Low Voltage Circuit Breakers - Design and Construction Basics

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In this article, we are going to study the basics of low voltage circuit breaker design and construction. The operation and arc extinction of different types of circuit breakers were already discussed. The basic design principles of low rating circuit breakers (MCB, MCCB, GFCI, etc.) are explained here. Variations to these design principles will be discussed later.

Circuit breakers are electrical switchgear which are constructed from the following five major components.

Frame
Contacts
Arc Chute Assembly
Operating Mechanism
Trip Unit

The functions and features of each component are explained with pictures.

Table of Contents

1. 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.

2. 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.

Contacts are of two types depending on the interrupting rating.

Straight-Through Contacts
Blow-Apart Contacts

Straight-Through Contacts

Some 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

Blow-apart contact design is commonly used by circuit breakers with higher interrupting ratings. With this design, the two contact arms are positioned parallel to each other.

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.

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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.

3. Arc Chute Assembly

The arc is extinguished in this assembly. 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.

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.

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.

Arcing phenomenon and methods of arc extinction were discussed in previous articles.

Operating Handle

A circuit breaker must provide a manual means for energizing and de-energizing a circuit and must be capable of being reset after a fault condition has been cleared.

These capabilities are typically provided through the use of an operating handle.

Molded case circuit breakers (MCCBs) are trip free, meaning that they cannot 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, contacts open). The circuit breaker is reset after a trip by moving the handle to the “OFF” position and then to the “ON” position.

4. Operating Mechanism

The operating handle is connected to the moveable contact arm through an operating mechanism.

In the following illustration, the operating handle is moved from the “OFF” to the “ON” position (Figure 1). 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.

Operating Mechanism of Low Rating Circuit Breakers

The contacts are opened by moving the operating handle from the “ON” to the “OFF” position (Figure 2). In this process, 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.

5. 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.