Setting of overload, Short circuit & Ground Fault Protection of MCCB (PART-1)

Introduction:

  • There are various types of protections setting in MCCB, which define various protection of Electrical Network.
  • In MCCB we can set most of protection are adjustable according to Electrical Load profile.
  • The main adjustable Setting in MCCB are
  • Over current Setting
  • Short Circuit Setting
  • Ground Fault Setting

Meaning of each selector switches of MCCB

  • As Per Standard IEC 60947-2 defines the names of the selector switches.
Setting Adjustment Protection For
Ir Long time Pick up Current Setting (or thermal Setting). This is a multiplication coefficient of the rating of the device. (Ir=xIn) Protection against overloads
tr Long time delay Setting in seconds, enabling in particular the starting current of a motor to be tolerated. (tr=Sec) Protection against overloads
Im / Isd Short time (Magnetic Setting). This is a multiplier of the Ir setting, often 1.5 to 10 times the Ir current (im=xIr) Protection against short circuits.
tm / tsd Short time delay Setting, enabling in particular the discrimination (time) to be increased with downstream feeders and the magnetization peaks of a transformer or a motor to be tolerated. It is recommended that the I²t selector switch is set to the ON position.(tm=Sec) Protection against short circuits.
Ii Instantaneous current Setting. Protecting the installation against strong short circuits (dead short circuits) by instantaneous tripping without Time Delay and self-protection of the circuit breaker.

The Ii > Isd.

Protection against Dead Short circuits.
Ig for monitoring the earth fault current circulating in the Phase and Earth conductor in TNS systems Earth protection
tg Earth protection time delay Earth protection
I delta n Adjustment of the sensitivity of the earth leakage protection Earth leakage protection
delta t Earth leakage protection delay. Earth leakage protection

  Setting of each Protection switch of MCCB

 (1) For Low level Fault / Over Current Protection (Thermal Setting):

  (A) Long-Pickup Current Setting (Ir):

  • It is determines the continuous ampere rating of the breaker.
  • Long time protection is time-dependent.
  • Long Pickup (Ir) value (multiplied by the ampere rating (In) of MCCB) sets the maximum current level which the circuit breaker will carry continuously.
  • If MCB is 1000A Rating but Full Load current is 800A than MCCB Rating can be changed from 1000A to 800A by setting it 0.8, Now Ir=0.8XIn =0.8×1000=800Amp
  • If the current exceeds this value for longer than the circuit breaker will trip at the set delay time.
  • Long time protection is inverse time type (with I2t constant)
  • The long-time pickup (Ir) is adjustable from 4 to 1.0 times the sensor plug rating (In)
  • Standard Practice for Setting:
  • No trip for a current below 105% of Ir
  • Trip in less than two hours for a current equal to for
  • 120% of Ir for an electronic trip unit and for
  • 130% of Ir for a thermal-magnetic trip unit
  • For a higher fault current, the trip time is inversely proportional to the fault current value.

(B) Long-Time delay Setting (tr):

  • Long time delay (tr) sets length of time that the circuit breaker will carry a sustained overload before tripping.
  • The delay bands are labeled in seconds of over current at six times the ampere rating.
  • Long-time delay is an inverse time characteristic in that the tripping time decreases as the current increases.
  • The long-time delay (tr) sets the length of the time that the circuit breaker will carry an over current (below the short-time or instantaneous pickup current level) before tripping.
  • The Long time delay can be set to I2t On and I2t OFF settings.
  • (A) I2t Response:I2t Out ,For coordination with other circuit breakers with electronic trip devices and for coordination with thermal-magnetic circuit breakers.
  • (B) I2t Response: I2t In ,For coordination with fuses and upstream transformer

 

 

 

 

 

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About Jignesh.Parmar
Jignesh Parmar has completed M.Tech (Power System Control), B.E(Electrical) from Gujarat University. He has more than 14 years experience in Power Transmission-Power Distribution-Electrical energy theft detection-Electrical Maintenance-Electrical Projects(Planning-Designing-coordination-Execution). He is Presently associate with one of the leading business group as a Deputy Manager at Ahmedabad,India. He is Freelancer Programmer of Advance Excel and design useful Excel Sheets of Electrical Engineering as per IS,NEC,IEC,IEEE codes. He is technical Author for "Electrical Mirror" and "Electrical India" Magazines. He is Technical Blogger and Familiar with English, Hindi, Gujarati, French languages. He wants to Share his experience & knowledge and help technical enthusiasts to find suitable solutions and updating themselves on various Engineering Topics.

8 Responses to Setting of overload, Short circuit & Ground Fault Protection of MCCB (PART-1)

  1. Bikram Paudel says:

    Thanks a lot. I appreciate for your consideration

    Regards,

    Bikram Paudel Electrical Engineer Nepal Electricity Authority

  2. mohammed says:

    good infirmation

  3. mohammed says:

    All notes are very much valuable

  4. mohammed says:

    While preparing schematics, what are the factors to be considered in designing a flat,floor,bldg.

  5. Vishal bhatkar says:

    Electrical related

  6. Raj Dipak says:

    Dear Sir,
    I am Raj Dipak. I have got some problems and I would like to get the advice from you.
    The problem is neutral cable is burnt at only incoming side without any tripping the MCCB and even the outgoing neutral is nothing happened.

    The main MCCB is 800A and incoming cable sizes are 150 sqmm (2 sets). From this breaker, the supply is taken to 2 nos 150A MCCB ( Cable Size are 95 sqmm) from bus bars respectively. The neutral cables for both 150A MCCB at 150A MCCB neutral bar are burnt but there is nothing happened to 800A MCCb’s neutral bus bar and not even to the outgoing neutral cable for both 150A MCCB.

    I found the cable insulation and, cable lug , bolt & nut melted like welding. I checked whether it’s loose connection or what, but the the termination is still tight. I am not sure that it is because of melting.

    I hope you will help me to the best solution.
    And I am really looking forward for your answer.Thanks.

    Sincerely
    Raj Dipak

  7. firoj says:

    Sir i am having doubt on single line diagram can u connect with linked in

  8. Orane Buck... says:

    Hi good day, I would like to know, why is that the 220v wire at the pothead is curled? ________________________________

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