CIRCUIT BREAKERS

CIRCUIT BREAKERS

A circuit breaker is a device that is used to completely disconnect a circuit when any  abnormal  condition  exists.   The  circuit  breaker  can  be  designed  to  actuate under any undesirable condition.

Introduction

The purpose of a circuit breaker is to break the circuit and stop the current flow when the current exceeds  a  predetermined  value  without  causing  damage  to  the  circuit  or  the  circuit  breaker. Circuit breakers are  commonly used in place of fuses  and sometimes eliminate  the need for a switch. A   circuit   breaker   differs from a fuse in that it "trips" to break the circuit and may be reset, while a fuse   melts   and   must   be   replaced. Air   circuit   breakers   (ACBs)   are breakers  where  the  interruption  of the breaker contacts takes place in an air environment.  Oil circuit breakers (OCBs)  use  oil  to  quench  the  arc when the breaker contacts open.

 

 

 

 

A cutaway view of the molded case circuit breaker

 

 

A  circuit  can  be  connected  or  disconnected  using  a  circuit  breaker  by  manually  moving  the operating handle to the ON or OFF position.  All breakers, with the exception of very small ones, have a linkage between the operating handle and contacts that allows a quick make (quick break contact action) regardless of how fast the operating handle is moved.  The handle is also designed so that it cannot be held shut on a short circuit or overload condition.  If the circuit breaker opens under one of these conditions, the handle will go to the trip-free position.  The trip-free position is midway between the ON and OFF positions and cannot be re-shut until the handle is pushed to the OFF position and reset. A circuit breaker will automatically trip when the current through it exceeds a pre-determined value.  In lower current ratings, automatic tripping of the circuit breaker is accomplished by use of thermal tripping devices.   Thermal trip elements consist of a bimetallic element that can be calibrated  so  that  the  heat  from  normal  current  through  it  does  not  cause  it  to  deflect.    An abnormally  high  current,  which  could  be  caused  by  a  short  circuit  or  overload  condition,  will cause the element to deflect and trip the linkage that holds the circuit breaker shut.   The circuit breaker will then be opened by spring action.   This bimetallic element, which is responsive to the  heat  produced  by  current  flowing  through  it,  has  an  inverse-time  characteristic.    If  an extremely high current is developed, the circuit breaker will be tripped very rapidly.

For moderate overload currents, it will operate more slowly.   Molded case breakers with much larger current ratings also have a magnetic trip element to supplement the thermal trip element. The magnetic unit utilizes the magnetic force that surrounds the conductor to operate the circuit breaker tripping linkage. When the separable contacts of an air circuit breaker are opened, an arc develops between the two contacts.  Different manufacturers use many designs and arrangements of contacts and their surrounding chambers.   The most common design places the moving contacts inside of an arc chute.  The construction of this arc chute allows the arc formed as the contacts open to draw out into the arc chute.   When the arc is drawn into the arc chute, it is divided into small segments and quenched.  This action extinguishes the arc rapidly, which minimizes the chance of a fire and also minimizes damage to the breaker contacts. Molded case circuit breakers come in a wide range of sizes and current ratings.   There are six frame sizes available:   100, 225, 400, 600, 800, and 2,000 amps.   The size, contact rating, and current  interrupting  ratings  are  the  same  for  all  circuit  breakers  of  a  given  frame  size.    The continuous  current  rating  of  a  breaker  is  governed  by  the  trip  element  rating.    The  range  of voltage available is from 120 to 600 volts, and interrupting capacity ranges as high as 100,000 amps.