The Basics Of Molded Case Circuit Breakers You MUST Know

Molded Case Circuit Breakers (MCCB) //

Molded case circuit breakers are interrupting devices with self-contained, current-responsive elements. These breakers are assembled as an integral unit in a supported and enclosed housing of insulating material.

Depending on the amount of protection required, these breakers can sense internally and then clear undervoltage, overcurrent, and short circuit conditions.

The Basics Of Molded Case Circuit Breakers You MUST Know
The Basics Of Molded Case Circuit Breakers You MUST Know (on photo: Old Schneider Electric's MCCB, type Compact NS; credit:

Some tripping elements are also externally accessible through control wiring, and other circuit protection can be added. Breakers may rely solely on outside information to perform their prime function. Several trip units may be available for a particular frame size. A specific assembled breaker may have a lower continuous current rating than the current designation of the frame manufacturers.

Arc chutes, extinguishers, and quenchers define the interrupting-current capacity of the assembly, in conjunction with the insulating and heat-dissipation properties of the molded-case chutes.

Contacts often begin to part during the first cycle of a fault. The current breaker must be capable of interrupting the maximum allowable first-cycle asymmetrical current. The operating mechanism provides a means of opening and closing the toggle mechanism of quick-make/quick-break type contacts (snap open or closed independent of the speed of handle movement).

NOTE // The breaker is trip free and cannot be prevented from tripping by holding the breaker handle in the “on” position during a fault condition.

Molded Case Circuit Breaker Principle Components
Figure 1 – Molded Case Circuit Breaker Principle Components

Molded case circuit breakers use is restricted to low voltage and medium voltage systems.

With alternating current (AC), this breaker provides high interrupting capacity for short circuits in minimum space. On AC or direct current (DC) systems, this breaker is often the first protection device to handle electrical problems.

For low voltage and medium voltage circuit breakers, interrupting and close-and-latch ratings are usually the same.

MCCB Trip elements //

Trip elements trip the operating mechanism of a circuit breaker during either a prolonged overload or a short circuit current. Some molded case circuit breakers have a screwdriver slot located on the front of the trip unit used for adjusting sensitivity.

The maximum setting is established by protection of the minimum conductor size in the circuit.

Instantaneous Magnetic Trip

Magnetic trips work by using an electromagnet in series with the load current. When the current reaches the set point, the electromagnet instantaneously trips. This type of trip is commonly found in low voltage breakers (e.g., household circuit breakers).

Thermal Trip

Considered the industry standard, these trip elements work using a bimetal heated by the load current. When overheated, indicating an overload, the bimetal will detect, which causes the operating mechanism to trip.

Thermal magnetic circuit breakers employ a bi-metalic strip to sense overload conditions.
Thermal magnetic circuit breakers employ a bi-metalic strip to sense overload conditions.

Thermal Magnetic Trip

A thermal magnetic trip, in addition to providing short circuit protection, guards against long-term current overloads existing longer than roughly 10 seconds. Because bimetal de ection is dependent on current and time, the thermal unit provides long-time delay for light overloads and fast response for heavy overloads.

The thermal magnetic unit may be ambient-temperature sensitive (breaker trips at a lower current as ambient temperature rises).

The National Electrical Code (NEC) defines the current at which a long-time-delay thermal element must initiate circuit-clearing operation. The NEC specifies a point that is 125% of the rated equipment or conductor ampacity.

TM thermal magnetic and MA magnetic trip units
TM thermal magnetic and MA magnetic tripping units

The circuit breaker will take no action below this current. For conventional (non-compensating) thermal magnetic elements, the thermal portion defines the continuous current rating of the breaker (specified as 100% at 40°C). The thermal element current rating cannot exceed the frame rating.

Manufacturers recommend that continuous current through the breaker be limited to 80% of the frame size.

Electronic Trip

Current transformers and solid-state circuits are used to monitor the current. When an overload or short circuit is detected, the monitors initiate a trip. Electronic trip elements can include trip features and exibility not present in other types of trip elements (e.g., adjustable pickup, time delays, instantaneous pickup, selective interlocking).

Solid-state components have replaced electromechanical-magnetic and thermal magnetic trip elements in some molded-case breakers.

Microprocessor tripping unit
Figure 3 – Microprocessor tripping unit (Overloads: Long time protection (Ir); Short-circuits: Short-time protection (Isd); Short-circuits: Instantaneous protection (Ii), Additional ground fault protection (Ig) and Neutral protection)

Shunt Trips

A shunt trip relay completes the circuit between the control-power source and the solenoid coil. Shunt trips are used to trip a circuit breaker electrically from a remote location and consist of a momentary-rated solenoid tripping device mounted inside a molded case.

The shunt trip can remotely trip the breaker, but cannot remotely reclose the breaker. To reclose the breaker, the breaker handle must first be moved to the reset position and then to the “on” position.

Reference // Electrical Safety: Systems, Sustainability, and Stewardship by Martha J. Boss (Purchase paperback from Amazon)

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About Author //


Edvard Csanyi

Edvard - Electrical engineer, programmer and founder of EEP. Highly specialized for design of LV/MV switchgears and LV high power busbar trunking (<6300A) in power substations, commercial buildings and industry fascilities. Professional in AutoCAD programming. Present on

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