Differences Between Shunt Relay and Undervoltage Relay

When looking at voltage, shunt relays and undervoltage relays are often used alongside compact key switches for similar purposes. Although these two devices may appear similar physically, they are distinguishable by specific codes displayed on their bodies.

What are the valid differences between shunt relays and undervoltage relays?

Each of these relays has unique functions that determine their operational differences. Below, the functions of these two relays are explained.

Shunt Relay:

The shunt relay is a crucial peripheral device alongside a compact key switch that mechanically disconnects the breaker when force is applied to its terminals. This force is not supplied from within the compact key switch and requires an external power source.

To select the appropriate shunt relay for AC or DC voltages, examine the type of compact key switch in use, and then acquire the suitable shunt relay based on its specifications.

A shunt trip relay, also known as a shunt trip, is used to disconnect the compact key switch in error conditions. This relay is incapable of detecting errors from within the breaker; instead, it utilizes an external power source. For example, in the event of incidents like a fire, thermal sensors detect the heat of the fire, activate the relay coil, and disconnect the breaker. Therefore, the relay uses an external power source to detect faults and is not dependent on the breaker.

What is an Undervoltage Relay?

An undervoltage relay is an automatic device directly connected to the main circuit. In the event of a voltage interruption or loss in the circuit, it automatically deactivates the automatic switch.

Undervoltage Relay Installation Locations:

Undervoltage relays are usually installed inside compact key switches in specific locations. Breakers or compact key switches often provide the possibility of installing one or two relays, but if more are needed, it is possible to request custom production of breakers that comply with specific requirements.

Structure and Operation of Relays:

The shunt relay essentially consists of an electric coil and a disconnect unit used for thermal and electromagnetic disconnection. A specific voltage is applied to the coil of the shunt trip relay, causing the current breaker to disconnect.

In emergency conditions, the shunt relay, by applying an electrical force, disconnects the automatic switches to prevent harm to individuals and equipment. This relay acts as a protective unit for the breaker and enables remote control of the breaker.

Compact key switches have a combination of thermal and magnetic sensors for protection. Older generations of these switches used electromechanical trip units with two types, fixed and replaceable.

Various criteria exist for shunt relays and undervoltage relays, selected based on the needs of the electrical circuit.

Difference Between Shunt Relay and Undervoltage Relay

The shunt trip relay functions as an electrical device that, upon the application of immediate pressure, facilitates the opening of the breaker. This relay depends on an external force to act on the breaker and controls the supply voltage.

Shunt relays have normally open (NO) contacts that immediately interrupt the supply voltage after the switching device is disconnected or lost. This relay is mostly used for opening the switching device through an electrical signal or remotely. Additionally, it can be used in parallel for local or control signal-based opening or disconnecting of the switching device.

The shunt relay cannot provide the operation of disconnecting the switching device in the case of voltage drop or voltage interruption. For this purpose, the undervoltage relay is used.

The undervoltage relay is an electrical device that is always powered and requires the application of force to keep the breaker or switching device in a closed state.

Upon power disconnection, through a local button or interlock contacts (arranged in series wires to prevent human errors), the switching device immediately operates and disconnects the breaker.

The shunt relay has a specific function and is recommended for situations requiring breaker trip/disconnection, reacting quickly to voltage drops. This type of voltage drop usually covers up to 80% of the allowable voltage for the circuit. This feature has made it popular, especially when compared to voltage-sensitive breakers like motor feeders.

The main difference between the shunt trip relay and the undervoltage relay lies in their mode of operation. The shunt relay responds to the application of force or electricity, while the undervoltage relay, which is always powered, reacts to the disconnection or reduction of voltage.

Are these relays always powered?

The undervoltage relay, as an electrical device, requires continuous power to detect changes and operate correctly in the event of a voltage drop or loss. Therefore, having a constant power supply is considered part of its functionality. However, in the case of a shunt relay, continuous power may lead to an increase in temperature and potential relay failure. Additionally, it does not require constant power as it operates as expected by applying power when needed.

Can they reconnect the circuit, and is this functionality possible?

In situations where sudden breaker trips need to be avoided, such as when the voltage has dropped, or when a human operator is not available to manually reopen the circuit (a situation associated with reconnecting breakers), a specific type of undervoltage relay is considered. This type of relay utilizes an energy storage unit called a capacitor. By leveraging this capacitor to prevent energy loss, the undervoltage relay uses the stored energy when voltage decreases or disappears, reconnecting the breaker. This reconnection capability, in addition to common features shared with other relays, provides a practical advantage.

Final Thoughts

This article delves into the examination and comparison of shunt relays and undervoltage relays. Each of these relays possesses unique features that are applied based on the requirements of the circuit in use. Furthermore, the discussion has touched upon whether these relays are always powered and if they provide the capability of reconnecting the circuit. The application of these relays in the control and protection of electrical circuits is highly valuable, and the precise characteristics of each have been thoroughly explained.

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