EEE 4508 Switchgear and Protection
Lecture Notes 7
7.0 Power Circuit Breakers:
The power circuit breakers that shall be considered in this lecture are the Vacuum, Metal clad
and Sulphur hexa-fluoride types of circuit breakers. The circuit breakers shall be discussed in
their mode of arc extinction and general operation including application.
7.1 Vacuum Circuit breakers
Although the benefits of interrupting the arc in a vacuum circuit breakers were recognized as
early as the nineteenth century, the use of such circuit breakers did not find wide application
till a few years ago. This was because of limitations in the knowledge of problems in material
science, vacuum technology and plasma physics being insufficiently advanced to provide
solution to the many technological problems encountered in the design and construction of a
reliable Vacuum Circuit Breaker.
High vacuum has two outstanding properties:
(i) It has the highest insulating strength known
(ii) When an a, c. circuit is opened by the separation of contacts in a vacuum, interruption
occurs at the first current zero with the dielectric strength across the contacts building
up at a rate thousands of times higher than that obtained with conventional circuit
breakers.
These properties obviously make the Vacuum Circuit Breaker more efficient, less bulky and
cheaper. The service life also is much greater than that of conventional equipment and hardly
any maintenance is necessary. Vacuum breakers are ideally suited for most duties encountered
in typical electric utility and industrial applications. Their voltage and interrupting rating is
such that with little modifications, they may be made to perform specific switching duties on
high-voltage a. c. systems. Field experience has proved the unique performance and reliability
characteristic of vacuum devices.
7.1.2 The Vacuum Medium
Every medium that has a pressure below atmospheric (760 mm of mercury) is known as
vacuum. Torricelli is known to be the first man who succeeded in evacuating a space by
building his mercury barometer. Low pressures are measured in torr, where 1 torr =1 mm of
1
, mercury. Today, with various improved techniques pressures as low as 10-7 torr can be
achieved.
The value of the pressure of the medium has a marked effect and relationship on the molecular
structure of the medium. In the high pressure ranges of vacuum systems, the mean free path is
very small and the molecules are in a constant state of inter-collision, the gas behaves as a fluid
and is known to be in a state of viscous flow. As the pressure is reduced the mean free path
increases. Eventually a point is reached at which the mean free path is equal to or greater than
the dimensions of the confining chamber. Under this condition the molecules will collide more
frequently with the walls of the chamber than with each other. In this region the gas is said to
be in a state of molecular flow.
The division between the two regions is specified by a dimensionless parameter called
the Knudsen number. For a cylindrical tube the Knudsen number is defined as the ratio of the
mean free path (Lm) of the gas molecules to the radius (R). When Lm/R is less than 0.01 gas
flow is viscous; if the ratio is greater than 1, the flow is molecular. The range between these
two limits is called the transition range. Table 7.1 shows the value of mean free path for N 2 at
various values of pressure. The gaseous medium in vacuum interruption devices is usually in
the molecular flow range.
Table 7.1 Gas Kinetic Data for N2 at 25oC
Pressure in (Torr) Density at 25oC (molecules/m3) Mean free path (cm)
760 2.5 x1019 6.3 x 10-6
10-3 3.3 x1013 4.8
10-4 3.3 x1012 48
10-5 3.3 x1011 480
10-6 3.3 x1010 4.8 x 103
10-7 3.3 x109 4.8 x 106
7.1.3 The Vacuum Arc
When contact separation takes place in air the ionized molecules are probably the main carriers
of electric charges and responsible for low breakdown value. In vacuum arc the neutral atoms,
ions and electrons must ultimately come from the electrodes themselves, and not from the
medium in which the arc is drawn. As the current carrying contacts are parted, the current
concentrates at a few local high spots on the contact surfaces. Normal conduction through metal
ceases when the last bridge between the two contacts is vaporized. The phenomena at the active
2
Lecture Notes 7
7.0 Power Circuit Breakers:
The power circuit breakers that shall be considered in this lecture are the Vacuum, Metal clad
and Sulphur hexa-fluoride types of circuit breakers. The circuit breakers shall be discussed in
their mode of arc extinction and general operation including application.
7.1 Vacuum Circuit breakers
Although the benefits of interrupting the arc in a vacuum circuit breakers were recognized as
early as the nineteenth century, the use of such circuit breakers did not find wide application
till a few years ago. This was because of limitations in the knowledge of problems in material
science, vacuum technology and plasma physics being insufficiently advanced to provide
solution to the many technological problems encountered in the design and construction of a
reliable Vacuum Circuit Breaker.
High vacuum has two outstanding properties:
(i) It has the highest insulating strength known
(ii) When an a, c. circuit is opened by the separation of contacts in a vacuum, interruption
occurs at the first current zero with the dielectric strength across the contacts building
up at a rate thousands of times higher than that obtained with conventional circuit
breakers.
These properties obviously make the Vacuum Circuit Breaker more efficient, less bulky and
cheaper. The service life also is much greater than that of conventional equipment and hardly
any maintenance is necessary. Vacuum breakers are ideally suited for most duties encountered
in typical electric utility and industrial applications. Their voltage and interrupting rating is
such that with little modifications, they may be made to perform specific switching duties on
high-voltage a. c. systems. Field experience has proved the unique performance and reliability
characteristic of vacuum devices.
7.1.2 The Vacuum Medium
Every medium that has a pressure below atmospheric (760 mm of mercury) is known as
vacuum. Torricelli is known to be the first man who succeeded in evacuating a space by
building his mercury barometer. Low pressures are measured in torr, where 1 torr =1 mm of
1
, mercury. Today, with various improved techniques pressures as low as 10-7 torr can be
achieved.
The value of the pressure of the medium has a marked effect and relationship on the molecular
structure of the medium. In the high pressure ranges of vacuum systems, the mean free path is
very small and the molecules are in a constant state of inter-collision, the gas behaves as a fluid
and is known to be in a state of viscous flow. As the pressure is reduced the mean free path
increases. Eventually a point is reached at which the mean free path is equal to or greater than
the dimensions of the confining chamber. Under this condition the molecules will collide more
frequently with the walls of the chamber than with each other. In this region the gas is said to
be in a state of molecular flow.
The division between the two regions is specified by a dimensionless parameter called
the Knudsen number. For a cylindrical tube the Knudsen number is defined as the ratio of the
mean free path (Lm) of the gas molecules to the radius (R). When Lm/R is less than 0.01 gas
flow is viscous; if the ratio is greater than 1, the flow is molecular. The range between these
two limits is called the transition range. Table 7.1 shows the value of mean free path for N 2 at
various values of pressure. The gaseous medium in vacuum interruption devices is usually in
the molecular flow range.
Table 7.1 Gas Kinetic Data for N2 at 25oC
Pressure in (Torr) Density at 25oC (molecules/m3) Mean free path (cm)
760 2.5 x1019 6.3 x 10-6
10-3 3.3 x1013 4.8
10-4 3.3 x1012 48
10-5 3.3 x1011 480
10-6 3.3 x1010 4.8 x 103
10-7 3.3 x109 4.8 x 106
7.1.3 The Vacuum Arc
When contact separation takes place in air the ionized molecules are probably the main carriers
of electric charges and responsible for low breakdown value. In vacuum arc the neutral atoms,
ions and electrons must ultimately come from the electrodes themselves, and not from the
medium in which the arc is drawn. As the current carrying contacts are parted, the current
concentrates at a few local high spots on the contact surfaces. Normal conduction through metal
ceases when the last bridge between the two contacts is vaporized. The phenomena at the active
2
