2.   The traditional molded-case circuit breaker uses electromechanical (thermal magnetic) trip units that may be fixed or interchangeable.  An MCCB provides protection by combining a temperature sensitive device with a current sensitive electromagnetic device.  Both these devices act mechanically on the trip mechanism. Circuit Breaker Basics

3.   Depending upon the application and required protection, an MCCB will use one or a combination of different trip elements that protect against the following conditions:  Thermal overloads;  Short circuits; and  Ground faults. Circuit Breaker Basics (cont.)

4.   Thermal overload:  In an overload condition, there's a temperature buildup between the insulation and conductor.  If left unchecked, the insulation's life will drastically reduce, ultimately resulting in a short circuit. Thermal Overload

5.   Usually, a short circuit occurs when abnormally high currents flow as a result of the failure of an insulation system.  This high current flow, termed short-circuit current, is limited only by the capabilities of the distribution system.  To stop this current flow quickly so that major damage can be prevented, the short circuit or instantaneous element of an MCCB is used. Short Circuit

6.   A ground fault actually is a type of short circuit, only it's phase-to-ground, which probably is the most common type of fault on low-voltage systems (600V or less).  Usually, arcing ground-fault currents are not large enough to be detected by the standard MCCB protective device. But, if left undetected, they can increase sufficiently to trip the standard protective device. Ground Fault

7.   Prior to the introduction of electronic CBs, separate ground fault protection devices were used to provide this additional level of protection. Today's modern electronic CB has the ground fault protection as an integral part of the trip unit. Ground Fault (cont.)

8.   Overload, or thermal trip action uses a piece of bimetal heated by the load current. This bimetal is actually two strips of metal bonded together, with each having a different thermal rate of heat expansion. They are factory-calibrated and not field- adjustable.  To trip the CB, this bimetal must deflect enough to physically push the trip bar and unlatch the contacts. Overload Trip Action

9.   Uses an electromagnet having a winding that's in series with the load current.  When a short circuit occurs, the current flowing through the circuit conductor causes the magnetic field strength of the electromagnet to increase rapidly and attract the armature.  When this happens, the armature rotates the trip bar, causing the CB to trip. Short Circuit Trip Action

10.   The only time delay factor involves the time it takes for the contacts to physically open and extinguish the arc; this usually is less than one cycle.  Magnetic elements are either fixed or adjustable, depending upon the type of CB and frame size. For example, most thermal magnetic breakers above the 150A frame size have adjustable magnetic trips. Short Circuit Trip Action

11.  Circuit Breaker Ratings

12.   The automatic operation of a circuit breaker occurs when there is an abnormal condition such as an overcurrent or short circuit.  Overcurrent is electrical current in excess of the equipment limit, total amperage load of a circuit, or conductor or equipment rating.  A short circuit is an unintentional connection of two ungrounded conductors that have a potential difference between them. Arc Conditions

13.  Arc Interruption

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15.  Racking LVPCB

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18.   DC circuit breakers work with the same principle of thermal protection and magnetic protection which is found in AC circuit breakers.  Thermal protection trips: This protection mechanism is based on a bimetallic contact that heats, expands and trips the circuit breaker.  Magnetic protection trips: This protection in a DC circuit breaker protects against short circuits and faults, which are drastically larger than an overload. DC Circuit Breaker

19.   A very important difference when interrupting alternating current and direct current is that the arc extinguishing point is higher for a DC circuit breaker.  In direct current where voltage is continuous, the electric arc is constant and more resistant to interruption.  For this reason, DC circuit breakers must include additional arc extinguishing measures: they typically have a mechanism to elongate and dissipate the electric arc in order to simplify interruption. DC Circuit Breaker

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