26-11-2012, 11:43 AM
Circuit Breakers
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Definition
A circuit breaker is defined as a piece of
equipment which can do any one of the
following tasks:
Makes or breaks a circuit either manually or
by remote control under normal conditions
Breaks a circuit automatically under fault
conditions
Makes a circuit either manually or by
remote control under fault conditions
Switching Function
Thus a circuit breaker is used for
incorporating manual as well as
automatic control for the switching
function.
The automatic control of the circuit
breaker is incorporated with the
help of relays.
The automatic control is only done
in case of fault conditions
Advantage over Fuse
A fuse operates once and then has
to be replaced.
The main advantage associated
with the use of circuit breaker is
that a circuit breaker can be reset
(either manually or automatically)
to resume normal operation.
Circuit Breaker Contacts
The primary contact is always
made of a high conductive material
such as copper (Cu).
The arcing contact is made of arc
resistance material such as
tungsten or molybdenum, which
has a much lower conductivity than
those used for primary contacts.
Arc Phenomenon
When the contacts of a circuit
break are separated under fault
conditions, an arc is struck
between them. The current is thus
able to continue until the discharge
ceases.
Arc is Useful?
The arc is useful in a way as it provides a
low resistance path for the current after
contact separation. It prevents current
chopping and associated abnormal
switching over-voltages in the system.
The arc provides a gradual, but quick,
transition from the current-carrying to the
current-breaking states of the contacts.
Based on Interruptiing
Mediia
The circuit breakers may be classified
into following categories:
Oil Circuit Breakers
Air Circuit Breakers
Air-blast Circuit Breakers
Sulphur Hexafluoride (SF6) Circuit
Breakers
Vacuum Circuit Breakers
Short-time Rating
The period for which the circuit
breaker is able to carry fault
current while remaining closed is
known as short-time rating.
The short-time rating of a circuit
breaker depends upon its ability to
withstand:
1. The electromagnetic force effects
2. The temperature rise