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Circuits are designed to stay within their amp rating. There are instances where excess current is drawn and can cause catastrophic damage. This is the reason every circuit must have protection from overcurrents. Overcurrents include overloads, short circuits, arc faults and ground faults.
This protection is provided by an overcurrent protective device (OCPD). The main purpose of a OCPD is to protect the wire from overheating in an overcurrent situation, therefore preventing or limiting the damage to people or equipment downstream of the OCPD. OCPDs include both circuit breakers and fuses.
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.
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 shortcut around the load and back to the power source. A short circuit is an overcurrent but not an overload.
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.
The five universal circuit breaker components are:
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.
Moulded case frames are made from strong insulating materials such as glass polyester or thermoset composite resins (plastic frames). Easton offers miniature circuit breakers, moulded case circuit breakers, insulated case circuit breakers and low voltage power circuit breakers in moulded case frames.
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 moulded case circuit breakers utilise over-toggle mechanism.
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 re-closing and safety. Rapid re-closing 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 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
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:
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
Electromechanical trip units utilise bimetals and electromagnets to provide overload and short circuit protection and do not include any specialist protection such as ground fault. They are commonly used in low voltage circuit breakers.
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 moulded case circuit breakers and low voltage power circuit breakers utilise electronic trip units (ETU).
