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Measuring / Motor Protective Relays

Introduction Features
Principles Classifications
Engineering Data Further Information
Explanation of Terms Troubleshooting

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Incorrect Phase Sequence Protection

Preventing motor reverse operation due to incorrect wiring of a three-phase power supply (three wires).

Phase Loss Protection

Monitoring a three-phase power supply (three wires) to prevent a motor from burning out due to a disconnected wire.


When a current above the rated value flows through a device (motor).


The operating status of a device (motor) under an abnormally low load (such as idle running of a submersible pump).


Applying a voltage that is above the rated value to a device.


When the rated voltage is not being applied to the device.

Voltage Asymmetry

A voltage imbalance in a three-phase power supply (three wires).

Liquid Level Control

Controlling the level of a liquid with electrodes
(e.g., the function of the 61F).

* A relay that performs this type of control is called a Conductive Level Controller or Floatless Level Controller.

Temperature Monitoring Relay

A relay that produces an alarm for an abnormal temperature
(it does not support PID control).

Single-phase Power Supply

A power supply that uses two wires.
Example: The power supply used for a household fan.
The fan motor runs with two wires on single-phaseresidential power.
A single-phase power supply uses two wires.

Three-phase Power Supply

A power supply that uses three wires.
Examples: Almost all belt conveyors, cranes, and other industrial motors use three-phase power supplies.
The majority of power equipment (motors and heaters) used in factories runs on three-phase AC power (400 V or 200 V).

* In addition to motors, there are single-phase and three-phase heaters too.

Incorrect Phase Sequence

If the power wires to a three-phase motor are connected in the wrong sequence, the motor operates in reverse.

Phase Loss

If a wire to a three-phase motor is disconnected or was not connected, the motor cannot achieve its specified output.