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    Monitoring Relay Basics: Definition, Working Principle, Functions & Wiring

    In industrial operations, ensuring the smooth functioning of electrical equipment is paramount. One of the critical components in this effort is the phase monitoring relay. In this post, let's explore how this relay functions and why it is vital for maintaining the integrity of electrical systems.

    What is a monitoring relay?

    A monitoring relay is a type of relay protection device for phase and voltage monitoring of a power system or equipment. It protects 3-phase devices from any potential damage caused by phase loss or sequence change.

    Monitoring relay definition

    How does it work?

    Phase failure relays operate on the principle of monitoring the voltage levels of each phase in a three-phase electrical system. Internally, when the phase sequence is correct, and there is no phase failure, the relay operates smoothly as the voltage passing through is sufficient to activate the relay's NO contact to close, thus conducting the circuit and allowing the load to function properly. In the event of a phase failure or voltage imbalance, the voltage supplied is insufficient to trigger the relay.

    Consequently, the relay remains in an off state when the NO contacts remain open, disrupting the external circuit and preventing the system from operating. By interrupting the power flow, these relays help prevent motor burnouts, overheating, and other electrical malfunctions that can occur due to unstable voltage conditions.

    What can a monitoring relay do?

    The relay can be used to monitor important electrical parameters in the power supply system including phase failure/loss, phase reversal, three-phase voltage imbalance, phase asymmetry, overvoltage and undervoltage.

    Phase Loss Protection

    Phase loss protection involves detecting when a phase in a three-phase electrical system experiences a voltage drop due to reasons like circuit breaks or blown fuses, and then cutting off the circuit to prevent motors from operating against phase failure, which could lead to burnout.

    Phase Sequence Protection

    Additionally, phase sequence protection ensures that the three-phase power supply is correctly sequenced, preventing motors from running in reverse or malfunctioning due to incorrect phase sequences. This feature is crucial for equipment like elevators and compressors that should not operate in reverse.

    Over/Under Voltage Protection

    What's more, phase monitoring relays also offer overvoltage and undervoltage protection. They can safeguard against voltage imbalances and out-of-range voltages in three-phase systems, preventing issues such as overheating, increased vibration, and other motor problems caused by voltage irregularities.

    Monitoring relay functions

    Wiring

    The wiring of phase monitoring relays involves connecting L1, L2, and L3 of the relay to the three-phase power supply. Pins NO and COM are connected in series in the control circuit to control an AC contactor.

    When the 3-phase power supply has appropriate voltages and sequencing, the NO contact will close to energize the contactor coil, completing the circuit. In this case, the monitoring relay indicator will turn green.

    If any phase line is disconnected, the Normal indicator light goes off and the Phase Loss indicator turns red. If the phase sequence is incorrect, the indicator light will turn red. In this condition, the NO contact remains open and cannot conduct, preventing the contactor from triggering and the motor from starting.

    Monitoring relay wiring

    In conclusion, phase monitoring relays are essential components in safeguarding electrical systems against phase variations and voltage irregularities. Implementing phase monitoring relays is a proactive measure toward maintaining the integrity and reliability of electrical installations.

    At ATO.com, we currently offer different phase and voltage monitoring relays for 3-phase power as well as single-phase and DC power systems. As a leading relay manufacturer, ATO understands the vital significance of selecting the right relay for your system.

    Monitoring relay
    If you have any questions, please watch the following video for further learning.

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