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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
As Per Standard IEC 60947-2 defines the names of the selector switches.
Setting
Adjustment
Ir
Long time Pick up Current Setting (or thermal Setting). Protection This is a multiplication coefficient of the rating of the overloads device. (Ir=xIn)
tr
Long time delay Setting in seconds, enabling in Protection particular the starting current of a motor to be tolerated. tol erated. overloads (tr=Sec)
Im / Isd
Short time (Magnetic Setting). This is a multiplier of the Protection against Ir setting, often 1.5 to 10 times the Ir current (im=xIr) short circuits.
tm / tsd
Short time delay Setting, enabling in particular the discrimination (time) to be increased with downstream Protection against feeders and the magnetization peaks of a transformer or short circuits. a motor to be tolerated. It is recommended that the I²t selector switch is set to the ON position.(tm=Sec)
Ii
Protection For
against
against
Instantaneous current Setting. Protecting the installation against strong short circuits (dead short circuits) by instantaneous tripping without Time Delay and self- Protection against protection of the circuit breaker. Dead Short circuits. The Ii > Isd.
Ig
for monitoring the earth fault current circulating in the Earth protection Phase and Earth conductor in TNS systems
tg
Earth protection time delay
I delta n
Adjustment of the sensitivity of the earth leakage Earth leakage protection protection
delta t
Earth leakage protection delay.
Earth protection
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 continuousl y. 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 propo rtional 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) b efore 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
(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 Dela y). (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
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.
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 c arry 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.
