What is Over Current Relay and How Does It Work?

The overcurrent relay is defined as the relay, which operates only when the value of the current is greater than the relay setting time.

An overcurrent relay is a protective device used in electrical systems to detect and respond to excessive current flowing through a circuit. It is designed to protect the equipment and wiring from damage caused by overloads or short circuits. The relay operates by sensing the current in the circuit and activating a trip mechanism when the current exceeds a predetermined threshold.

How Does an Overcurrent Relay Works?

Current Sensing: The overcurrent relay is connected in series with the circuit it is intended to protect. It has a current transformer or a current sensor that measures the current flowing through the circuit. The current sensor generates a proportional signal, which is used by the relay for further processing.

Current Setting: The relay has an adjustable current setting, which is set based on the maximum allowable current for the protected circuit. This setting determines the threshold at which the relay should trip and disconnect the circuit.

Time-Current Characteristic: The overcurrent relay has a time-current characteristic curve, also known as the trip curve. This curve defines the relationship between the magnitude of the current and the time it takes for the relay to operate and initiate a trip. The trip curve is typically chosen based on the characteristics of the circuit being protected and the desired level of protection.

Operation: When the current exceeds the set threshold, the relay compares the measured current with the trip curve. If the current is above the curve, the relay initiates a trip signal. The trip signal can be used to open a circuit breaker or activate other protective devices, disconnecting the faulty circuit from the power supply.

Time Delay: Overcurrent relays often include an adjustable time delay feature. This delay allows the relay to tolerate temporary current surges, such as inrush currents during motor starting, without tripping. The time delay is set based on the specific characteristics of the circuit and the type of loads connected.

Resetting: Once the fault or overload condition is cleared, the overcurrent relay can be manually or automatically reset to restore power to the circuit. Some relays have an automatic reset feature that waits for a predetermined time before re-energizing the circuit to ensure that the fault has been cleared.

Overcurrent relays are commonly used in electrical distribution systems, motor control circuits, and various industrial applications. They provide an important layer of protection by detecting abnormal current conditions and taking appropriate actions to prevent damage to the equipment and ensure the safety of the electrical system.

Types of Overcurrent Relay Used in Electrical Substations

Overcurrent relays are protective devices used in electrical systems to detect and respond to abnormal current conditions. There are several types of overcurrent relays, each designed to address specific applications and fault conditions. Here are some commonly used types of overcurrent relays:

Instantaneous Overcurrent Relay: This type of relay operates based on the magnitude of the current and provides a quick response to faults. When the current exceeds a preset threshold, the relay trips instantaneously, initiating a protective action.

Inverse Time Overcurrent Relay: These relays operate based on the duration of the current above a specific threshold. The tripping time is inversely proportional to the magnitude of the fault current. In other words, higher fault currents cause faster tripping times.

Definite Time Overcurrent Relay: Definite time relays operate with a fixed time delay after the current exceeds the set threshold. They provide a constant time delay regardless of the magnitude or duration of the fault current.

Directional Overcurrent Relay: Directional overcurrent relays are used to protect specific sections of a power system. These relays consider the direction of current flow and operate only when the current flows in a specific direction, indicating a fault in the protected zone.

Ground Overcurrent Relay: Ground overcurrent relays are designed to detect faults between the power system conductors and the ground. They are used to protect against ground faults and can operate based on either instantaneous or inverse time characteristics.

Differential Overcurrent Relay: Differential relays compare the currents entering and leaving a protected zone. If there is a significant difference between the two currents, indicating a fault, the relay trips to isolate the faulty section.

Thermal Overload Relay: Thermal overload relays are used to protect motors and other equipment from overheating due to excessive current flow. These relays have a built-in thermal element that heats up with the current flow and triggers a trip when a certain temperature is reached.

It's important to note that the specific features and characteristics of overcurrent relays can vary based on the manufacturer and the intended application. Additionally, some relays may incorporate multiple functions, combining overcurrent protection with other protective elements such as voltage or frequency monitoring. Different Types of Relays and Their working Procedure are discussed in detail in another episode.