Phase Failure Relay Wiring Diagram

three phase monitoring relay

If the electrical energy is not well managed by the final consumer, there could be losses and serious problems to the loads and user alike. With the recent increase in the use of power in our homes and industries, there is the need to adequately protect our homes, industries, and every other appliance that makes use of electricity. Thus a phase failure relay is extremely important for the protection of the life and properties of the electricity consumers.

One of the common faults faced in industrial plants is overheating and damaging loads due to phase failures. Even though devices like overload relays or circuit breakers are used to protect, electrical loads need something fast and electronic.

What is a phase failure relay?

The phase failure relay is a special type control device that monitors phase sequence, phase loss, phase unbalance, overvoltage, and under-voltage in three-phase electrical systems.

In the electrical market, they are also known as phase protection relay, phase control relay, line monitoring relay, or phase monitoring relay.

How does a phase failure relay work?

The basic function of a phase failure relay is to receive input signals and determines them. If a pre-set value is reached, an output contact will change its position. This contact output is connected to devices such as contactors and switches that can open the circuit. (Or an alarm output is sent to PLC)

How three phase monitoring relay works 1

How three phase monitoring relay works 2

Functions of phase failure relay

Phase failure relay has various types of protection functions. Some of them are:

Phase unbalance

If the supply by the three-phase system is unbalanced due to an uneven distribution of the load, the motor will convert a part of the energy into reactive power. This energy gets lost unexploited; also the motor is exposed to higher thermal stress. Phase unbalance is causing a strong derating of AC motors. Above 5% unbalance it is strongly recommended to shut down the motor. Only monitoring relays (with phase unbalance monitoring function) can automatically stop before the motor is damaged. Reliable monitoring for unbalance extends the life of a motor and prevents expensive damage.

Phase sequence

An incorrect phase sequence applied at start-up or a change in the phase sequence during operation will cause a three-phase motor to run with reverse rotation. Operating in the reverse direction will cause damage to some motors or loads such as pumps, screw compressors, and fans. Operating with an incorrect phase sequence can loosen parts of the machine or workpieces and cause extremely dangerous situations. This can be avoided by permanently monitoring the phase sequence. All you need is a monitoring device and a contactor to switch off the devices.

Phase loss (Phase failure)

A phase loss can result from, for example, a blown fuse, mechanical failure within the equipment, a broken power line, a defect on a transformer winding, or lightning. As soon as one phase is lost, a loaded three-phase motor cannot start or may stall under load. But it might also keep on running asymmetrically. If a motor stalls the electrical resistance is considerably less than in a rotating motor. This causes an increased current of up to 600% of the nominal current of the motor. Such a high current will destroy the motor windings within seconds. Motors should never be started during phase loss. If a motor stalls it must be disconnected immediately.

Voltage monitoring

All electric devices can be damaged by continuous operation at incorrect voltage levels. Voltage peaks can destroy electronic components and, in the worst case, the insulation of an electronic or electric device can be damaged by an electrical breakdown. Determining factors are the voltage level, time, and the resulting over temperature. Under voltage might be the reason for a motor not starting or a contactor not make. Again under-voltage results in heating causing thermal damage and an undefined status of the equipment.

Advantages of phase failure relay

Using a phase failure relay has the below benefits:

Increases motor lifetime.
Reduces maintenance and repair costs of motors.
Minimizes downtime due to motor problems.
Avoids the risk of electric shock or fire due to the short circuit of the motor windings.
Increases the safety of the electrical circuit.
Saves space in the cabinets.

Selection parameters of phase failure relay

When selecting a phase failure relay the below parameters should be considered:

Control voltage.
Functions. (Undervoltage, phase sequence, phase failure, etc..)
Number and type of output contacts.
Max and min setting range of the voltage threshold values.
Setting range for the response delay.

Applications of phase failure relay

It can be used in the following applications.

Control for connection of moving equipment such as air conditioning compressors, refrigerated trucks and containers, and cranes.
Control against reverse motor operation (lifting, handling, elevators, escalators, etc.)
Control of sensitive three-phase supplies.
Overheating of the motor due to asymmetrical voltage.
Protection of a plant against destruction due to overvoltage.
The direction of rotation of the drive.