Coordination of MCCB and magnetic contactor

MCCB and magnetic contactor

MCCB and magnetic contactor are products with basic purposes, and this must be understood.

The magnetic contactor is intended to provide protection when the motor is in an overload or constrained state. With its basic performance it has a long life with normal start-stop switching operations. Thus, the current that can set or interrupt the magnetic contactor is specified in various standards as 8 to 12-times the rated current. It does not have the capability to interrupt large currents such as a short-circuit current. In other words, this larger range must rely on MCCB, and a coordination style suitable for both products is required.

Requirements for protection coordination

The following conditions must be satisfied for favorable protection coordination to be attained between MCCB and magnetic contactor.

             q) The thermal relay and MCCB operation characteristics must have an intersecting point. There must be seamless protection operating characteristics in all current areas, and the thermal relay’s characteristics must be lower at currents lower than the cross point.

             w) The operating characteristics intersecting point must be a current value less than the breaking capacity of the magnetic contactor.

e)  If a short-circuit current flows to the magnetic contactor, the magnetic contactor must not break until MCCB interrupts the current

Of course, MCCB, magnetic contactor and thermal relay must satisfy the following conditions with their basic functions.

r) MCCB must have a breaking capacity that can accurately interrupt the short-circuit current, and must protect the wires from short-circuits and overloads. It must not malfunction with the motor’s starting current.

The magnetic contactor must accurately close and interrupt the maximum current the could occur in the motor’s normal state.

           y)  The thermal relay must have operating characteristics that can accurately protect when the motor is in the overload or constrained state.

Fig. 4. 25 shows the above coordination requirements. Fig.4. 25 (a) shows a state with the conditions satisfied. Fig. 4. 25 (b) shows the state in which the protection range is cut off, and the protection coordination is not complete. In region (D), the thermal relay melts. However, the width of this region (D) is usually narrow. However, it is rare that the accident current here will develop into a large current region accident, or that it is caused when there is a rare short or ground fault in the motor coil. Thus, the necessity of a complete coordination and the cost efficiency must be considered.

4. 5. 3 Magnetic contactor short-circuit protection by MCCB 

If a short-circuit accident occurs, the short-circuit current is interrupted by MCCB. The peak value of the current that passes at that point and I2· t relies on the circuit conditions such as the voltage and power factor, and tend to increase when the short-circuit current increases. If a short-circuit current exceeding a certain level flows, MCCB protection to prevent the magnetic contactor from breaking is difficult unless the generation of an arc between the magnetic contactor’s contacts is prevented (contacts are prevented from lifting up) or the arc is suppressed to a minimal level.It may be possible to prevent damage to the magnetic contactor if the short-circuit point is at the end of the load side and the short-circuit current is small.The required degree of protection coordination must be determined by the necessity and cost effectiveness. IEC 60947-4-1 “Contactors and motor-starters” lists the “Type of Coordination” as shown in Table 4. 10 according to the degree of magnetic contactor damage when a short-circuit occurs. Type 1 is the most inexpensive type that does not require any consideration for most protection coordination. Type 2 requires various consideration, and is expensive.