What is a Three Phase Voltage Monitoring Relay?

A three phase voltage monitoring relay, or phase monitor, is a common type of monitoring relay in the protection of high voltage and high current electrical systems. It monitors the voltage of the three phases and trips if any of them is outside of a predetermined range. This prevents damage to equipment. If you’re looking to use a 3 phase voltage monitoring relay, here’s what you need to know about its application and installation.

What you’ll learn:

• What is a phase monitor?
• What does a phase monitor do?
• Phase monitoring relay working principle
• 3 phase monitoring relay price and installation rules

What is a Phase Monitor?

Also called a phase sensing relay or phase failure relay, a 3 phase voltage monitor is an electrical device that’s used to protect equipment in a 3 phase system.

A three phase network carries three alternating voltages of the same frequency, but with different phases. These phases must balance in order for the system to function properly.

If one of the phases in a three-phase system isn’t working correctly, it can cause an imbalance in the system. This can lead to equipment damage and power outages.

A three phase voltage monitor helps to prevent this by monitoring the voltages of all three phases and tripping if any of them are outside of a predetermined range. This keeps a check on the system’s stability and prevents damage to equipment.

Phase Monitoring Relay Working Principle

A 3 phase monitoring relay works by constantly checking the voltages of all three phases in a system. If any of the phases goes outside the acceptable range, the relay will trip and disconnect the equipment from the power supply.

The predetermined voltage ranges that will cause the relay to trip can be customized depending on the needs of the system. For example, you can set the phase relay to trip if any of the phases fall below 90% of the nominal voltage or above 110% of the nominal voltage.

The phase monitoring relay can also be set to trip if the phase difference between any two phases exceeds a certain value. This helps to prevent damage from phase imbalance.

When the relay trips, it may also send a signal to an alarm or control panel so that the problem can be addressed. Let’s now see why the 3 phase voltage monitoring relay is such a vital device when using a three-phase electrical system.

Three Phase Monitoring Relay Function

A three phase voltage monitor is a vital piece of equipment in any three phase system. It helps to prevent damage from overvoltages and undervoltages, and also protects against phase unbalance, among other functions. The three phase monitoring relay function is explained in summary below.

Overvoltage Monitoring Relay

An overvoltage is when the voltage in a system exceeds the maximum voltage that the equipment can handle. This can damage equipment and cause power outages. The phase voltage monitoring device trips if any of the phases are above the predetermined maximum voltage, preventing damage from overvoltage.

Undervoltage Monitoring Relay

The 3 phase voltage monitor trips if any of the phases are below the predetermined minimum voltage, preventing damage from undervoltage. An undervoltage situation can occur when the voltage in a system drops below the level that is needed to power the equipment.

Phase Sequence Protection Relay

Incorrect phase sequences can cause motor loads to rotate in the wrong direction, among other problems. Installing a phase voltage monitor can help to prevent this by tripping if the phase sequence is incorrect.

Phase Unbalance Protection Relay

A phase imbalance can occur when one of the phases in a three-phase system isn’t working correctly. This can lead to equipment damage and power outages. A three phase voltage monitoring relay trips if the phase difference between any two phases exceeds a certain value, preventing damage from phase imbalance.

Phase Loss Relay Protection

In a three phase system, if one of the phases is lost, it can cause a motor to fail to start or, if running, to stall. Such situations can cause the motor to be overloaded leading to damage. A three phase voltage monitor can help prevent that.

Three Phase Monitoring Relay Applications

The 3 phase voltage monitoring devices are used in a variety of industries and applications. Some common examples include:

• Industrial machinery
• HVAC systems
• Pumps and compressors
• Motors and generators
• Solar power systems

In these applications the 3 phase voltage monitor is used to protect equipment from damage due to overvoltages, undervoltages, and other voltage irregularities such as phase imbalance, phase sequence, and phase loss.

3 Phase Monitoring Relay Price

Three phase voltage monitoring relays are available from a variety of manufacturers. Prices can range from $50 to $500, depending on the features and specifications that you require.

As you can see, the three phase voltage monitoring relay price range is generally higher than that of a single phase voltage monitoring relay. This is because the 3 phase relay serves high voltage loads and, therefore, must be priced accordingly.

When sizing this type of monitoring relay, it’s important to ensure the voltage and amperage ratings are appropriate for your application. Some important considerations include:

• Whether the voltage monitor is UL listed,
• The maximum current rating,
• The environmental conditions (such as temperature and humidity) that the voltage monitor can withstand, and
• The size and weight of the 3 phase monitoring relay.

Conclusion

In a 3 phase electrical network, voltage or phase problems can cause damage to the equipment in the system. Most of the time, the equipment includes motors, pumps, compressors etc. that have a high starting torque. A three phase voltage monitoring relay is used in order to protect the equipment from these voltage irregularities. You can use the relay in a broad range of industries, including solar, HVAC, data centers, and most industrial applications.