Low-voltage circuit breaker basics

A circuit breaker is designed to keep an undesirably large amount of current, voltage, or power out of a given part of an electrical circuit. Industrial circuit breaker categories tend to follow voltage classes, which are divided according to magnitude. The IEEE divides voltage systems into four classes listed in the table titled "IEEE voltage classifications.


Key concepts
Circuit breaker construction
Trip elements
Trip-free and nontrip-free circuit breakers
Circuit breaker maintenance
IEEE voltage classifications

A circuit breaker is designed to keep an undesirably large amount of current, voltage, or power out of a given part of an electrical circuit.

Industrial circuit breaker categories tend to follow voltage classes, which are divided according to magnitude. The IEEE divides voltage systems into four classes listed in the table titled "IEEE voltage classifications."

Circuit breakers found in industrial plants accommodate all voltage levels. However, low and medium-voltage circuit breakers comprise the lion's share of switchgear used in industrial manufacturing plants. The focus of this article is limited to low-voltage circuit breakers.

The main classifications of low-voltage circuit breakers are "toggle" mechanism and two-step stored energy mechanism circuit breakers. The molded-case circuit breaker (MCCB) (Fig. 1) has a toggle mechanism with a distinct tripped position, which is typically midway between on and off.

The low-voltage power circuit breaker (LVPCB) (Fig. 2) has a two-step stored energy mechanism. This type of mechanism uses an energy storage device, such as a spring, that is "charged" and then released, or "discharged," to close the circuit breaker. The LVPCB is older technology. Therefore the trend is away from LVPCB and toward insulated case circuit breakers (ICCB) because of reduced maintenance. No dust or contaminants can get into the sealed compartments of the ICCB and components are designed to ensure longer life.


Circuit breaker construction

As shown in Fig. 3, most circuit breakers have five main components:

  • Frame or molded case

  • Operating mechanism

  • Arc extinguishers and contacts

  • Terminal connectors

  • Trip bar or element.

    • The frame provides an insulated housing and is used to mount the circuit breaker components. The frame determines the physical size of the circuit breaker and the maximum allowable voltage and current. The operating mechanism provides a means of opening and closing the breaker contacts. In addition to indicating whether the breaker is open or closed, the operating mechanism handle indicates when the breaker has opened automatically (tripped) by moving to a position between on and off. To reset the circuit breaker, first move the handle to the "off" position, and then to the "on" position.

      The arc extinguisher confines, divides, and extinguishes the arc drawn between contacts each time the circuit breaker interrupts current. The arc extinguisher is actually a series of contacts that open gradually, dividing the arc and making it easier to confine and extinguish (Fig. 4). Arc extinguishers are generally used in circuit breakers that control a large amount of power, such as those found in power distribution panels. Small power circuit breakers, such as those found in lighting panels, may not have arc extinguishers.

      Terminal connectors are electrically connected to the contacts of the circuit breaker and provide the means of connecting the circuit breaker to the circuit. The trip element is the part of the circuit breaker that senses the overload condition and causes the circuit breaker to trip or break the circuit. Some circuit breakers use solid-state trip units, which use current transformers and solid-state circuitry.

      Trip elements

      The thermal trip element circuit breaker, like a delay fuse, protects a circuit from a small overload that continues for a long time (Fig. 5). The larger the overload, the faster the circuit breaker trips. The thermal element also protects the circuit from temperature increases. A magnetic circuit breaker trips instantly when the preset current is present. In some applications, both types of protection are desired. Rather than use two separate circuit breakers, a single trip element combining thermal and magnetic trip elements is used.

      A magnetic trip element circuit breaker uses an electromagnet in series with the circuit load. With normal current, the electromagnet does not have enough attraction to the trip bar to move it; the contacts remain closed. The strength of the magnetic field of the electromagnet increases as current through the coil increases. As soon as the current in the circuit becomes large enough, the trip bar is pulled toward the magnetic element (electromagnet), the contacts are opened, and the current stops.

      The amount of current needed to trip the circuit breaker depends on the size of the gap between the trip bar and the magnetic element. On some circuit breakers, this gap, and therefore the trip current, is adjustable.

      In the thermal-magnetic trip element circuit breaker, a magnetic element is connected in series with the circuit load, and the load current heats a bimetallic element. Thermal-magnetic trip element operation is detailed in Fig. 6a and 6b.

      Trip-free and nontrip-free circuit breakers

      Circuit breakers are classified as being trip free or nontrip free. A trip-free circuit breaker is a circuit breaker that trips even if the operating mechanism is held in the "on" position. A nontrip-free circuit breaker can be reset and/or held "on" even if an overload or excessive heat condition is present. In other words, a nontrip-free circuit breaker can be bypassed by holding the operating mechanism "on."

      Trip-free circuit breakers are used on circuits that cannot tolerate overloads and on nonemergency circuits. Examples of these are precision or current sensitive circuits, nonemergency lighting circuits, and nonessential equipment circuits. Nontrip-free circuit breakers are used for circuits that are essential for operations. Examples of these circuits are emergency lighting, required control circuits, and essential equipment circuits.

      Circuit breaker maintenance

      Circuit breakers that can be accessed for maintenance require careful inspection and periodic cleaning. Before you attempt to work on circuit breakers, check the applicable technical manual carefully. Remove power to the circuit breaker before you work on it. Tag the switch that removes the power from the circuit breaker to ensure that power is not applied while you are working.

      Manually operate the circuit breaker several times to ensure the operating mechanism works smoothly. Inspect the contacts for pitting caused by arcing or corrosion. If pitting is present, smooth the contacts with a fine file or number 00 sandpaper.

      Be certain the contacts make proper contact when the operating mechanism is in the "on" position.

      Check the connections at the terminals to ensure the terminals and wiring are tight and free from corrosion. Check all mounting hardware for tightness and wear. Check all components for wear. Clean the circuit breaker completely.

      When you have finished working on the circuit breaker, restore power and remove the tag from the switch that applies power to the circuit.

      PLANT ENGINEERING magazine extends its appreciation to Eaton | Cutler-Hammer, E-T-A Circuit Breakers, Rockwell Automation, Schneider Electric, and Siemens Energy & Automation, Inc., for the use of their materials in the preparation of this article.

      IEEE voltage classifications

      Low-voltage systems

      &1000 Vac

      Medium-voltage systems

      >1000 Vac to

      100,000 Vac*

      High-voltage systems

      >100,000 Vac to

      230,000 Vac

      Extra-high voltage systems

      >230,000 Vac to 800,000 Vac


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