Setting of overload, Short circuit & Ground Fault Protection of MCCB

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

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 0,4 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