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A motor protection circuit breaker (MPCB) is used to protect from overload and/or short circuit currents. An overload relay is used for overload only, and as such will have a relatively long delay before tripping. Also, I believe some overload relays are a one-use device (once tripped, they must be replaced), and I don't believe this is the case with MPCBs (could be wrong though).
If you look at a manual for a MPCB, you'll see the tripping curve is two curves in one - at low currents, it is very slow (time in hours), but once you get above load currents, it speeds up significantly down to a few cycles, which is what you want for faults / short circuit conditions. An overload relay will have the first (slow) curve only.
Do a Google search for Cutler - Hammer (or go to their website) for some examples.
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What is MPCB in motors?
Motor Protection Circuit Breakers.
What is full form of MPCB?
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WHAT IS type 2 coordination chart?
To understand various considerations for proper co-ordination, time-current characteristics of thermal overload relay (curve B), H.R.C. fus (curve C), MCCB with only instantaneous release (curve D) and MPCB (curve E) are superimposed on motor starting characteristics (curve A) in Fig. 3. Intersection of characteristics of thermal overload relay and Fuse / MCCB is termed as 'cross-over point' and corresponding current as 'cross-over current' lco. Following points are to be ensured while selecting components to have properly co-ordinated motor protection: lContactor rating (AC-3) should be more than or equal to motor full load current (if application is AC-3 duty) Thermal overload relay of appropriate 'Trip Class' is selected. Time current characteristics of the relay should remain above motor starting characteristics as shown in Fig. 3. For fault currents lower than 'cross-over current lco', relay will respond faster than SCPD and hence contactor will interrupt the fault current. Fault currents higher than lco will be interrupted by SCPD. Hence, rating of contactor is so chosen that lco is less than rated breaking capacity of the contactor Relay and contactor should be able to withstand lco for a duration equal to trip time of the relay. IEC / BS / EN standards require that the contactor should be able to withstand at least current equal to 8 times AC-3 rating (6 times for ratings higher than 630A) for 10 seconds While using MCCB or MPCB, attention needs to be given to motor peak starting current. To avoid operation of MCCB / MPCB during starting, instantaneous release is chosen as 12 times the full load current of the motor. This thumb rule assumes motor starting current equal to 6 times full load current In case of high efficiency motors, starting currents could be about 8 times full load current. For such application, MCCB rating need to be selected such that instantaneous release setting is higher than 12 (about 14) times full load current to avoid nuisance tripping during starting. Similarly, while using MCCB as a SCPD for Star-Delta starter, consideration needs to be given to peak current associated with change over from Star to delta. Instantaneous release of MPCB is normally set at 12 times the rating. Hence, possibility of nuisance tripping needs to be considered while using MPCB for protection of high efficiency motors or for star delta starter.
Where is an mpcb used?
Motor protection circuit breakers(MPCB) are a specialized type of electrical protection device that is designed specifically for electric motors, like their name implies. Electric motors have plenty of applications and are used to drive mechanical devices of all types, so it is very important to protect them adequately with MPCBs.
What is a break in a circuit?
It is a fuse or a breaker. The wire inside the fuse will burn out and will need to be replaced once the fault has be corrected. A breaker is designed to click open, so breaking the circuit.
How does mpcb work?
The basic working principle is similar to all other circuit breakers.• Thermal protection is used to guard the electric motor against overload. It is based on an expanding and contracting contact that disconnects the motor if excessive current is detected. It is very important to know that thermal protection has a delayed response, to allow the high inrush currents when a motor starts. However, if the motor is unable to start for some reason, thermal protection will trip in response to the extended inrush current.• Magnetic protection is used when there is a short circuit, line fault, or other high current electric fault. Unlike thermal protection, magnetic protection is instantaneous, to immediately disconnect the dangerous fault currents.• The main difference between the MPCB and other circuit breakers is that the MPCB can provide protection against phase unbalance and phase loss. Three-phase circuit motors require three live conductors with balanced voltages in order to operate effectively. An unbalance of more than 2% will be detrimental to the motor's service life. If one of the phase voltages is suddenly lost, the effect is even more damaging because the motor will keep on running with only two phases. The motor protection circuit breaker is capable of detecting these conditions by measuring the differences among phase voltages, and disconnects the motor immediately when they occur. It is important to note that phase current unbalance is normal in three-phase systems that power separate single-phase loads, but is unacceptable when the three-phase circuit powers an electric motor.• MPCBs are also equipped with a manual interruption mechanism, allowing disconnection of electric motors for replacement or maintenance.Motor protection circuit breakers are available in a wide variety of current ratings, and one of their best features is that many models allow the current rating to be adjusted. This means that the same MCPB can be configured to protect motors of different capacities.
Difference between fuse and mcb?
CB MCB (i) Mechanism of CB is tripping relay. (i) Mechanism of MCB is tripping release (ii) It is used in HV system. (ii) It is used in LV system. (iii) Rupturing capacity is high . (iii) Rupturing capacity is low.
