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Relay protection classified by signal source

Fibre interconnection and data room reference Published 01 Mar 2026

Use this reference to prepare product questions for FD SOLUCIONES. Check interfaces, capacity, protection and installation requirements against the current product documentation.

Protective relays are classified by the type of signal they respond to, such as current, voltage, power, reactance, impedance, or frequency, and further divided into over and under relays based on threshold conditions. Classification by Signal SourceProtective relays operate by sensing changes in an electrical quantity. Based on the actuating signal, relays can be classified as follows: Current Relays: Operate when the current exceeds or falls below a set value.

Overcurrent relays trip when current is too high, while undercurrent relays respond to abnormally low currents. Voltage Relays: Respond to voltage deviations. Overvoltage relays act when voltage rises above a threshold, and undervoltage relays operate when voltage drops below a set level.

Power Relays: Triggered by changes in active or reactive power. They are used to protect generators, transformers, and feeders from abnormal power conditions. Reactance and Impedance Relays: Measure the ratio of voltage to current to detect faults based on line impedance or reactance, commonly used in distance protection schemes.

Frequency Relays: Respond to deviations in system frequency, protecting against over- or under-frequency conditions. Directional Relays: Detect the direction of current or power flow to ensure correct operation in complex network configurations.

Over and Under Relays

Relays are also categorized by the direction of deviation they respond to: Overrelays: Operate when the measured quantity exceeds a predetermined value. Underrelays: Operate when the measured quantity falls below a predetermined value. For example, an overcurrent relay trips when current exceeds a set limit, while an undervoltage relay trips when voltage drops below a threshold.

Primary and Secondary Relays

Primary Relays: Sensing elements are directly connected to the circuit or equipment being protected. Secondary Relays: Sensing elements receive signals through transformers (current or voltage transformers), allowing isolation from high-voltage circuits and enabling accurate measurement.

Functional Roles

Relays can also be classified by their function in the protection scheme: Main Relays: Directly detect abnormal conditions and initiate tripping of circuit breakers. Auxiliary Relays: Operate under the control of other relays to perform functions like time delays, signal amplification, or alarm activation. Signal Relays: Provide indication of relay operation, trigger alarms, or communicate status to control systems.

This classification ensures that protective relays respond accurately to specific electrical conditions, maintain system stability, and minimize unnecessary outages while providing selective protection for critical components of the power system.

Relay protection classified by signal source - FD SOLUCIONES OPTICAL SYSTEMS [PDF]

Relay protection classified by signal source
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