Breakdown in solids, liquids and gases

AJITH VIJAYAN, ASST PROFESSOR, EEE DEPT., GEC WAYANAD
MECHANISM OF BREAKDOWN IN GASES, LIQUIDS AND SOLIDS
MECHANISM OF BREAKDOWN IN GASES
At normal temperature and pressure, the gases
are excellent insulators. The current conduction
is of the order of 10–10
A/cm2
. This current
conduction is due to the ionization. At higher
fields, charged particles may gain sufficient
energy between collisions to cause ionization
on impact with neutral molecules
Explanation
Gaseous dielectrics contain free electrons which
may be caused by irradiation or field emission
and this can lead to a breakdown process. On
the application of an electric field these free
electrons are accelerated from the cathode to
the anode. They acquire a kinetic energy (½
mv2
) as they move through the field. These free
electrons, moving towards the anode collide
with the gas molecules present between the
electrodes. In these collisions, part of the
kinetic energy of the electrons is lost and part is
transmitted to the neutral molecule. This
molecule gains sufficient energy it may ionize
by collision. The newly liberated electron and
the impinging electron are then accelerated in
the field and an electron avalanche is set up.
Further increase in voltage results in additional
ionizing processes. Ionization increases rapidly
with voltage.
Ionization can occur in any of the following
ways:
(i) Ionization by simple collision: When the
kinetic energy of an electron (½ mu²), in
collision with a neutral gas molecule exceeds
the ionization energy of the molecule, then
ionization can occur.
M + e
-
(½ mu²) → M
+
+ 2 e
-
(ii) Ionization by Double electron impact : If a
gas molecule is already raised to an excited
state (with energy Ee) by a previous collision,
then ionization of this excited molecule can
occur by a collision with a relatively slow
electron. This electron would need less energy
than the ionization energy, but the energy must
exceed the additional energy required to attain
the ionization energy.
(iii)Photo-ionization: This ionization by
radiation or photons involves the interaction of
radiation with matter. Photo ionization occurs
when the amount of radiation energy absorbed
by an atom or molecule exceeds its ionization
energy and is represented as
A + hν → A+
+ e
where A represents a neutral atom or molecule
in the gas and hν the photon energy.
(iv) Electron Attachment : If a gas molecule has
unoccupied energy levels in its outermost
group, then a colliding electron may take up
one of these levels, converting the molecule
into a negative ion M-.
M + e-
→ M-
(vi) Electron detachment: This occurs when a
negative ion gives up its extra electron, and
becomes a neutral molecule.
M-
→ M + e-
(vi) Thermal Ionization: The term thermal
ionization is the ionizing actions of molecular
collisions, radiation and electron collisions
occurring in gases at high temperatures. When
a gas is heated to high temperature, some of
the gas molecules acquire high kinetic energy
and these particles after collision with neutral

particles ionize them and release electrons.
These electrons and other high-velocity
molecules in turn collide with other particles
and release more electrons. Thus, the gas gets
ionized.
In short: if the applied voltage/field is large, the
current flowing through the insulation increases
very sharply an electrical breakdown occurs. A
strongly conducting spark formed during
breakdown practically produces a short circuit
between the electrodes. The breakdown in a
gas, called spark is the transition of non-
sustaining discharge into a self-sustaining
discharge. The build-up of high currents in a
breakdown is due to the process known as
ionization in which electrons and ions are
created from neutral atoms or molecules' and
their migration to the anode and cathode
respectively leads to high current. There are
two theories related to breakdown of gases
which are of importance.
(i) Electron Avalanche Mechanism (Townsend
Breakdown Process)
(ii) Streamer theory
TOWNSEND BREAKDOWN MECHANISM:
It is based on the generation of successive
secondary avalanches to produce breakdown.
Suppose a free electron exist in a gas which is
under the influence of an electric field. If the
field strength is sufficiently high, then it is likely
to ionize a gas molecule by simple collision
resulting in 2 free electrons and a positive ion.
These 2 electrons will be able to cause further
ionization by collision leading in general to 4
electrons and 3 positive ions. The process is
cumulative, and the number of free electrons
will go on increasing as they continue to move
under the action of the electric field. The swarm
of electrons and positive ions produced in this
way is called an electron avalanche. In the
space of a few millimeters, it may grow until it
contains many millions of electrons.
Townsend's first ionization coefficient
Consider a parallel plate capacitor having gas as
an insulating medium and separated by a
distance d.
When no electric field is set up between the
plates, a state of equilibrium exists between the
state of electron and positive ion generation
due to the decay processes. This state of
equilibrium will be disturbed when a high
electric field is applied. The variation of current
as a function of voltage was studied by
Townsend. He found that the current at first
increased proportionally as the voltage is
increased and then remains constant, at I 0
which corresponds to the saturation current. At
still higher voltages, the current increases
exponentially. To explain the exponential rise
in current, Townsend introduced a coefficient
α known as Townsend’s first ionization
coefficient and is defined as the number of
electrons produced by an electron per unit
length of path in the direction of field.

When the voltage applied across a pair of
electrodes is increased, the current throughout
the gap increases slowly as the electrons
emitted from the cathode move through the
gas. Let
n0 = number of electrons/second emitted from
the cathode,
nx = number of electrons/second moving at a
distance x from the cathode
[nx > n0 due to ionizing collisions in gap]
α = number of ionizing collisions [Townsend's
first ionization coefficient]
1/α = average distance traversed in the field
direction between ionizing collisions.
Consider a laminar of thickness dx at a distance
x from the cathode. The nx electrons entering
the laminar will traverse in the presence of the
applied field E. The ionizing collisions generated
in the gas gap will be proportional to both dx
and to nx. Say dnx electrons are formed due to
collisions.
Thus dnx ∝ nx
Also dnx ∝ dx
Therefore dnx = α. nx . dx
Rearranging and integrating gives
If the anode is at a distance x = d from the
cathode, then the number of electrons nd
striking the anode per second is given by
nd = n0 . eαd
In the steady state, the number of positive ions
arriving at the cathode/second must be exactly
equal to the number of newly formed electrons
arriving at the anode. Thus the circuit current
will be given by
I = I0 . eαd
where I0 is the initial photo-electric current at
the cathode. The term eαd
is called the electron
avalanche and it represents the number of
electrons produced by one electron in travelling
from cathode to anode.
Townsend's second ionization coefficient
We have the current growth equation,
I = I0 . eαd
Taking log on both the sides, ln I = ln I0 + αd
This is a straight line equation with slope α and
intercept (ln I0) if pressure p and E is kept
constant as shown in fig.
Townsend in his earlier investigations had
observed that the current in parallel plate gap
increased more rapidly with increase in voltage
as compared to the one given by the above
equation. To explain this departure from
linearity, Townsend suggested that a second
mechanism must be affecting the current. He
postulated that the additional current must be
due to the presence of positive ions and the

photons. The positive ions will liberate
electrons by collision with gas molecules and by
bombardment against the cathode. Similarly,
the photons will also release electrons after
collision with gas molecules and from the
cathode after photon impact. Let
γ = number of secondary electrons (on average)
produced at the cathode per ionizing collision in
the gap. [Townsend's second ionization
coefficient]
n0 = number of primary photo-electrons/second
emitted from the cathode
n0' = number of secondary electrons/second
produced at the cathode
n0" = total number of electrons/second leaving
the cathode
Then n0” = n0 + n0’
On an average, each electron leaving the
cathode produces [eαd
- 1] collisions in the gap,
giving the number of ionizing collisions/second
in the gap as n0" (eαd
- 1). Thus by definition
Similar to the case of the primary process (with
α only), we have
Thus, in steady state, the circuit current I will be
given by
This equation describes the growth of average
current in the gap before spark breakdown
occurs.
As the applied voltage increases, eαd
and γeαd
increase until γeαd
→ 1 , and the denominator of
the circuit current expression becomes zero and
the current I → ∞ . In this case, the current in
practice is limited only by the resistance of the
power supply and the conducting gas.
This condition may thus be defined as the
breakdown and can be written as
γ.(eαd
- 1) = 1
This condition is known as the Townsend
criteria for spark breakdown.
When γeαd
> 1, the ionization produced by
successive avalanche is cumulative. The spark
discharge grows more rapidly
When γeαd
< 1, the current I is not self-
sustained i.e., on removal of the source the
current I0 ceases to flow
Determination of Townsend's Coefficients α
and γ
Townsend's coefficients are determined in an
ionization chamber which is first evacuated to a
very high vacuum of the order of 10-4 and 10-6
torr before filling with the desired gas at a
pressure of a few torr. The applied direct
voltage is about 2 to 10 kV, and the electrode
system consists of a plane high voltage
electrode and a low voltage electrode
surrounded by a guard electrode to maintain a

uniform field. The low voltage electrode is
earthed through an electrometer amplifier
capable of measuring currents in the range 0.01
pA to 10 nA. The cathode is irradiated using an
ultra-violet lamp from the outside to produce
the initiation electron. The voltage current
characteristics are then obtained for different
gap settings. At low voltage the current growth
is not steady. Afterwards the steady Townsend
process develops as shown in figure
From the Townsend mechanism, the discharge
current is given by
I = I0 . eαd
This can be written in logarithmic form as
ln I = αd + ln I0
y = m x + c
From a graph of ln I vs d, the constants α and I0
can be determined from the gradient and the
intercept respectively.
Once I0 and α are known, γ can be determined
from points on the upward region of the curve.
PASCHEN’S LAW
When electrons and ions move through a gas in
a uniform field E and gas pressure p, their mean
energies attain equilibrium values dependant
on the ratio E/p
For a uniform field gap, the electric field E =
V/d. Thus applying Townsend's criterion for
spark breakdown of gases gives
which may be written in terms of the functions
as
This equation shows that the breakdown
voltage V is an implicit function of the product
of gas pressure p and the electrode separation
d.
In the above derivation the effect of
temperature on the breakdown voltage is not
taken into account. Using the gas equation
pressure . volume = mass. R . Temperature
(PV=nRT)
Or pressure = density . R . Temperature. (ρRT)
Thus the correct statement of the above
expression is V = f(ρ.d). This shows that the
breakdown voltage of a uniform field gap is a
unique function of the productof gas pressure
and the gap length for a particular gas and
electrode material.
This is the statement for Paschen’s law.

With very low products of (pressure x spacing),
a minimum breakdown voltage occurs, known
as the Paschen's minimum.
The breakdown voltage varies linearly with the
product pd. The variation over a large range is
shown in fig
(ii) STREAMER OR KANAL MECHANISM
This type of breakdown mainly arises due to the
added effect of the space-charge field of an
avalanche and photo-electric ionization in the
gas volume.
Townsend mechanism when applied to
breakdown at atmospheric pressure was found
to have certain drawbacks.
 Firstly, according to the Townsend
theory, current growth occurs as a
result of ionization processes, only. But
in practice, breakdown voltages were
found to depend on the gas pressure
and the geometry of the gap.
 Secondly, the mechanism predicts time
lags(The time that elapses between the
application of the voltage to a gap
sufficient to cause breakdown) of the
order of 10-5
s, while in actual practice
breakdown was observed to occur at
very short times of the order of 10-8
s
 While the Townsend mechanism
predicts a very diffused form of
discharge, in actual practice, discharges
were found to be filamentary and
irregular.
The Townsend mechanism failed to explain all
these observed phenomena and as a result,
around 1940, Rather and, meek and.
Loeb
independently proposed the Streamer theory.
The term 'Kanal' is taken from German language
which means a canal or a channel.
The growth of charge carriers in an avalanche in
a uniform field is described by eαd
. This is valid
only as long as the influence of the space charge
due to ions is very small compared to the
applied field. In his studies on the effect of
space charge on avalanche growth, Rather
observed that when charge concentration was
between 106
and 108
, the growth of the
avalanche became weak. On the other hand,
when the charge concentration was higher than
108
, the avalanche current was followed by a
steep rise in the current between the electrodes
leading to the breakdown of the gap.
For simplicity, the space charge at the head of
the avalanche is assumed to have a spherical
volume (See fig below) containing negative
charge at its top because of the higher electron
mobility. The space charge produced in the
avalanche causes sufficient distortion of the
electric field that those free electrons move
towards the avalanche head, and in so doing
generate further avalanches in a process that
rapidly becomes cumulative. As the electrons
advance rapidly, the positive ions are left
behind in a relatively slow-moving tail. The field
will be enhanced in front of the head. Just
behind the head the field between the
electrons and the positive ions is in the opposite

direction to the applied field and hence the
resultant field strength is less. Again between
the tail and the cathode the field is enhanced.
Due to the enhanced field between the head
and the anode, the space charge increases,
causing a further enhancement of the field
around the anode. The process is very fast and
the positive space charge extends to the
cathode very rapidly resulting in the formation
of a streamer. The breakdown process is shown
below.
Thorough experimental investigation developed
an empirical relation for the streamer spark
criterion of the form
where Er is the radial field due to space charge
and E0 is the externally applied field.
BREAKDOWN IN LIQUID DIELECTRICS
Liquid dielectrics are used for filling
transformers, circuit breakers and as
impregnates in high voltage cables and
capacitors. For circuit breaker, again besides
providing insulation between the live parts and
the grounded parts, the liquid dielectric is used
to quench the arc developed between the
breaker contacts. The most important factors
which affect the dielectric strength of oil are
the, presence of fine water droplets and the
fibrous impurities.
Liquids which are chemically pure, structurally
simple and do not contain any impurity even in
traces of 1 in 109
, are known as pure liquids.Eg
Paraffin Hydrocarbons, n-Heptane.. In contrast,
commercial liquids used as insulating liquids are
chemically impure and contain mixtures of
complex organic molecules.
There are two schools of thought regarding the
breakdown in liquids.
 The first one tries to explain the
breakdown in liquids on a model which
is an extension of gaseous breakdown,
based on the avalanche ionization of
the atoms caused by electron collision
in the applied field. It has been
observed that conduction in pure
liquids at low electric field (1 kV/cm) is
largely ionic due to dissociation of
impurities and increases linearily with
the field strength. At moderately high
fields the conduction saturates but at

high field (electric), 100 kV/cm the
conduction increases more rapidly and
thus breakdown takes place. Fig. 1.11
(a) shows the variation of current as a
function of electric field for hexane.
This is the condition nearer to
breakdown.
The type of breakdown process in pure
liquids is called the electronic
breakdown, involves emission of
electrons at fields greater than
100kV/cm.
 The second school of thought
recognizes that the dielectric strength
of liquid dielectrics is affected by the
presence of foreign particles in liquid
insulations( like gas bubbles, suspended
particles as in commercial liquids).
When breakdown occurs in these
liquids, additional gases and gas
bubbles are evolved and solid
decomposition products are formed.
The breakdown mechanism depends on
the nature and condition of the
electrodes, physical properties of the
liquid and impurities and gases present
in the liquid.
The breakdown mechanism is classified
into 4 (for commercial liquids) :
1. Suspended Particle Mechanism
2. Cavitation and Bubble Mechanism
3. Thermal Mechanism
4. Stressed oil volume theory
ELECTRONIC BREAKDOWN (IN PURE LIQUIDS) :
Once an electron is injected into the liquid, it
gains energy from the electric field applied
between the electrodes. It is presumed that
some electrons will gain more energy due to
field than they would lose during collision.
These electrons are accelerated under the
electric field and would gain sufficient energy to
knock out an electron and thus initiate the
process of avalanche. The threshold condition
for the beginning of avalanche is achieved when
the energy gained by the electron equals the
energy lost during ionization (electron emission)
and is given by
e λ E = C.hv
where λ is the mean free path, hv is the energy
of ionization and C is a constant
1.SUSPENDED SOLID PARTICLE MECHANISM
Commercial liquids will always contain solid
impurities either as fibers or as dispersed solid
particles. The permittivity of these solids (ε1)
will always be different from that of the liquid
(ε2).
Let us assume these particles to be sphere of
radius r. These particles get polarized (acquire
polarity) in an electric field E and experience a
force which is given as
The force will tend the particle to move towards
the strongest region of the field. If the particles
present are large, they become aligned due to
these forces and form a bridge across the gap.
The field in the liquid between the gap will
increase and if it reaches critical value,
breakdown will take place.

If the number of particles is not sufficient to
bridge the gap, the particles will give rise to
local field enhancement and if the field exceeds
the dielectric strength of liquid, local
breakdown will occur near the particles and
thus will result in the formation of gas bubbles
which have much less dielectric strength and
hence finally lead to the breakdown of the
liquid.
Liquids with solid impurities have lower
dielectric strength as compared to its pure
form. The larger the size of the particle impurity
the lower is the overall dielectric strength of the
liquid.
2. CAVITATION AND BUBBLE MECHANISM:
The dielectric strength of liquid depends upon
the hydrostatic pressure above the gap length.
The higher the hydrostatic pressure, the higher
the electric strength.
The smaller the head of liquid, the more are the
chances of partially ionized gases coming out of
the gap and higher the chances of breakdown.
This means a kind of vapour bubble formed is
responsible for the breakdown. The following
processes lead to formation of bubbles in the
liquids:
a) Gas pockets on the surface of
electrodes.
b) Changes in temperature and pressure.
c) Gaseous products due to the
dissociation of liquid molecules by
electron collisions.
d) Vapourization of the liquid by corona
type discharge due to irregular surface
of electrodes.
The bubble will elongate in the direction of the
electric field under the influence of electrostatic
forces. When the field Eb equals the gaseous
ionization field, discharge takes place which will
lead to decomposition of liquid and breakdown
occurs. An expression for the breakdown
field/bubble breakdown strength is given as
o σ is the surface tension of the liquid,
o E0 is the field in the liquid in absence of
the bubble.
o ε2 and ε1 are the permittivities of the
liquid and bubble respectively,
o r - the initial radius of the bubble
o Vb- the voltage drop in the bubble.
This theory does not take into account the
production of the initial bubble and hence the
results given by this theory do not agree well
with the experimental result.
3. THERMAL BREAKDOWN:
Extremely large currents are produced just
before breakdown. The high current pulses
originate from the tips of the microscopic
projections on the cathode surface with
densities of the order of 1 A/cm3
. These high
density current pulses give rise to localized
heating of the oil which may lead to the
formation of vapour bubbles.
When a bubble is formed, breakdown follows,
either because of its elongation to a critical size
or when it completely bridges the gap between
the electrodes. In either case it will result in the
formation of a spark. The breakdown strength
depends on the pressure and the molecular
structure of the liquid. The theory doesn’t
explain the reduction in breakdown strength
with increased gap length.
4. STRESSED OIL VOLUME THEORY:
In commercial liquids where minute traces of
impurities are present, the breakdown strength
is determined by the “largest possible impurity”
or “weak link”. The electrical breakdown

strength of the oil is defined by the weakest
region in the oil, that is, the region which is
stressed to the maximum and by the volume of
oil included in that region. In non-uniform
fields, the stressed oil volume is taken as the
volume which is contained between the
maximum stress contour (Emax) and (0.9 Emax)
contour. According to this theory the
breakdown strength is inversely proportional to
the stressed oil volume.
The breakdown voltage is highly influenced by
the gas content in the oil, the viscosity of the
oil, and the presence of other impurities. These
being uniformly distributed, increase in the
stressed oil volume consequently results in a
reduction in the breakdown voltage. The
variation of the breakdown voltage stress with
the stressed oil volume is shown in fig below
BREAKDOWN IN SOLID DIELECTRICS
Solid insulating materials are used almost in all
electrical equipments, be it an electric heater or
a 500 MW generator or a circuit breaker, solid
insulation forms an integral part of all electrical
equipments especially when the operating
voltages are high. When breakdown occurs the
gases regain their dielectric strength very fast,
the liquids regain partially and solid dielectrics
lose their strength completely. The breakdown
of solid dielectrics not only depends upon the
magnitude of voltage applied but also it is a
function of time for which the voltage is
applied. the product of the breakdown voltage
and the log of the time required for breakdown
is almost a constant, i.e.
Vb.ln tb = constant
The various mechanisms of break down are
classified based on the time scale of voltage
application.
The various mechanisms are:
1. Intrinsic Breakdown
2. Electromechanical Breakdown
3. Breakdown due to Treeing and Tracking
4. Thermal Breakdown
5. Electrochemical Breakdown
6. Chemical deterioration
1. INTRINSIC BREAKDOWN:
If the voltage is applied for a very short time of
the order of 10–8
second, the dielectric strength
of the specimen increases rapidly to an upper
limit known as intrinsic dielectric strength. The
intrinsic strength, therefore, depends mainly
upon the structure of the material and
temperature. The stresses required are of the
order of 5-10 million volt/cm.
Intrinsic breakdown depends upon the
presence of free electron which is capable of
migration through the lattice of the dielectric.

Usually small numbers of conduction electrons
are present, with some structural imperfections
and small amounts of impurities. The impurity
atoms or molecules act as traps for the
conduction electrons up to certain ranges of
electric fields and temperatures. When these
ranges are exceeded, additional electrons are
trapped and released which participate in the
conduction process.
There are two types of intrinsic breakdown
mechanisms:
i. Electronic breakdown:
The intrinsic breakdown is obtained in times
of the order of 10–8
sec and therefore is
considered to be electronic in nature. The
intrinsic strength is generally assumed to have
been reached when electrons in the valance
band gain sufficient energy from the electric
field to cross the forbidden energy band to the
conduction band. As an electric field is applied,
the electrons gain energy and due to collisions
between them the energy is shared by all
electrons. Finally the temperature of electrons
will exceed the lattice temperature and this will
result into increase in the number of trapped
electrons reaching the conduction band and
finally leading to complete breakdown.
ii. Avalanche or streamer breakdown:
This is similar to breakdown in gases due to
cumulative ionization. Conduction electrons
gain sufficient energy above a certain critical
electric field and cause liberation of electrons
from the lattice atom by collisions.
The electrons moving from cathode to anode
will gain energy from the field and losses it
during collisions. When the energy gained by an
electron exceeds the ionization potential, an
additional electron will be liberated due to
collision of the first electron. This process
repeats itself resulting in the formation of an
electron avalanche, and breakdown will occur
when the avalanche exceeds a certain critical
value. In practice, breakdown does not occur by
the formation of a single avalanche, but occurs
as a result of many avalanches formed and
extending step by step through the entire
thickness of the material.
2. ELECTROMECHANICAL BREAKDOWN:
When a dielectric material is subjected to an
electric field, charges of opposite nature are
induced on the two opposite surfaces of the
material and hence a force of attraction is
developed and the specimens is subjected to
electrostatic compressive forces and when
these forces exceed the mechanical withstand
strength of the material, the material collapses.
If the initial thickness of the material is d0 and is
compressed to a thickness d under the applied
voltage V then the compressive stress
developed due to electric field is
where εr is the relative permittivity of the
specimen. If γ is the Young’s modulus, the
mechanical compressive strength is
Equating the two under equilibrium condition,
we have
On differentiating the above equation w.r.t d
and then solving we get
For any real value of voltage V, the reduction in
thickness of the specimen cannot be more than
40%. If the ratio V/d is less than the intrinsic
strength of the specimen, a further increase in
V shall make the thickness unstable and the
specimen collapses. The highest apparent stress
is obtained as

3.BREAKDOWN DUE TO TREEING AND
TRACKING (SURFACE BREAKDOWN)
Treeing : The strength of a chain is given by the
strength of the weakest link in the chain.
Similarly whenever a solid material has some
impurities in terms of some gas pockets or
liquid pockets in it the dielectric strength of the
solid will be more or less equal to the strength
of the weakest impurities.
Suppose some gas pockets are trapped in a
solid material during manufacture, the gas has a
relative permittivity of unity and the solid
material εr, the electric field in the gas will be εr
times the field in the solid material. As a result,
the gas breaks down at a relatively lower
voltage. The charge concentration here in the
void will make the field more non-uniform. The
charge concentration in such voids is found to
be quite large to give fields of the order of 10
MV/cm which is higher than even the intrinsic
breakdown.
These charge concentrations at the voids within
the dielectric lead to breakdown step by step
and finally lead to complete rupture of the
dielectric. Since the breakdown is not caused by
a single discharge channel and assumes a tree
like structure as shown in fig below, it is known
as breakdown due to treeing.
Treeing occurs due to the erosion of material at
the tips of the spark and results the roughening
of the surface and becomes dirt and
contamination. Breakdown channels spread
through the insulation in an irregular “tree” and
leading to the formation of conducting channel.
Tracking: Suppose we have two electrodes
separated by an insulating material and the
assembly is placed in an outdoor environment.
Some contaminants in the form of moisture or
dust particles will get deposited on the surface
of the insulation and leakage current starts
between the electrodes through the
contaminants. The current heats the moisture
and causes breaks in the moisture films. These
small films then act as electrodes and sparks are
drawn between the films. The sparks cause
carbonization and volatilization of the insulation
and lead to formation of permanent carbon
tracks on the surface of insulations.
Thus, tracking is the formation of a permanent
conducting path usually carbon across the
surface of insulation. The conduction (carbon
path) results from degradation of the insulation
itself leading to a bridge between the
electrodes.
For tracking to occur, the insulating material
must contain organic substances. The rate of
tracking can be slowed down by adding filters to
the polymers which inhibit carbonization.
Usually tracking occurs even at very low
voltages, whereas treeing requires high voltage.
The numerical value of voltage that initiates or
causes the formation of a track is called the
“tracking index” and this is used to qualify the
surface properties of dielectric material. Treeing
can be prevented by having clean, dry and
undamaged surfaces and clean environment.
Usually treeing phenomena is observed in
capacitors and cables.
4. THERMAL BREAKDOWN:
When an insulating material is subjected to an
electric field, the material gets heated up due to
conduction current and dielectric losses due to
polarization. The conductivity of the material
increases with increase in temperature and a

condition of instability is reached when the heat
generated exceeds the heat dissipated by the
material and the material breaks down.
In practice, although the heat lost may be
considered somewhat linear, the heat
generated increases rapidly with temperature,
and at certain values of electric field no stable
state exists where the heat lost is equal to the
heat generated so that the material breaks
down thermally.
The variation of heat generated by a device for
2 different applied fields and the heat lost from
the device with temperature is shown below.
For the field E2, a stable temperature θA exists.
For the field E1, the heat generated is always
greater than the heat lost so that the
temperature would keep increasing until
breakdown occurs.
Heat generated is proportional to the frequency
and hence thermal breakdown is more serious
at high frequency. Thermal breakdown stresses
(MV/cm) are lower under a.c. condition then
under d.c.
5.Electrochemical breakdown:
Whenever cavities are formed in solid
dielectrics, the dielectric strength in the
specimen decreases. Some of the electrons
dashing against the anode with sufficient
energy shall break the chemical bonds of the
insulation surface. Similarly, positive ions
bombarding against the cathode may increase
the surface temperature and produce local
thermal instability. Similarly, chemical
degradation may also occur from the active
discharge products e.g., O3, NO2 etc. formed in
air. The net effect of all these processes is a
slow erosion of the material and a consequent
reduction in the thickness of the specimen.
6. Chemical deterioration :
Progressive chemical degradation of insulating
materials can occur in the absence of electric
stress from a number of causes.
a) Oxidation: In the presence of air or oxygen,
especially ozone, materials such as rubber
and polyethylene undergo oxidation giving
rise to surface cracks, particularly if
stretched and exposed to light. Polythene
also oxidizes in strong day light unless
protected by opaque filler.
b) Hydrolysis : When moisture or water
vapour is present on the surface of a solid
dielectric, hydrolysis occurs and the
materials lose their electrical and
mechanical properties. Electrical properties
of materials such as paper, cotton tape,
and other cellulose materials deteriorate
very rapidly due to hydrolysis. Polyethylene
film may lose its mechanical strength in a
few days if kept at 100 % relative humidity.
c) Chemical Instability: Many insulating
materials, especially organic materials,
show chemical instability. Under normal
operating conditions, this process is very
slow, but the process is strongly
temperature dependant. The logarithm of
the life t of paper insulation can be
expressed as an inverse function of the
absolute temperature θ.
log10 t = A/θ + B
where A & B are constants.

Breakdown in solids, liquids and gases

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