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Circuit breaker fundamentals

What is a circuit breaker

A circuit breaker is an electrical switch designed to protect an electrical circuit from damage caused by overcurrent/overload or short circuit. Its basic function is to interrupt current flow after protective relays detect a fault.

Circuit breaker technology

Take an indepth look at circuit breakers with a special emphasis on the Eaton's most technologically advanced circuit breaker, the Power Defense molded case circuit breaker.

Differences amoung overcurrents, overloads and short circuits


NEC defines and overcurrent as any current in excess of the rated current of equipment of the ampacity of a conductor. An over current is the result of an overload, short circuit, arc or ground fault. Effects of overcurrent include fires, conductor insulation damage and equipment damage.


According to NEC, an overload is the operation of equipment in excess of normal, full-load rating, or of a conductor in excess of rated ampacity that, when it persists for a sufficient length of time would cause damage or dangerous overheating. An overload is NOT a short circuit, ground or arc fault.

Short Circuits

A short circuit is an overcurrent which greatly exceeds the normal full load current of the circuit. Also, as the name infers, a short circuit leaves the normal current carrying path of the circuit and takes a short-cut around the load and back to the power source. A short circuit is an overcurrent but not an overload.

Circuit breakers vs fuses

Common OCPDs include circuit breakers and fuses. While the purpose of the fuses and circuit breakers is the same, there are fundamental differences, which are important to understand when making selections for protection of equipment and personnel.


  • Must be replaced after it has interrupted an overcurrent event.
  • Has less initial cost
  • No maintenance required
  • Only opens on overcurrent events
  • No capability for optional protective features

Circuit breakers

  • May be reset after interrupting an overcurrent event
  • Has a higher initial cost
  • Requires maintenance
  • Optional protective features (i.e. ground fault)

Circuit breaker components

Although low and medium voltage circuit breakers have unique designs that are specific to amperage, voltage and application, there are five main components that are universal across the different types of circuit breakers.

Five universal circuit breaker components

The five universal circuit breaker components are:

  1. Frame – Protects internal parts of the circuit breaker from outside materials
  2. Operating mechanism – Provides a means of opening and closing the circuit breaker
  3. Contacts – Allows the current to flow through the circuit breaker when closed.
  4. Arc extinguisher – Extinguishes an arc when the circuit breaker interrupts a fault.
  5. Trip unit – Opens the operating mechanism in the event of a prolonged overload or short circuit.
molded case circuit breaker with components labeled

Frames - circuit breaker component

The circuit breaker frame provides the rigidity and strength required to successfully deal with the interruption process and achieve the desired interrupting ratings. It insulates and isolates the electrical current in order to protect people and equipment during use or operation.  Frames can be made from  metal or molded insulating materials.

Metal Frame

A metal frame is assembled from precise metal pieces that are bolted and welded together to form the frame. Eaton offers low voltage power circuit breakers and medium voltage vacuum breakers with metal frames.


Molded Insulated Frame (Molded Case)

Molded case frames are made from strong insulating materials such as glass-polyester or thermoset composite resins (plastic frames). Eaton offers miniature circuit breakers, molded case circuit breakers, insulated case circuit breakers and low voltage power circuit breakers in molded case frames.

Operating mechanism - circuit breaker component

There are two types of operating mechanisms, over toggle and two step stored energy.

Over Toggle

The function of the operating mechanism is to provide a means of opening and closing the circuit breaker. This toggle mechanism is the quick-make, quick-break type, meaning that the speed with which the contacts snap open or close is independent of how fast the handle is moved.

In addition to indicating whether the breaker is ON or OFF, the operating mechanism handle indicates when the breaker is tripped by mobbing to a position midway between the ON and OFF.

Eaton’s residential, miniature and molded case circuit breakers utilize over-toggle mechanism.


Two Step Stored Energy

The two-step stored energy mechanism is used when a large amount of energy is required to close the circuit breaker and when it needs to close rapidly. The major advantages of this mechanism are rapid reclosing and safety. Rapid reclosing is achieved by storing charged energy in a separate closing spring. Safety is achieved by providing remote charging of the spring.

The two-step stored energy process is designed to charge the closing spring and release energy to close the circuit breaker. It uses separate opening and closing springs. This is important because it permits the closing spring to be charged independently of the opening process. This allows for an open-close-open duty cycle. The closing spring can be charged (or recharged) manually via a charging handle or electrically via a motor. The motor can be operated remotely, allowing maximum safety for the operator.

Contacts - circuit breaker component

Contacts are found in the arc interruption chamber (in low voltage circuit breakers) and in the vacuum interrupter (in medium voltage vacuum circuit breakers).

The contact assembly consists of the movable contact, the movable contact arm, the stationary contact and the stationary conductor.

As the circuit breaker opens or closes, the fixed contact moves to close (make) or open (break) the circuit. The contacts are designed to protect against two fault conditions

  1. Overcurrent (thermal overload)
  2. Short Circuit (magnetic)

Arc extinguisher - circuit breaker component

An arc is a discharge of electric current crossing a gap between two contacts. An arc extinguisher is the component of the circuit breaker that extinguishes an arc when the contacts are opened. Circuit breakers must be designed to control them because arcs cannot be prevented.

The six methods used in the electrical industry to manage electrical arcs:

  1. Arc chute
  2. Vacuum interrupter
  3. SF6
  4. Minimum oil
  5. Magnetic Coil
  6. Puffer

What is an arc flash

Arcs are formed when the contacts of a circuit breaker open due to larger than normal current. Arcing is a condition that must be dealt with quickly and effectively by a circuit breaker. The ability of the circuit breaker to control the arc is key to its short circuit interrupting capability

Trip unit - circuit breaker component

The function of a trip unit is to trip or open the operating mechanism in the event of a prolonged overload or short circuit fault condition such as thermal overload, short circuit currents and specialty faults. To accomplish this, an electromechanical or a solid state trip unit is provided.

Electromechanical trip units

Electromechanical trip units utilize bimetals and electromagnets to provide overload and short circuit protection and do not include any specialty protection such as ground fault. They are commonly used in low voltage circuit breakers.


Electronic trip units

Electronic trip units offer capabilities such as programming monitoring diagnostics communications system coordination and testing that are not available on thermal magnetic trip units. Eaton’s molded case circuit breakers and low voltage power circuit breakers utilize Electronic Trip Units (ETU).