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Relay protection circuitry

This handbook covers the code of practice in protection circuitry including standard lead and device numbers, mode of connections at terminal strips, colour codes in multicore cables, dos and donts in execution. Also principles of various protective relays and schemes including special protection schemes like differential, restricted, directional and distance relays are explained with sketches.

Practical handbook for relay protection engineers
Practical handbook for relay protection engineers

The norms of protection of generators, transformers, lines and capacitor banks are also given. The procedures of testing switchgear, instrument transformers and relays are explained in detail. The close and trip, indication and alarm circuits for variety of circuit breakers indicating ferrule numbers are also included.

All relevant information and circuit diagrams necessary for troubleshooting are also given.

Relay protection

The objective of relay protection is to quickly isolate a faulty section from both ends so that the rest of the system can function satisfactorily.

The functional requirements of the relay:


The most important requisite of the protective relay is reliability since they supervise the circuit for a long time before a fault occurs. If a fault then occurs, the relays must respond instantly and correctly.


The relay must be able to discriminate (select) between those conditions for which prompt operation is required and those for which no operation, or time delayed operation is required.


The relaying equipment must be sufficiently sensitive so that it operates reliably when required under the actual conditions that produces least operating tendency.


The relay must operate at the required speed. It should neither be too slow which may result in damage to the equipment nor should it be too fast which may result in undesired operation.

Basic connections of protectiverelay
Basic connections of protectiverelay

Important elements used in protection relaying


Circuit breaker bulk oil, minimum oil, SF6, airblast, vacuum etc. depending on medium used for quenching the arc. Different operating mechanisms such as solenoid, spring, pneumatic, hydraulic etc. are employed.

Protective gear

Relays (current, voltage, impedance, power, frequency, etc.) based on operating parameter, definite time, inverse time, stepped etc. as per operating characteristic, logic wise such as differential, over fluxing etc.

Station Battery

A Station battery containing a number of cells accumulate energy during the period of availability of A.C supply and discharge at the time when relays operate so that relevant circuit breaker is tripped.

Station battery connection scheme
Station battery connection scheme

Table of handbook contents:

  1. Code of Practice:
    1. Standard number for devices
    2. Types of panels
    3. Protective relay – connection and zones of protection
    4. Norms of protection for generator, transformers and lines
    5. Current transformers
    6. Voltage transformers
    7. Energy meters
    8. Synchronising panel
  2. Generator and their protection
  3. Transformers and their protection
  4. Distance relays in A.P. system, relay indications and their meanings
  5. Busbars – arrangements and protection
  6. O/L & E/F relays
  7. Circuit breakers
  8. Station battery
  9. Earthing practices
  10. Excitation and voltage regulation
Title:Practical handbook for relay protection engineers – Mohammed Fasil
Size:3.2 MB
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Practical handbook for relay protection engineers
Practical handbook for relay protection engineers

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    Oct 08, 2020

    Nice Book. This is a good hand book for understanding the basics of protection system.

  2. Madhav
    Jan 18, 2020

    This is a good hand book for understanding the basics of protection system. Although references are drawn from the practice followed in AP Electricity Distribution(In southern India) system, the protection protocol for feeders is generally the same elsewhere in India. However, we in Maharashtra (Central India) often have to adopt very low settings as far as Earth Fault Protection is concerned.. e.g. for a typical 11kV feeder with connected CT ratio of 200/5A having 3 O/C and 1 E/F IDMT relay we have the adopted relay settings as follows:
    IDMT curve: 1.3 sec.
    O/C settings: Plug setting (current setting) 80%, Time Dial (TMS) setting 0.07
    E/F settings: Plug setting (current setting) 10%, Time Dial (TMS) setting 0.07
    and the Instantaneous or High Fault settings for O/C and E/F are set to 300% for both. In Numerical relays we also use the Cold Load Pickup feature in order to block the relay from mal-operating at the time of charging a feeder.

    Jan 16, 2020


  4. garba mohd
    Jan 16, 2020

    I am very excited, as on a project of installing relay today.

  5. Jerry Padayhag
    Jan 16, 2020

    Thanks a lot.!

    • ariel balcos
      Jan 16, 2020

      Jerry, you know Nono Padayhag?

  6. NguyenCao Ky
    Jan 16, 2020

    Thanks so much!

  7. Davis Mwakyusa
    Jan 15, 2020

    The various articles have been very useful to me.

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