Circuit breakers are constructed in five major components:
Frame (Molded Case) Contacts, Arc Chute Assembly,Operating Mechanism and Trip Unit.
The frame provides an insulated housing to mount the circuit breaker components. The construction material is usually a thermal set plastic such as glass-polymer. The construction material can be a factor in determining the interruption rating of the circuit breaker. Frame ratings indicate several pieces of important information such as; maximum voltage, ampere rating, interrupting rating, and physical size.
Circuit breakers use contacts to break the circuit and stop the flow of energy. Some conventional circuit breakers use a straight-through contact arrangement. The electrical path through the contacts is a straight line.a magnetic field is developed around a current carrying conductor. The magnetic fields developed around the contact arms of a straight-through contact arrangement have little or no effect on the contacts arms. During a fault, the contacts are only opened by the mechanical operation of the circuit breaker spring.
But current causes heat, which is destructive to electrical equipment. A rise in current causes a corresponding rise in heat. In reality, the thermal energy the circuit will see is proportional to the square of the current multiplied by the time the current flows (I2T). This means that the higher the level of current, the shorter the time it takes for heat to damage equipment. In the following illustration, IP represents the peak level the fault current rises before the breaker contacts open.
The blow-apart contacts are two contact arms are positioned parallel to each other as shown in the following illustration. As current flows through the contact arms, magnetic fields are set up around each arm. Because the current flow in one arm is opposite in direction to the current flow in the other arm, the two magnetic fields oppose each other. The strength of the magnetic field is directly proportional to the amount of current. During normal current conditions, the magnetic field is not strong enough to force the contacts apart.
When a fault develops, current increases which increases the strength of the magnetic field. The increased strength of the opposing magnetic fields actually helps to open the contacts faster by forcing them apart.
As the contacts open a live circuit, current continues to flow for a short time by jumping the air space between the contacts in the form of an arc. When the contacts open far enough the arc is extinguished and the current flow stops.
The arc can cause burning on the contacts. In addition, ionized gases form inside the molded case. If the arc isn’t extinguished quickly the pressure from the ionized gases could cause the molded case to rupture. An arc chute assembly is used to quench the arc. This assembly is made up of several “U” shaped steel plates that surround the contacts. As the arc is developed it is drawn into the arc chute where it is divided into smaller arcs, which are extinguished faster.
An operating handle is provided to manually open and close the contacts. Molded case circuit breakers (MCCBs) are trip free, meaning that they can’t be prevented from tripping by holding or blocking the operating handle in the “ON” position. There are three positions of the operating handle: “ON” (contacts closed), “OFF” (contacts open), and “TRIPPED” (mechanism in tripped position). The circuit breaker is reset after a trip by moving the handle to the “OFF” position and then to the “ON” position.
The operating handle is connected to the moveable contact arm through an operating mechanism. Molded case circuit breakers use an over-center toggle mechanism that is a quickmake and quick-break design. In the following illustration, the operating handle is moved from the “OFF” to the “ON” position. In this process a spring begins to apply tension to the mechanism. When the handle is directly over the center the tension in the spring is strong enough to snap the contacts closed. This means that the speed of the contact closing and opening is independent of how fast the handle is operated.To open the contacts, the operating handle is moved from the “ON” to the “OFF” position. In this process a spring begins to apply tension to the mechanism. When the handle is directly over the center the tension in the spring is strong enough to snap the contacts open. As in closing the circuit breaker contacts, contact opening speed is independent of how fast the handle is operated.
The trip unit is the “brain” of the circuit breaker. It consists of components that will automatically trip the circuit breaker when it senses an overload or short circuit. The tripper bar is moved by a manual “PUSH TO TRIP” button, a thermal overcurrent sensing element or an electromagnet.
A trip mechanism is held in place by the tripper bar. As long as the tripper bar holds the trip mechanism, the mechanism remains firmly locked in place.The operating mechanism is held in “ON” position by the trip mechanism. When a trip is activated, the trip mechanism releases the operating mechanism, which opens the contacts.
MCCBs, heavy duty and above, can be manually tripped by depressing the red “PUSH TO TRIP” button on the face of the circuit breaker. When the button is pressed the tripper bar rotates up and to the right. This allows the trip mechanism to “unlock” releasing the operating mechanism. The operating mechanism opens the contacts. The “PUSH TO TRIP” button also serves as a safety device by not allowing access to the circuit breaker interior in the “ON” position. If an attempt is made to remove the circuit breaker cover while the contacts are in the closed (“ON”) position, a spring located under the pushbutton will cause the button to lift up. This action will also trip the breaker.
Frame (Molded Case) Contacts, Arc Chute Assembly,Operating Mechanism and Trip Unit.
The frame provides an insulated housing to mount the circuit breaker components. The construction material is usually a thermal set plastic such as glass-polymer. The construction material can be a factor in determining the interruption rating of the circuit breaker. Frame ratings indicate several pieces of important information such as; maximum voltage, ampere rating, interrupting rating, and physical size.
But current causes heat, which is destructive to electrical equipment. A rise in current causes a corresponding rise in heat. In reality, the thermal energy the circuit will see is proportional to the square of the current multiplied by the time the current flows (I2T). This means that the higher the level of current, the shorter the time it takes for heat to damage equipment. In the following illustration, IP represents the peak level the fault current rises before the breaker contacts open.
The blow-apart contacts are two contact arms are positioned parallel to each other as shown in the following illustration. As current flows through the contact arms, magnetic fields are set up around each arm. Because the current flow in one arm is opposite in direction to the current flow in the other arm, the two magnetic fields oppose each other. The strength of the magnetic field is directly proportional to the amount of current. During normal current conditions, the magnetic field is not strong enough to force the contacts apart.
When a fault develops, current increases which increases the strength of the magnetic field. The increased strength of the opposing magnetic fields actually helps to open the contacts faster by forcing them apart.
As the contacts open a live circuit, current continues to flow for a short time by jumping the air space between the contacts in the form of an arc. When the contacts open far enough the arc is extinguished and the current flow stops.
The arc can cause burning on the contacts. In addition, ionized gases form inside the molded case. If the arc isn’t extinguished quickly the pressure from the ionized gases could cause the molded case to rupture. An arc chute assembly is used to quench the arc. This assembly is made up of several “U” shaped steel plates that surround the contacts. As the arc is developed it is drawn into the arc chute where it is divided into smaller arcs, which are extinguished faster.
An operating handle is provided to manually open and close the contacts. Molded case circuit breakers (MCCBs) are trip free, meaning that they can’t be prevented from tripping by holding or blocking the operating handle in the “ON” position. There are three positions of the operating handle: “ON” (contacts closed), “OFF” (contacts open), and “TRIPPED” (mechanism in tripped position). The circuit breaker is reset after a trip by moving the handle to the “OFF” position and then to the “ON” position.
The operating handle is connected to the moveable contact arm through an operating mechanism. Molded case circuit breakers use an over-center toggle mechanism that is a quickmake and quick-break design. In the following illustration, the operating handle is moved from the “OFF” to the “ON” position. In this process a spring begins to apply tension to the mechanism. When the handle is directly over the center the tension in the spring is strong enough to snap the contacts closed. This means that the speed of the contact closing and opening is independent of how fast the handle is operated.To open the contacts, the operating handle is moved from the “ON” to the “OFF” position. In this process a spring begins to apply tension to the mechanism. When the handle is directly over the center the tension in the spring is strong enough to snap the contacts open. As in closing the circuit breaker contacts, contact opening speed is independent of how fast the handle is operated.
The trip unit is the “brain” of the circuit breaker. It consists of components that will automatically trip the circuit breaker when it senses an overload or short circuit. The tripper bar is moved by a manual “PUSH TO TRIP” button, a thermal overcurrent sensing element or an electromagnet.
A trip mechanism is held in place by the tripper bar. As long as the tripper bar holds the trip mechanism, the mechanism remains firmly locked in place.The operating mechanism is held in “ON” position by the trip mechanism. When a trip is activated, the trip mechanism releases the operating mechanism, which opens the contacts.
MCCBs, heavy duty and above, can be manually tripped by depressing the red “PUSH TO TRIP” button on the face of the circuit breaker. When the button is pressed the tripper bar rotates up and to the right. This allows the trip mechanism to “unlock” releasing the operating mechanism. The operating mechanism opens the contacts. The “PUSH TO TRIP” button also serves as a safety device by not allowing access to the circuit breaker interior in the “ON” position. If an attempt is made to remove the circuit breaker cover while the contacts are in the closed (“ON”) position, a spring located under the pushbutton will cause the button to lift up. This action will also trip the breaker.
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