Class 5: Quick trip time (5 seconds), used in applications where rapid response is necessary.

Class 10: Trips within 10 seconds, often used for small motors.

Class 20: Standard trip time for most general-purpose motors, trips within 20 seconds.

Class 30: Longer trip time, used for heavy-duty applications with high thermal capacity, such as large motors.

Selecting the correct trip class ensures that the relay responds appropriately to overloads without tripping too quickly or too slowly for the application.

• Adjustable Settings

Many overload current relays come with adjustable settings that allow you to fine-tune the trip point based on your system’s requirements. For example, adjustable thermal relays let you adjust the current threshold at which the relay trips, offering flexibility in different load conditions. These adjustments ensure the relay can be tailored to the exact needs of the motor or equipment it is protecting.

Adjustable features are particularly useful in industrial applications, where different machines may have varying load characteristics and require customized protection.

• Reset Mode

Overload relays generally come with two reset options: manual and automatic. In manual reset mode, an operator must physically reset the relay after it trips, which provides an extra layer of safety by ensuring the fault is checked before restarting the system.

Automatic reset relays, on the other hand, will reset themselves after the overload condition subsides. These are useful in situations where immediate reactivation is needed, though they may pose a risk if the underlying fault is not properly addressed.

Read Also: Thermal Overload Relay Types and Use Cases

• Thermal vs. Magnetic Protection

Overload current relays can be categorized into two main types based on their protection mechanism: thermal and magnetic.

Thermal Overload Relays: These relays use a bimetallic strip that bends and trips the relay when excessive heat is generated due to overcurrent. They are commonly used for motor protection and offer a time-delayed trip response based on the heat generated by the overload.

Magnetic Overload Relays: These relays trip instantaneously when the current exceeds the preset limit, using electromagnetic force rather than heat. Magnetic overload relays are better suited for applications where immediate tripping is necessary, such as in short-circuit protection.

• Phase loss sensitivity

In three-phase electrical systems, phase loss or imbalance can cause significant damage to motors and equipment. Many overloads current relays are designed with phase loss sensitivity, which detects a phase failure and trips the relay to prevent motor damage. If you are operating in a three-phase environment, it is crucial to select a relay that can respond effectively to phase loss conditions.

• Ambient Temperature Compensation

The performance of an overload current relay can be affected by the ambient temperature, especially in thermal relays. Ambient temperature compensation ensures that the relay functions correctly regardless of the surrounding temperature, maintaining accurate protection levels.

This feature is particularly useful in industrial environments where temperature fluctuations are common and can influence the relay’s response to overload conditions.

Types of Overload Current Relays

There are several types of overload current relays available, each with its specific use case. Understanding the different types will help you choose the best relay for your application.

• Thermal Overload Relays

As mentioned earlier, thermal overload relays rely on the heat generated by the current to trigger a trip. These relays are ideal for applications where a time-delayed trip is required, such as in motors that experience temporary current surges during startup. Thermal relays are cost-effective and widely used in various motor protection applications.

• Magnetic Overload Relays

Magnetic overload relays are instantaneous and respond immediately to overcurrent conditions. They are ideal for applications where a fast response is needed to protect against short circuits. Magnetic relays are commonly used in circuit breakers and systems where precision tripping is crucial.

• Electronic Overload Relays

Electronic overload relays offer advanced protection by using microprocessors to monitor current conditions and respond more precisely to overloads. These relays are highly adjustable and offer features such as phase imbalance detection, motor temperature monitoring, and communication interfaces for integration into control systems. They are more expensive but offer greater accuracy and flexibility, making them suitable for critical industrial applications.

• Electromechanical Overload Relays

Electromechanical overload relays use both thermal and magnetic principles to provide protection. These relays offer both time-delay and instantaneous trip options, making them versatile for a wide range of applications.

Specifications to Consider

When selecting an overload current relay, it’s important to evaluate its specifications to ensure compatibility with your system. Key specifications include:

• Rated current (in): The maximum continuous current the relay can handle.
• Voltage rating (Un): The operating voltage range of the relay.
• Contact configuration: Determines whether the relay can open or close contacts (normally open or normally closed).
• Breaking capacity: The maximum current the relay can interrupt without damage.
• Operating temperature range: Ensures the relay functions properly in your specific environmental conditions.

Conclusion

Choosing the right overload current relay is essential for protecting your electrical systems from damage caused by overcurrent conditions. By carefully considering key features such as current rating, trip class, reset mode, and protection mechanism, you can select the optimal relay for your specific needs. Understanding the different types of overload relays—thermal, magnetic, electronic, and electromechanical—will help you match the relay to your application.

For reliable and high-quality overload current relays, Lauritz Knudsen Electrical & Automation offers a wide range of products designed to meet the needs of residential, commercial, and industrial applications. Their relays provide superior performance, ensuring the safety and longevity of your electrical systems. Trust Lauritz Knudsen for top-tier electrical solutions that deliver both quality and reliability.