MPCBs for Motor Protection
Kashif Jamal
Switchgear | Busways | Power Distribution | Experienced Sales Leader with a Passion for Driving Business Growth
Introduction
As per the standard two types of co-ordination are permissible, Type “1” and “2”. Type “1” co-ordination requires that under short-circuit conditions; the contactor or the starter shall cause no danger to persons or installation. The motor feeder may not be suitable for further service without repair and replacement of parts (Not remaining suitable is NOT a requirement). Type “2” co-ordination requires that under short-circuit conditions; the contactor or the starter shall cause no danger to persons or installation and shall remain suitable for further use. However, contact welding is recognized. Also, the time-current characteristics of the overload protection device should not change. This in other words means safety, low down time and continued protection.
But co-ordination of contactors and overload relays with miniature circuit breakers (MCBs) is challenging. At times it poses problems.
Problem while using an MCB for Motor protection
Different MCBs with various curve types have different magnetic settings. C type MCBs with magnetic setting of 5-10 times In are seen deployed for motor protection applications.
Unlike a Fuse unit the MCB is a peak sensing device. While providing Short Circuit protection to the motor it is imperative that the MCB does not trip on the starting transients of the motor. This care must be taken while selecting the rating of the MCB. These transients are usually of the tune of 12 times the full load current. Now suppose a C curve MCB is selected, to ensure it does not trip during the starting of the motor, 12 times the motor full load current should be lesser than 5 times the MCB’s nominal current.
Let us study an example of a motor with full load current of 6A to understand the difficulty of coordination of contactor with MCB.
for a Motor having a full load current of 6 A; inrush current will be 12 x 6 =72 A.
72A should be lesser than 5 times nominal current of MCB. So, C curve MCB with a rating of 75/5 = 15A will have to be selected.
6A AC3 rated contactor will have to be selected along with 4-6A range relay. Let us now consider a situation when fault occurs. Motor suddenly starts drawing 60A current. In this situation, MCB will not trip as fault current 60A is lesser than 5 x 15A = 75A. As a result, the overload relay will have to give a trip signal to the contactor to break this current.
The IEC standard specifies the breaking capacity of a contactor to be 8 times its AC3 rating. 60 A is greater than 8*6=48A as a result the contactor will get damaged.
The second more serious problem can be described by considering the below case.
Consider a 0.16 hp motor with a Full load current of 0.45A. The initial starting current will be around 5.4A. As in the earlier case a C curve MCB of 2 A will have to be selected. With proper de rating, an 18 A Contactor is selected with a relay having rating of 0.3-0.5A. Now in this case, the crossover between the relay and the MCB will take place at 5x2=10A which is 20 times the upper limit of the relay. This will cause permanent damage to the relay. There is no solution to this problem as de rating a relay is not possible.
Let us consider another combination. Suppose a D curve MCB is selected, then for the above case, a 72/10= 7.2 A i.e., an 8 A MCB will have to be selected. Now the MCB has to trip for currents between 10-20 times (D Type) its nominal current. For the worst case in which the MCB trips at 20 times (i.e., 160A), for a fault current of 140 A, the overload relay will have to give a tripping command to the MCB and there will be similar consequences as in the previous case.
Thus, in conclusion, while selecting an MCB for motor protection which may be a cost effective solution, one must be fully aware of the possible damages that might be caused to the contactor and overload relay. Therefore, it is recommended that for protection of motor feeder, MPCB should be used.
What is MPCB?
MPCB stands for Motor Protection Circuit Breaker. As the name suggests, MPCB is specially designed to offer reliable protection for motor applications. MPCB has the benefit of having both overload and short circuit protection in a single compact unit. Hence no separate overload relay is required when used in DOL motor starters. This makes the solution compact and economical. Another important advantage one gets with using MPCB in motor feeder applications is protection as per Type 2 Coordination. Besides, various accessories like bus bars, Trip-Alarm contacts can be used with MPCB for ease of wiring and indication and interlocking purposes.
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Working Principle of MPCB
The motor protection circuit breaker can be considered a subtype of a thermal magnetic circuit breaker, but with additional functions that are specially designed to protect electric motors. 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. Threephase 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 can detect these conditions by measuring the differences among phase voltages and disconnects the motor immediately when they occur.
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.
Advantages of MPCB
Conclusion
Complete reliability of contactors and relays is not ensured when MCB is used. This is the main reason why MCB is not recommended in Type 2 co-ordination. The correct solution is to use MPCB with contactor as per the recommended type 2 co-ordination. As there is no overload relay required while using MPCB, possibility of damage is out of question. This ensures both safety and reliability of entire motor feeder application.
Motor protection circuit breakers have a very important role in electrical safety, since the motors they protect have a wide variety of applications in commercial buildings and industry. Asynchronous motors, the most common type of electric motor in industrial and commercial settings, have special protection requirements that can only be met by a motor protection circuit breaker. It is also possible to complement MPCB with other protection or automation devices such as undervoltage protection, timers, and reduced voltage motor starters. Adequate selection of the MPCB is key to provide reliable motor protection.
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