Setting of overload, Short circuit & Ground Fault Protection of MCCB (PART-2)


(2) For Short Circuit Protection (Magnetic Setting):

  (C) Short Time pickup Current Setting (Im):

  • Short time protection is time-independent.
  • It is determines or sets the level of fault current at which the short-time trip delay countdown is actuated.
  • Short Time Pick up Value (Im) (multiplied by the ampere rating) sets the short circuit current level at which the circuit breaker will trip after the set time delay.
  • The short-time pickup (Isd) sets current level (below instantaneous trip level) at which circuit breaker will trip after the preset time delay.
  • Standard Practice for Setting:
  • No trip for a current below 80% of the short time setting
  • Trip for a current equal to 120% of the short time setting
  • The trip time is Less than 0.2 s for a short time protection with no time delay and equal to the value of the time delay tsd for a protection with time delay

(D) Short time delay Setting ™:

  • Short Time delay sets the amount of time the breaker will carry both a low level and high fault currents before tripping.
  • tm sets length of time the circuit breaker will carry a short circuit within the short-time pickup range.
  • Delays bands are labeled in seconds of short-circuit current at 10 times the ampere rating.
  • The short time delay can be set to I2t On and I2t OFF settings (Inverse Time Delay).
  • (A) I2t OFF: Gives Constant time delay usually in multiplication of 0.5 sec. It has not inverse-time delay Characteristic. It is used for coordination with other circuit breakers with electronic trip devices and for coordination with thermal-magnetic circuit breakers.
  • (B) I2t ON :Gives an inverse-time delay that resembles the time/current characteristics of fuse It is used for coordination with fuses and upstream transformer

(3) For Instantaneous Trip (Short Circuit Protection):

(E) Instantaneous Pickup Setting (Ii):

  • Instantaneous protection is time-independent.
  • It is determines the level of fault current that will actuate a trip with no time delay.
  • Ii value (multiplied by the ampere rating (In)) sets the short-circuit current level at which the circuit breaker will trip with no intentional time delay.
  • This protection trips to eliminate quickly high value currents and its trip times cannot be set
  • The instantaneous function will override the short-time function if the instantaneous Pickup is adjusted at the same or lower setting than the Short Time Pickup.
  • Standard Practice for Setting:
  • No trip for a current below 80% of the instantaneous setting
  • Trip for a current equal to 120% of the instantaneous setting
  • The trip time is less than 0.2 second.

 (4) For Ground Fault Protection:

(F) Ground Fault Pickup Setting (Ig):

  • It is determines the level of fault current at which the ground fault trip delay countdown is actuated.

(G) Ground Fault Delay Setting (Itg) :

  • It is determines the amount of time the breaker will carry a ground fault before tripping.
  • It 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

 Example for Settings of MCCB for Protections

Example1: We have Sub feeder MCCB Size of 2000A, Short Circuit Current is 4000A. Maximum Load Current is 1000A .What is Over current (Long Time) and Short Circuit (Short Time) and Instantaneous setting of MCCB.

  • Here In=2000A

Over current Setting:

  • Dial Setting=(Load current / MCB Rating) =1000/2000=0.5 , set Dial at 0.5
  • Over current Setting Ir=0.5xIn =0.5X2000=1000A
  • For Set Over Current Time Delay: It is necessary to overview Over current Time delay of downstream MCCB and Upper stream MCCB for better coordination otherwise for downstream fault upper stream MCCB gives tripping.
  • Downstream Over current Time Delay< Over current Time Delay< Upper stream Over current Time Delay.
  • In Our example it is last MCCB in circuit and upper stream MCCB Over current delay setting is 2 Sec so Select tr=1 Sec.

Short Circuit Current Setting:

  • Dial Setting=( Short Circuit Current/ MCB Rating) =4000/2000=2
  • Short Circuit Setting Isd=2xIn=2×2000=4000A
  • Downstream Short current Time Delay< Short  current Time Delay < Upper stream Over current Time Delay Short  current Time Delay
  • Our example it is last MCCB in circuit and upper stream MCCB Short Circuit current delay setting is 0.4 Sec so Select Isd=0.2Sec.
  • For coordination of other MCCB we need I2t ON with 0.2 Sec delay.

Instantaneous Tripping Setting:

  • Instantaneous Tripping gives instant tripping without any delay for switching or short Circuit current.
  • Instantaneous Tripping >= Short Circuit Tripping Setting
  • If We can set Ii=3xIn =3×2000=6000A , It will full fill our requirement
  • Here Ii>Isd=6000A>4000A.

 

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