Are you still using MCB for your motor applications?
MCB is commonly used for motor applications. However, such a solution is not reliable as MCB when used for short circuit protection in a feeder does not offer complete protection. Hence there is always a possibility of damage to relay or contactor because of incorrect selection.
The correct solution is to go for recommended Type 2 selection with MPCB and contactor. This ensures complete safety to the personnel along with reliability of switching devices.
So are you still using MCB for your motor applications? If yes, read further to know the benefits of MPCB and how it offers a safe, reliable and technically correct solution for all your motor applications.
What is an MPCB?
MPCB stands for Motor Protection circuit breaker. As the name suggests, MPCB is specially designed to offer reliable protection for your 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 different accessories like bus bars, Trip-Alarm contacts can be used with MPCB for ease of wiring and indication and interlocking purposes.
Problem of using MCB in Motor Feeder Applications
An MCB is a peak sensing device. The starting transients of the motor are usually up to 12 times the motor rated current (Peak Value). Hence when a MCB is used, it must be ensured that it does not trip during motor starting. If a C-Curve MCB is used it must be ensured that 12 times the rated motor current must be lesser than 5 times the MCB’s nominal rating to prevent nuisance tripping of MCB at start. (Since magnetic trip setting of C-Curve MCB is 5 – 10 times the rated current)
Let us understand with the help of an example,
Consider an 11KW motor with DOL starting,
Rated motor full load current = 21A
Peak current at starting = 12 x 21 = 252A
Peak starting current must be less than 5 times the MCB nominal rating.
Hence, MCB rating must be greater than 252A/5 i.e. greater than 51A.
In this case we have to select a C-Curve 63A MCB.
If we select a 25A contactor with relay range of 14 – 23A and considering the worst case scenario, MCB will trip at 10 times i.e. 630A. Just below this value it may happen that the MCB wont trip and the thermal overload relay has to give tripping signal to the contactor to break this current
As per IS breaking capacity of a contactor is 8 times its AC3 current. Considering this, a 25A contactor which would have breaking capacity of about 200A, it may get damaged in such a scenario. This can be addressed by using higher rating contactor. The contactor selected must have a breaking capacity almost equivalent to 630A. Hence, we would need a contactor with AC3 rating of 630 / 8 i.e 80A
However, even than the reliability is not fully ensured as the relay is still susceptible to damage if the current exceeds it’s withstand capacity. Such kind of damage is common when MCB is used for motor applications.
Hence 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. This will ensure both safety and reliability of your entire motor feeder application.