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anansit · May 27, 2007, 10:04 PM

Hello friends
:confused: My question is what is motor protection circuit breakers? How it is different from MCB and MCCB's??
Can anybody please provide thorough technical details of it?? How it works?? >?

tkrussell · May 28, 2007, 04:54 AM

Standard MCCB, Molded Case Circuit Breakers, are designed for protection circuits, specifically the wiring and devices, from overheating due to overload of the circuit. If a circuit is rated 20 amps, a MCCB breaker will monitor the current and trip if the current rises above 20 amps, with allowances of time.

A MPCB, Motor Protection Circuit Breaker is designed to protect motors. These will serve as short circuit protection of the circuit wiring and overload of the motor. In large industrial installations, this can be a huge cost savings by elimination the separate overloads for each motor.

A motor needs to be more closely protected. Typically the rating is anywhere from 115% to 125% of normal running current. A MPCB will be adjustable to be more closely rated and matched to the motor running current. If a motor is rated to draw 14.2 amps at full load and is allowed to be protected at no more than 125%, a MPCB can be adjusted exactly to 17.75 amps.

anansit · May 28, 2007, 09:54 PM

Thanks tkrussel...
But MCCB too have short circuit protection in it in addition with thermal protection so why don't we use MCCB alone... rather than using it with overload relays. I mean where exactly it differs from MCCB... all MCCB's I have seen are good for Short circuit protection...

tkrussell · May 29, 2007, 02:36 AM

A MCCB cannot be adjusted as accurately as a MPCB for overload.

ZCCG · May 30, 2007, 11:51 AM

That is the exact reason we use MPCB's on ALL of our motors.
When a motor is first installed, we take current readings and adjust the breaker to match the inrush current.. . plus 1 or 2 %. Then if the breaker starts to trip on start ups, we know that something has changed (mechanically or electrically) and we need to investigate the cause.
This helps us from having to change motors prematurely.
Short circuit protection in both styles is designed to prevent a sudden, uncontrolled rise in current.
There is a huge difference between a motors inrush current (controlled)
And a short circuit (uncontrolled).