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What are Monitoring Relays? Working and Function

Monitoring relays
Monitoring relays
Resource: https://www.designworldonline.com

Monitoring relays are important protection devices. When installed in an electrical system, they provide protection by constantly monitoring the system parameters and switching off power in case of any abnormal condition. How do these relays work and what are their different types? See below.

What are Monitoring Relays?

A monitoring relay, as the name suggests, is a type of protection relay that is used to monitor various conditions of an electrical system. In other words, it is an electrical switch that is triggered when a certain preset parameter is exceeded. That way, the relay can provide protection by switching off the system in case of any abnormal condition.

A typical monitoring relay, whether a line or motor monitoring relay, is a combination of two main components – a sensing circuit and a trip element. The sensing element is used to sense the parameter that is to be monitored, while the trip element is used to provide the switching action.

Monitoring relays are also usually provided with user-adjustable set points, which can be used to tailor the protection to the specific needs of the electrical system. These include knobs and screws or, in the case of a digital relay, some buttons and a backlit screen. Important features may include:

  • Time delay – to avoid false trips due to momentary surges or transients
  • Manual/auto reset – to allow the electrical system to be manually reset after a trip
  • Remote reset – to allow the electrical system to be remotely reset after a trip
  • Alarm contacts – to provide an alarm signal in case of a trip
  • Visual indicators – to provide a visual indication of the status of the relay
A voltage monitoring relay
A voltage monitoring relay
Resource: https://www.electriciansforums.net

Monitoring Relay Categories

Monitoring relays fall into different categories. Their classification is generally based on the parameter that they are designed to monitor, which can vary from voltage and current to temperature. The most common types of monitoring relays are:

  • Voltage monitoring relays
  • Phase monitoring relays
  • Current monitoring relays
  • Frequency monitoring relays
  • Temperature monitoring relays
  • Level monitoring relays

1. Voltage Monitoring Relay

Voltage monitoring relays are designed to protect the electrical system against voltage surges. They are triggered when the voltage exceeds or drops below the preset level. As such, these devices are available to detect either high voltage or low voltage but mostly both high and low voltages.

For example, an undervoltage monitoring relay will trip when the voltage drops below a certain point. On the other hand, an overvoltage relay will protect equipment from unexpected and unwanted voltage spikes by opening the circuit. The most common type of overvoltage relay is the surge protection relay.

It’s also good to note that there are single phase and 3 phase voltage relays. A single phase voltage relay is normally used in low power systems such as those in domestic applications. Three phase voltage relays are common in industrial and commercial applications.

2. Phase Monitoring Relay

As the name suggests, phase monitoring relays are designed to monitor and protect against phase reversal, phase loss, and phase unbalance. They work by constantly checking the phase difference between two or more phases and tripping, or opening the circuit, when a phase anomaly is detected.

A phase loss occurs when one of the phases in a 3-phase system is lost or disconnected. This can happen due to a variety of reasons such as loose connections, blown fuses, or lightning strikes. A phase unbalance occurs when the currents in the three phases are not equal.

A phase reversal, on the other hand, happens when the phases are reversed. This can be caused by incorrect wiring. Phase monitoring relays are commonly used in 3-phase systems or industrial motor applications.

3. Current Monitoring Relay

Current monitoring relays protect the equipment in an electrical system against overcurrent or undercurrent events. Just like the voltage sensing types, current relays are triggered when the current exceeds or drops below the preset level.

An undercurrent situation may occur when a running motor is unloaded. Also, if there are faults within an electrical system. Either way, that can cause the connected electrical equipment to get damaged.

An overcurrent, on the other hand, is an abnormal condition that happens when the current exceeds the maximum limit. This can happen due to a short circuit or equipment failure. When this occurs, the current monitoring relay will immediately trip and disconnect the equipment from the power supply.

4. Frequency Monitoring Relay

The frequency monitoring relay is designed to protect against frequency anomalies. These types of monitoring devices are mostly used in alternating current (AC) systems where the monitoring of the power frequency is essential.

A frequency anomaly can occur due to a sudden change in the load or a system malfunction. Regardless of the cause, a frequency irregularity can damage equipment if left unchecked. This is where a frequency monitoring relay comes in as it will immediately trip when an abnormal frequency is detected.

Frequency monitoring relays can are also available for underfrequency, overfrequency, and differential frequency applications. Some industrial appliances, such as motors, depend on a constant frequency to function properly.

Thus, a sudden change in the frequency can cause changes to an industrial process due to a change in the motor’s speed. The frequency relay is meant to protect against this type of event.

5. Temperature Monitoring Relay

A temperature monitoring relay is primarily designed to protect against overheating. It does so by constantly monitoring the temperature of a system and that of the equipment it’s meant to protect. These devices are commonly used in industrial applications where extreme temperatures are a concern.

Temperatures may increase due to various reasons such as a sudden change in the load or a system malfunction. It could also be that the ambient temperature is too high.

A temperature relay will trip when the temperature exceeds the pre-determined level. This will help prevent equipment damage due to overheating.

6. Level Monitoring Relay

A level monitoring relay is commonly used in a variety of industries where the need to constantly monitor liquid levels is essential. These devices are used in tanks, reservoirs, and other similar applications.

A level monitoring relay may be used to prevent pumps from running dry and getting damaged, for example, or it may be installed to help with leak detection.

In these applications, the relay will trigger an alarm or shut down the system when the liquid level reaches a certain point, among other conditions. This is important to prevent equipment damage due to overfilling or low levels of liquids.


Monitoring relays play a crucial role in protecting electrical systems and equipment against various anomalies. By constantly monitoring the voltage, current, phase, frequency, and temperature of a system, these devices help to ensure that the equipment is operating within safe limits. In the event of an anomaly, the monitoring relay will immediately trip, thus helping to prevent further damage.


I am William, Electrical Engineering Author. Dedicated to writing technical articles on Timer Relay, Monitoring Relay, Surge Protection Device and other electrical devices. With 7 years of writing experience, I am committed to providing accurate and in-depth expertise to my readers.

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