Working of Numerical Relays:

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Numerical Relays working explained in this post with the help of a Numerical relay used in industrial applications.

Brief Introduction about Relay:

RELAY: A Relay is a protection device that senses the fault in the circuit and gives a trip signal to the Circuit Breaker to isolate the faulty circuit.

To understand simply it is like a brain in our human body, when the brain gives the sense the body parts are operate to do any work.

Relay is the heart of the protection circuit, as if any case relay fails to operate, it leads to crores rupees loss to the equipment and severe hazards take place.

Numerical Relay:

These are the advanced version of relays by using microprocessor and programming software these relays work.

Advantages of Numerical Relays:

  1. Numerical relay consists of many functions in one relay thus replace many traditional relays with one.
  2. One relay can be used in many ways. Users can configure relay according to their system requirement.
  3. Consists of no mechanical moving parts hence accurate operation.
  4. No maintenance is needed like electromechanical relays.
  5. Relays are compact in size and appearance is good.
  6. Many complicated functions can be achieved with developed logics using various Gates.
  7. Gives entire details of fault record including with the graphs.
  8. Some relays have different Setting groups. The advantage of setting groups is that user can change the entire settings of the relay by simply changing Group setting. Frequent configuration is not needed.
  9. The main advantage of Numerical relays is communication between relays and other equipment in the network is possible for doing automation of the system.

Common Features of Numerical Relays:

Any numerical relay usually consists of the following essential parts.

Numerical Relay Front Side:

Front side of any Numerical Relays usually have the following components.

  • LED Configuration Menu
  • LCD Display
  • Numerical Key pad
  • Led Reset Button
  • Menu Button
  • Control Button
  • Breaker ON/OFF buttons
  • Esc button
  • Run LED
  • Error LED
Working of Numerical Relay-Front Side

Relay Front Side

Numerical Relay Back Side:

Relay back side usually consist of different Terminal Blocks [TB] for giving the Digital and Analog inputs, outputs .

Working of Numerical Relay-Back Side

Relay Back Side

Binary/Digital Inputs:

Binary Inputs gives the status of the various equipment to the relay in the form of 1’s and 0’s, for e.g. the Circuit breaker status is given by BI is either ON/OFF

Binary/Digital Outputs:

Binary Outputs are the Commands generated by the relay in the form of 1’s and 0’s, for e.g. trip command given by the relay to breaker in the case of fault.

LED Configuration:

Any LED can be configured in two ways either Latch or Unlatch.

Latch LEDs:

            The Latch configured LED picks up as soon as the indication exists and remains picked up until the relay is reset at the device.

The reset has been done by either pressing Reset button manually or via the system interface (SCADA or DCS).

Unlatch LEDs:

            The Unlatch configured LED picks up as soon as the indication exists and drops as soon as the indication no longer applied.

It means the LED reset itself and no manual reset required.

Communication Protocols:

Relay communication is provided with the following protocols. These protocols helps to communicate relays among them and also with HMI through modem.

Following are the various protocols used.

Modbus RS485:

This uncomplicated, serial protocol is mainly used in industry and by power supply corporations.


DNP 3.0 (Distributed Network Protocol, version 3) is a messaging-based communication protocol.

IEC61850 with RJ45 interface or Fibre interface:

The Ethernet-based IEC 61850 protocol is the worldwide standard for protection and control systems used by power supply corporations.

Numerical Relay Types:

The following are the various types of numerical relays

1. Over current/Earth fault Relays

2. Motor Protection Relays

3. Differential Relays

4. Line/Cable Differential Relays

5. Directional Over Current Relays

6. Under/Over Voltage Relays

7. Distance Relays

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