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    Monitoring Relays

    Monitoring relays are electrical devices that are used to monitor various parameters in a power system, such as voltage, current, temperature, and frequency, and trigger an alarm or action if the values go beyond a preset threshold. They are designed to detect abnormal operating conditions in a system and alert operators or take corrective action to prevent damage to equipment or injury to personnel.

    RS New Zealand offers a selection of monitoring relays from top brands such as Siemens, Schneider Electric, ABB, Dold and more.

    Types of Monitoring Relays

    There are many types of monitoring relays, each designed to monitor a specific parameter. Here’s a breakdown of each type:

    Voltage Monitoring Relays

    Voltage monitoring relays are used to monitor the voltage levels in a circuit and trigger an alarm or action if the voltage levels go beyond a preset threshold.

    If the voltage falls below the lower threshold or exceeds the upper threshold, the voltage detection relay will trigger a pre-defined action (such as alarm, circuit disconnection, switching to backup power etc).

    Current Monitoring Relays

    Similar to voltage monitoring relays, current monitoring relays and current sensing relays are used to monitor the current levels in a circuit and trigger an alarm or action if the current levels go beyond the set thresholds for both overcurrent and undercurrent conditions.

    For instance, a 240V current sensing relay could be installed in a circuit powering a high-wattage heater. If the current draw from the heater exceeds the safe limit for the circuit (overload), your current monitoring relay could trip a circuit breaker to prevent overheating and potential fire hazards.

    Phase Monitoring Relays

    These specialised relays, often called 3-phase relays, are important for ensuring proper operation in 3-phase AC power systems. While some phase sensing relays can function in single-phase applications, their core functionality revolves around monitoring the three key aspects specific to 3-phase systems:

    • Phase Presence: Verifying that all three phases (voltage and current) are present and active.
    • Phase Sequence: Ensuring the correct sequence of the phases (typically Line-Line-Line or Line-Neutral-Line). Incorrect phase sequence can damage equipment.
    • Phase Balance: Monitoring the balance of voltage and current levels across all three phases. Unbalanced loads can lead to inefficiencies and potential equipment overheating.

    Phase monitoring relays often trigger alarms or initiate corrective actions when any of these conditions fall outside acceptable parameters. For simpler applications where only detecting a missing phase is crucial, a phase failure relay might be a more cost-effective solution.

    Frequency Monitoring Relays

    Frequency monitoring relays monitor the frequency of an AC (Alternating Current) power signal. They are crucial in maintaining grid stability and preventing equipment damage, as deviations from the standard frequency can indicate power system issues or problems with generators.

    Frequency monitoring relays can trigger actions like load shedding (disconnecting non-critical loads) or generator adjustments to maintain grid stability.

    Temperature Monitoring Relays

    Temperature monitoring relays monitor the temperature of equipment or processes. They often utilise temperature sensors (like thermistors or RTDs) to measure temperature and trigger actions based on preset thresholds.

    They are widely used in various industries to prevent overheating and equipment failure. Examples include monitoring motor bearings, transformer oil temperature, or controlling heating elements in industrial processes.

    Power Factor Monitoring Relays

    These monitoring relays measure the power factor, a value indicating how efficiently electrical power is being used in an AC circuit. A low power factor signifies wasted energy. Generally, power factor monitoring relays are used to improve energy efficiency and reduce electricity costs.

    By monitoring power factor, corrective actions can be taken, such as installing power factor correction capacitors, to optimise energy usage.

    Insulation Monitoring Relays

    Insulation monitoring relays (IMRs) continuously monitor the insulation resistance between electrical conductors and ground. They detect deterioration in insulation quality before it leads to a ground fault or equipment failure.

    IMRs are crucial for ensuring safety and reliability in applications with high-voltage equipment, transformers, and motors. Early detection of insulation issues allows for preventive maintenance and avoids potential breakdowns.

    Earth Leakage Monitoring Relays

    Earth leakage monitoring relays (ELMRs) specifically monitor for small current leakage to ground in an electrical circuit. This leakage can indicate potential faults or safety hazards. ELMRs are particularly important in environments with increased risk of electrical shock, such as swimming pools, construction sites, and medical facilities. They can quickly disconnect power in case of a ground fault, preventing serious accidents.

    Multi-Function Monitoring Relays

    Multi-function monitoring relays such as power monitoring relays are versatile relays that combine the functionalities of several monitoring types into a single unit. They can often monitor voltage, current, temperature, power factor, and even some combinations of these parameters.

    Multi-function monitoring relays offer a cost-effective solution for applications requiring comprehensive monitoring of various electrical parameters. They simplify installation and maintenance by reducing the need for multiple single-function relays.

    Speed Monitoring Relays

    Speed monitoring relays monitor the rotational speed of motors or other rotating equipment. They typically utilise sensors like tachometers to measure speed and trigger actions based on preset limits.

    Speed monitoring relays are essential for maintaining proper operation and preventing damage in machinery with critical speed requirements. They can be used to ensure motors operate within safe speed ranges, detect potential bearing issues, or initiate automatic shutdowns in case of overspeed conditions.

    Where are Monitoring Relays Used?

    Monitoring relays are used in a wide range of applications where it is important to monitor and protect electrical systems and equipment. They are commonly used in industrial settings, such as manufacturing plants and data centres, where reliable power is critical. They can also be used in residential or commercial settings to monitor electrical systems and equipment.

    In addition to monitoring parameters, some monitoring relays also have built-in communication capabilities, allowing them to send alerts or data to a remote monitoring system or operator. This can help operators quickly identify issues and take corrective action, improving system reliability and reducing downtime.

    Motor Control

    Monitoring relays are commonly used to protect motors from damage caused by overloads, phase loss, or phase imbalance. They can also monitor the temperature of the motor and trigger an alarm or action if it exceeds a preset threshold.

    Power Distribution

    Monitoring relays are used in power distribution systems to monitor voltage and current levels and trigger alarms or actions if they go beyond a preset threshold. This can help prevent damage to equipment and ensure that power is distributed safely and efficiently.


    Monitoring relays are used in heating, ventilation, and air conditioning (HVAC) systems to monitor temperature and humidity levels and trigger alarms or actions if they go beyond a preset threshold. This can help ensure that HVAC systems are operating efficiently and effectively.


    Monitoring relays are used in lighting systems to monitor voltage and current levels and trigger alarms or actions if they go beyond a preset threshold. This can help prevent damage to lighting fixtures and ensure that lighting systems are operating safely and efficiently.

    Renewable Energy

    Monitoring relays are used in renewable energy systems, such as solar and wind power, to monitor voltage, current, and frequency levels and trigger alarms or actions if they go beyond a preset threshold. This can help ensure that renewable energy systems are operating efficiently and effectively.

    How to Order a Monitoring Relay from RS?

    RS New Zealand is a trusted supplier and distributor of monitoring relays in NZ. Order your monitoring relay from our leading brands like Siemens, Carlo Gavazzi, Schneider Electric, Eaton, ABB, and more.

    Uncover the extensive selection of monitoring relays offered by RS NZ. Our detailed product descriptions and downloadable datasheets empower you to make informed choices. With our user-friendly filter system based on monitoring application, electrical phrase, supply voltage and more, finding the perfect monitoring relay for your needs is a breeze.

    Can’t find what you need? You can also explore other electrical relays to meet your specific requirements.

    Delivery Information for New Zealand

    At RS NZ, we prioritise getting your monitoring relays to you quickly. Check out our delivery services and fees for more information.

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