Protection setting of MCCB ( Over load , Shirt circuit & Ground Fault Protection (LESSON-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
