Download Low and Medium Voltage Circuit Breakers

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Circuit Breaker Basics
 The traditional molded-case circuit breaker uses
electromechanical (thermal magnetic) trip units that
may be fixed or interchangeable.
 A MCCB provides protection by combining a
temperature sensitive device with a current sensitive
electromagnetic device.
 Both these devices act mechanically on the trip
Circuit Breaker Basics (cont.)
 Depending upon the application and required
protection, a MCCB will use one or a combination of
different trip elements that protect against the
following conditions:
 Thermal overloads;
 Short circuits; and
 Ground faults.
Thermal Overload
 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.
Short Circuit
 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
 To stop this current flow quickly so that major
damage can be prevented, the short circuit or
instantaneous element of an MCCB is used.
Ground Fault
 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
Ground Fault (cont.)
 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.
Overload Trip Action
 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 fieldadjustable.
 To trip the CB, this bimetal must deflect enough to
physically push the trip bar and unlatch the contacts.
Short Circuit Trip Action
 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
 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.
Circuit Breaker Ratings
Arc Conditions
 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 Interruption
Arc Interruption
Racking LVPCB
Racking LVPCB
Racking LVPCB
Remote Racking
DC Circuit Breaker
 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
 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
 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
DC Circuit Breaker