NEWS

A moulded case circuit breaker (MCCB) is a type of electrical protection device that is used to protect the electrical circuit from excessive current, which can cause overload or short circuit. With a current rating of up to 1600A, MCCBs can be used for a wide range of voltages and frequencies with adjustable trip settings. These breakers are used instead of miniature circuit breakers (MCBs) in large scale PV systems for system isolation and protection purposes.

How the MCCB operates

The MCCB uses a temperature sensitiv‘e device (the thermal element) with a current sensitive electromagnetic device (the magnetic element) to provide the trip mechanism for protection and isolation purposes. This enables the MCCB to provide:
•Overload Protection,
•Electrical Fault Protection against short circuit currents
•Electrical Switch for disconnection.

Overload Protection

Overload protection is provided by the MCCB via the temperature sensitive component. This component is essentially a bimetallic contact: a contact which consists of two metals that expand at different rates when exposed to high temperature. During the normal operating conditions, the bimetallic contact will allow the electric current to flow through the MCCB. When the current exceeds the trip value, the bimetallic contact will start to heat and bend away due to the different thermal rate of heat expansion within the contact. Eventually, the contact will bend to the point of physically pushing the trip bar and unlatching the contacts, causing the circuit to be interrupted.

The thermal protection of the MCCB will typically have a time delay to allow a short duration of overcurrent which is commonly seen in some device operations, such as inrush currents seen when starting motors. This time delay allows the circuit to continue to operate in these circumstances without tripping the MCCB.

Electrical Fault Protection against short circuit currents

MCCBs provides an instantaneous response to a short circuit fault, based on the principle of electromagnetism. The MCCB contains a solenoid coil which generates a small electromagnetic field when current passes through the MCCB. During normal operation, the electromagnetic field generated by the solenoid coil is negligible. However, when a short circuit fault occurs in the circuit, a large current begins to flow through the solenoid and, as a result, a strong electromagnetic field is established which attracts the trip bar and opens the contacts.

Electrical Switch for disconnection

In addition to tripping mechanisms, MCCBs can also be used as manual disconnection switches in case of emergency or maintenance operations. An arc can be created when the contact opens. To combat this, MCCBs have internal arc dissipation mechanisms to quench the arc.

Types of MCCB