Molded Case Circuit Breakers or MCCB circuit breakers are black plastic–cased automatic switches. They are designed to primarily protect insulating materials from overheating. Molded case circuit breakers (MCCB) are by far the most commonly used breakers in residential, commercial, and industrial facilities. When an MCCB has opened (or “tripped”) to protect the circuit, it can be physically reset immediately. This allows it to be ready to protect the circuit again should the need arise.

MCCBs have current ratings in standard increments between 15 and 6,000 A. In low-voltage systems, circuit breakers come in one-pole for 120 and 277 VAC, two-pole for 208, 240, or 480 VAC, and three-pole for 208, 240, and 480 VAC. When the neutral conductor must be opened, four pole breakers are used.

Multipole breakers open all poles (switches) at the same time. A breaker must not be installed where its voltage rating will be exceeded. For example, a 120/240 slash-rated breaker should not be used to control a 277 V lighting circuit. Circuit breakers also have an ampere rating.

MCCB Circuit Breaker Working Principle

They have two types of sensing elements, one that operates to clear running overloads, and one to clear dangerous short circuits. The overload element, called a thermal element, adds an amount of time delay before opening the protected circuit. The element that protects against short circuits and acts without any intentional delay is called the magnetic element.

Internally, they may have either a thermal or magnetic trip element, or they may have both types of trip elements. Both miniature and molded case circuit breakers have contacts exposed to ambient air. They are either electromechanically controlled or microprocessor controlled and are often used for main power distribution in small and medium-sized facilities and industrial plants.

The thermal element in an MCCB has a varying amount of time delay built into it by design. This element is typically two pieces of very thin metal bonded together. Each metal has a different expansion and contraction rate. When bonded together, with the least expansive metal (invar) on the inside, the strip of metal will bend in an arc as it is heated and cooled. This bending action allows it to be used as a trip lever to open the circuit if an overload condition is sensed by the MCCB.

Breakers under about 200 amps are tested in free air. When they are placed in a metal box such as a load center or panelboard, they can carry continuously only 80 percent of their rating. Breakers rated above about 200 amps are 100 percent rated. Therefore, a 30-amp breaker can only carry 80 percent of its rating continuously.

80% of 30 is (0.8 × 3) 24 amps.

When the breaker is located in a much hotter location, say, in a poorly ventilated boiler room, the thermal element may trip the breaker sooner than it normally would. Where this occurs, an ambient compensated breaker should be considered for use. The operating handle of a breaker can be in either the on or off position or a little over the midpoint, which is the tripped position.

MCCBs can be mounted by either some type of stab-lock, push-in/pull-out method, or bolted in place. Power must be removed before attempting to remove a breaker. With bolted breakers, check to see if the bolt is still hot, most likely it may be necessary to kill power to the entire panel before the bolt(s) can be removed.

Breakers with a thermal element exhibit an inverse current and time relationship. That is, as the amount of current flowing in the circuit increases, the sooner the breaker will open to protect the circuit component from overheating.

Breakers can be considered as being high-temperature limit switches. When the temperature of the conductor and its electrical insulation increases, the breaker’s thermal element temperature increases, and when the point is reached that the insulation is in danger of being damaged, the breaker opens its contacts, stopping the current flow in the circuit. The size of an insulated electrical conductor is typically selected for 125 percent of the connected load. That is, breakers are sized to prevent the circuit from overheating.

As an example, for an appliance with a rated load of 16 amps, the circuit conductor would be rated at 20 amps, and the circuit breaker at 20 amps. Should the circuit begin to pull 20 amps for an extended time, the breaker’s thermal element would open the circuit. Should this same appliance short out internally and begin to pull short circuit amps of over 100 amps, the magnetic element would open the circuit without delay.

When a breaker trips out, the operating handle moves to a position about 60 percent of the way between the on and off positions. As a safety feature, when a breaker opens it means that something is wrong and should be corrected.

Motor Circuit Protectors

An MCCB that has only a magnetic element is called a motor circuit protector (MCP). MCPs are used to provide short-circuit protection, while a motor starter provides running overload protection.

While they physically look very much like an MCCB, they do not provide overload protection. They are used in conjunction with motor starters which provide the necessary running overload protection.

Supplemental Protectors

Supplemental protectors devices (SPD) are molded case overcurrent devices listed for use within a machine to provide overcurrent protection for only the components located downstream of the SPD. While they physically look very like an MCCB, they are not allowed to provide branch circuit protection.

Miniature Circuit Breakers

Miniature circuit breakers (MCBs) typically have a rating of less than 200 A and have either thermal or thermal and magnetic elements. Mounting options include flush, surface, or use of a 35 mm DIN rail. Connection options include lugs on both ends, bolt-on, or plug-in.

Some are rated for switch duty (SWD). Many are Heating, Air Conditioning, and Refrigeration (HACR)–rated and are also rated for use with high-intensity discharge (HID) lighting. Miniature circuit breakers are used extensively in Europe in residential panelboards known as consumer units (CU), and in commercial distribution boards.

MCBs should not be confused with a supplemental protector used extensively in the United States, as they physically look very much alike. Voltage ratings include 120/240 VAC, 125 VDC, and 277 VAC.

Difference Between MCB and MCCB Circuit Breaker

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