The Arc Extinction 4.5 Electric Arc, 4.6 Arc formation in A.C. (C.B), 4.7 Modes at Arc Extinction, 4.8 Arc interruption theories

1. Switchgears and Protective Devices (Sunil S Rao)
Presented By: Engr. FAZAL UR REHMAN
Lecturer Electrical KPTEVTA
Visiting Lecturer COMWAVE INSTITUTE ISLAMABAD

2. Today's Topics
Chapter 4.Characteristics of Circuit Breakers
 The Arc Extinction
 4.5 Electric Arc,
 4.6 Arc formation in A.C. (C.B),
 4.7 Modes at Arc Extinction,
 4.8 Arc interruption theories

3. Electric Arc
 The self sustained discharge of electricity between
electrodes in gas or vapour having voltage drop across
it is called an electric arc.
 This spark / arc formation resulted due small
clearances between electrodes (in order of few cm) is
also termed as discharge.
 This discharge is classified on the basis of V-A zones;
 Glow discharge,
 Townsend discharge,
 Arc discharge.

4.  In the case of an arc discharge, the discharge is
sustained by thermo-field electron emission from the
cathode and it is accompanied with the formation of a
tiny cathode spot.
 In the case of a glow discharge, the discharge is
sustained by secondary electron emission due to ion
bombardment.
 The Townsend discharge is a gas ionization process
where an initially very small amount of free electrons,
accelerated by a sufficiently strong electric field, give
rise to electrical conduction through a gas
by avalanche multiplication. Townsend discharge is
named after John Sealy Townsend.

7. Arc Discharge
 During opening of current carrying contacts in
a circuit breaker the medium in between opening
contacts become highly ionized.
 Through which the interrupting current gets low
resistive path
 And continues to flow through this path even the
contacts are physically separated. This is called arc
discharge.

8.  Separation of circuit breaker contact result in arc
production between the contacts as shown (arc
current).
 Due to high resistive air stream voltage drop takes place
across the arc which is called Arc Voltage.

9. Arc Extinction
 Air / Vacuum
Gas / Oil
 Used for Arc-
Extinction.
 E.g. SF6 gas
has been used
over here.

10. Arc Formation in AC C.B
 During separation of contacts, contact surfaces locally
got heated.
 Due to high temperature electron emission from the
surface takes place.
 Emission of electron result in ionization of gas
between the contact space due to thermal ionization
and collision.
 Mixture of ions and gas is formed between the contact.
(Plasma formation)
 The ionized gas carry the arc current between contact.

11. Modes at Arc Extinction
 There are two modes of arc interruption;
1. High Resistance interruption
2. Low Resistance / Zero point interruption.
Note. Artificial current zero principle is used in HVDC
system breakers

12. High Resistance Interruption
 As the name suggest ‘ In this method the arc is
interrupted by a high resistance path. As increase in
resistance decrease the current (Ohm’s Law), and
hence the arc.
 This method is used in low and medium voltage AC
and DC circuit breakers.

13.  High Resistance interruption can be done in many
ways, like;
1. Lengthening of arc by arc runner. Horn like blades
of conducting material connected to arcing contacts.
2. Splitting of arc. Resin bonded fibre glass plates
placed in the path perpendicular to the arc blown with
the help of magnetic field.
3. Cooling of arc. By bringing the arc in contact with
cooler air result in increase of arc resistance.

15. Low Resistance of zero point
interruption
 AC signal of 50Hz passes through zero point 100 times
every second. As arc is basically a current flow through
ionized gases.
 So in each second current pass 100 times through zero
point and hence arc vanishes for 100 times in each
second.
 The re appearance of arc can be interrupted in each
chance (zero point) by removing the ionized gases
with the help of some dielectric medium of high
dielectric strength.

16. Arc interruption Theories
 How arc interruption occur in a circuit breaker ?
 There are mainly two theories given by Two scientists.
So let us know about it. These two main theory explain
current zero interruption of arc. The two arc
interruption theory is as follows:.
1. Slepian’s Theory.
2. Cassies theory

17. Slepian’s Theory
 It describes the process as a race between the dielectric
strength and restriking voltage.
 After every current zero, there is a column of residual
ionized gas.
 This may cause the arc to strike again by developing
necessary restriking voltage and this voltage is stress is
sufficient to detach electron out of their orbit due to
which a great heat is released.

18.  According to this theory rate at which positive ion and
electrons combine to form neutral molecules is
compared with the rate of rising of restriking voltage.
 Due to the recombination dielectric strength of the
gap gets recovered.
 Hence the rate of recovery of dielectric strength is
compared with RRRV(rate of rising of restriking
voltage).
 If the restriking voltage rises more rapidly than
the dielectric strength, gap space breaks down
and arc strikes again and persist.

19.  According to the graph here (a), point denote rate
of dielectric strength is more than restriking voltage
and point (b) denotes the rate of dielectric strength is
less than the rate of rising of restriking voltage.

20.  Drawbacks:
 This assumption made while developing this theory is
that the restriking voltage and rise of dielectric strength
are comparable quantities which are not quite correct.
 This theory does not consider the energy relations in arc
extinction. The arcing phase is not covered by this
theory so it is incomplete.

21. Cassie’s Theory
 Cassie has given an alternative explanation of above
process. It is also called Energy Balance Theory.
 He suggested that the re-establishment of arc or
interruption of an arc both is an energy balance process.
 This means that if energy input to an arc continues to
increase, the arc restrikes and if not, arc gets interrupted.

22.  The assumption made in this theory:
 Arc consists of cylindrical column having a uniform
temperature at its cross-section. The energy
distributed in the column is uniform.
 The temperature remains constant.
 The cross-section of arc adjusts itself to accommodate
the arc current.
 Power dissipation is proportional to the cross-sectional
area of arc column.

23.  Energy equation given by Cassie is given as,
dQ/dt=EI-N
where Q= Energy content/ length of the arc in cm
E= Volts/cm
I=total current
N=Total power loss/cm

24.  Breakdown occurs if power fed to the arc is greater
than power loss.
 This theory holds true for High current. When it
reaches current zero immediately after that contact
space contain ionized gas and therefore has a finite
post zero resistance.
 Now there is RRV(rising restriking voltage). This
causes current to flow between the contacts. Due to
this, power gets dissipated in the form of heat.

25.  Initially when RV(restriking voltage is zero), automatically
current and power will be zero.
 It again gets zero when space has become fully deionize
and resistance between the contacts is infinitely high.
 In between these two limits, the power dissipated rise to a
maximum.
 If the heat so generated exceeds the rate at which heat can
be removed from contact space, Ionization will persist and
breakdown will occur, giving an arc for another half cycle.