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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