ARCING IN CIRCUIT BREAKERS

Switchgear is a general term covering a wide range of equipment concerned with switching operation. A C.B. is a switching and current interrupting device in switchgear, it serves two purposes: (i) Switching during normal working conditions for the purpose of operating and maintenance. (ii) Switching during abnormal conditions such as S.C. and interrupting the fault currents.

The Trip circuit: When fault occurs secondary C.T. current increases relay is actuated:

·         After a very short time the relay closes the contacts.

·         The circuit of trip coil of C.B. is completed.

·         So coil energizes and trips the C.B. contacts.

·         Thus equipment is isolated in case of fault conditions.

                         

Under the normal conditions these contacts are remained closed.

Arc Phenomenon: When he contacts of a C.B. are separated under fault conditions, an arc is struck between them. The production of arc decays current interruption proceeds and generates enormous heat which may cause damage to the system or to the breaker itself. So arc extinction is important. The contacts are placed in a closed chamber containing some insulating media like liquid or gas for distinguishing the arc.

The Plasma State: The plasma state (of a gas or air) consists of large number of charged particles which cause conduction.

Each substance consists of molecule formed of atom. Normally, molecules and atoms are electrically neutral (positive charge = negative charge). If the matter is ionized by some means the matter can be made to conduct. For e.g. consider a gas kept in a container whose temperature is gradually increased. The molecules now experience a force in all directions. At high temperature molecules velocity increases and they collide with particle coming in their way at 3000 K dissociation occurs. At 6000 K atom become truly charged and electrons manage to escape. Thus the matter is ionized. If temperature is further increases plasma state is reached. The plasma consists of a charged atoms. In CBs. The contact space is ionized by:

i)        Thermal ionization at 3000 K dissociation – 6000⁰C electrons become free and impact against each other causes mutual collision ionization.

ii)       Ionization by collision.

iii)      Thermal emission from surface contacts – when contact separate, few spots on the surface of contact produce high current density due to which high local temperature is there, this causes thermal emission.

iv)     Secondary emission of contact surface – fast moving electron strikes on the surface of contact causing impact which produces emission from contact surface.

v)      Photo emission.

Arc Formation in C.B.: Separation of contacts leads to high temperature on contact surface. The electrons are emitted from contact surface thermal, secondary, field and photo emission. The gas between contacts is ionized by thermal ionization and ionization by collision. Because of this the space between contacts is in plasma state and therefore it conducts. So are discharge takes place between contacts. The waveform of A.C. arc is shown in fig.

                                     

Arc Interruption Theories:

a)   Slepians theory: This theory describes the arc extinction as a process of race between the dielectric strength and re-striking voltage. After current zero, there exists a column of ionized gas. If dielectric strength between gaps, buildup rapidly so that it is greater than the re-striking voltage, the arc does not re-strike. It is clear from figure that it dialect strength is (a), the arc will not re-strike but if it is as shown by (c) the arc re-strikes.

                                                 

Drawbacks:

1.    This theory assumes that re-striking voltage and buildup of dielectric strength are comparable quantities. This is not quite correct. Because these two quantities are not identical.

2.    It does not cover arcing phase hence it is incomplete.

b) Energy balance theory/Cassie’s theory; Following are the assumptions:

i) Arc consists of cylindrical column. This column has substantial uniform temperature over its cross-section with well defined boundary. There is uniform distribution of energy in this column.

ii) The temperature is constant.

iii) Cross section of arc adjusts itself to accommodate the arc current.

iv) Power dissipation is proportional to arc cross sectional area of arc column. Cassie explained the energy relationship as:

dΦ/dt = EI – N

where Φ = Energy content/cm of arc length

           E = Volts/cm

            I = Total current

           N = Total power loss/cm

Breakdown occurs if power feed in the arc is more than power loss.