The following section presents some basics of circuit breaker design. Circuit breakers are constructed from the following five major components:
• Frame (Molded Case)
• Contacts
• Arc Chute Assembly
• Operating Mechanism
• Trip Unit
Straight-Through Contacts
The current flowing in a circuit controlled by a circuit breaker flows through the circuit breaker’s contacts. When a circuit breaker is turned off or is tripped by a fault current, the circuit breaker interrupts the flow of current by separating its contacts.
Many circuit breakers use a straight-through contact arrangement, so called because the current flowing in one contact arm continues in a straight line through the other contact arm.
Blow-Apart Contacts
As an improvement over the straight-through contact design, Siemens developed the blow-apart contact design now commonly used by circuit breakers with higher interrupting ratings. With this design, the two contact arms are positioned parallel to each other, as shown in the following illustration. As current flows through the contact arms, magnetic fields develop 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. Under normal conditions, the magnetic fields are not strong enough to force the contacts apart.
When a fault develops, current increases rapidly causing the strength of the magnetic fields surrounding the contacts to increase as well. The increased strength of the opposing magnetic fields helps to open the contacts faster by forcing them apart.
By reducing the time required to open circuit breaker contacts when a fault occurs, the blow-apart contact design reduces the damaging heat felt by the circuit protected by the circuit breaker.
Arc Chute Assembly
When circuit breaker contacts open, current continues to flow for a short time by arcing across the air space between the contacts. When the contacts open far enough, the arc is extinguished and the current stops.
Minimizing the arc is important for two reasons. First, arcing can damage the contacts. Second, the arc ionizes gases inside the molded case. If the arc isn’t extinguished quickly the pressure from the ionized gases can cause the molded case to rupture.
Circuit breakers commonly use an arc chute assembly to quench the arc. This assembly is made up of several “U” shaped steel plates that surround the contacts. As the arc developes, it is drawn into the arc chute where it is divided into smaller arcs, which are extinguished faster.
Operating Mechanism
The operating handle is connected to the movable contact arm through an operating mechanism. Siemens molded case circuit breakers use an over-center toggle mechanism that is a quick-make 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 is independent of how fast the handle is operated.
The contacts are opened by moving the operating handle 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. Therefore, contact opening speed is also independent of how fast the handle is operated.
Trip Unit
In addition to providing a means to open and close its contacts manually, a circuit breaker must automatically open its contacts when an overcurrent is sensed. The trip unit is the part of the circuit breaker that determines when the contacts will open automatically.
In a thermal-magnetic circuit breaker, the trip unit includes elements designed to sense the heat resulting from an overload condition and the high current resulting from a short circuit. In addition, some thermal-magnetic circuit breakers incorporate a “Push-to-Trip” button.
• Contacts
• Arc Chute Assembly
• Operating Mechanism
• Trip Unit
Frame
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. Typical frame ratings include: maximum voltage, maximum ampere rating, and interrupting rating.
Straight-Through Contacts
The current flowing in a circuit controlled by a circuit breaker flows through the circuit breaker’s contacts. When a circuit breaker is turned off or is tripped by a fault current, the circuit breaker interrupts the flow of current by separating its contacts.
Blow-Apart Contacts
As an improvement over the straight-through contact design, Siemens developed the blow-apart contact design now commonly used by circuit breakers with higher interrupting ratings. With this design, the two contact arms are positioned parallel to each other, as shown in the following illustration. As current flows through the contact arms, magnetic fields develop 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. Under normal conditions, the magnetic fields are not strong enough to force the contacts apart.
Arc Chute Assembly
When circuit breaker contacts open, current continues to flow for a short time by arcing across the air space between the contacts. When the contacts open far enough, the arc is extinguished and the current stops.
Minimizing the arc is important for two reasons. First, arcing can damage the contacts. Second, the arc ionizes gases inside the molded case. If the arc isn’t extinguished quickly the pressure from the ionized gases can cause the molded case to rupture.
Operating Mechanism
The operating handle is connected to the movable contact arm through an operating mechanism. Siemens molded case circuit breakers use an over-center toggle mechanism that is a quick-make 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 is independent of how fast the handle is operated.
Trip Unit
In addition to providing a means to open and close its contacts manually, a circuit breaker must automatically open its contacts when an overcurrent is sensed. The trip unit is the part of the circuit breaker that determines when the contacts will open automatically.
