Vacuum Interrupter Technology

• The continuing use of vacuum interrupter technology

1. Breaker Overview

The structure of an outdoor vacuum circuit breaker (VCB)
is shown in Fig.1. The three-phase ganged interrupter assembly
is mounted in a self-contained breaker module
containing the operating mechanism, auxiliary switch,
mechanical linkage, and vacuum interrupter (VI)
mounted in its own support insulator. The mechanical
linkage and supporting framework are isolated from
the high voltage interrupter by an insulated drive rod
and the interrupter support insulators.

• 2. Features of Vacuum Interrupters

2.1 The Property of Vacuum
The dielectric strength of a vacuum is superior to
other dielectric mediums such as air and SF6 gas as
shown in Fig. 2. Due to high dielectric strength of a
vacuum environment, it is possible to reduce the arcing
time of the interrupter as the dielectric capabilities of
a vacuum can better withstand the generated transient
recovery voltage (TRV). The net result of this is the the magnetic force generated by the current flow, this will
cause the arc to be forced from the point of origin across the
spiral petal to the outer diameter of the contact. Once the
arc is on the outer edge of the contact, the magnetic force
along the outer diameter will cause the arc to rotate around
the outside diameter of the contact until it is extinguished.
The AMF contact structure will generate an axial magnetic
field caused by the current flow through the contacts which
in turn will create a diffuse arc (low energy arc) over the
entire contact area dispersing the arc energy over the contact
surface area. The diameter of spiral contacts can be smaller
than that of the AMF contact structure for the same fault
current rating due to its ability to rapidly move and control
the arc energy. In comparison the AMF contacts are superior
for applications which require long arcing times and require
a high number of full fault interruptions.minimum arcing time of a 15 kV vacuum circuit breaker
is approximately 1 to 2 ms. The higher dielectric capability
of the vacuum will yield a lower arc voltage and therefore
reduce the amount of arc energy to be dissipated across the
contacts in comparison with other interrupting mediums.
This results in vacuum interrupters being able to break
large fault currents with a short interrupting time and with
minimal contact erosion. These properties allow vacuum
interrupters to have a very compact design.

• 2.2 Basic Structure of Vacuum Interrupters

The basic structure of a vacuum interrupter is shown in
Fig. 3. The main contacts are installed in a ceramic insulator
in which high vacuum approximately 10-5 Pa is maintained.
The movable terminal is connected to the breaker mechanism
through an insulated drive rod which will open and
close the interrupter contacts. A stainless steel bellows is
used to allow contact travel and still maintain the integrity of
the vacuum in the interrupter. The contact material strongly
affects the interrupting capability, service life, and reliability
of the vacuum interrupter. There are three types of contact
designs used in vacuum interrupters, and they are applied
based on their application as shown in Table 1. Generally,
in applications with fault currents below approximately 20
kA, a flat butt type of contact design is sufficient. In applications
where fault currents can exceed 20 kA, the design
limitation of the contact will manifest itself as a localized
high energy arc which results in large amounts of metal
vapor being placed in the contact zone and thereby limiting
the fault rating of the interrupter. In order to improve the
interrupter’s fault capacity either the spiral or axial magnetic
field (AMF) contact design can be applied. These contacts
are used in higher fault ratings as they are able to utilize the
force created by the magnetic fields associated with the fault
current. This enables the interrupter to effectively control
the generated arc and therefore create a more efficient interrupter
design. The spiral contacts will generate a magnetic
field caused by the current flow through the contact. Due to

• 2.3 Arc Control During Interruption

By using a high-speed video camera and a specially
adapted vacuum chamber, the motion of the arc movement
can be observed. Fig. 4 shows the movement of the
arc for a spiral contact configuration during fault current
interruption. At contact part plus 1.5 ms, the arc has been
established between the contacts. At contact part plus 2 ms
the arc is beginning to rotate around the outer edge of the
contacts as a result of the magnetic forces that are being
generated from the flow of the fault current through the
contacts. This movement of the arc around the outer diameter
of the contact surface provides the greatest amount of
surface for the arc to travel over thus providing a larger area
to distribute the heat generated from the arc. The net result
of this movement of the arc is less localized overheating of
the contact surfaces and therefore less metal vapor being
introduced into the area between the contact surfaces. This
greatly improves the interrupting capability of the interrupter
and also results in less erosion of the contact surfaces,
improving the life of the interrupter.
Fig. 5 shows the arcing that takes place in an AMF
contact configuration during fault interruption. At contact
part plus 1 ms, the arc is beginning to be dispersed as a
result of the axial magnetic fields generated by the contact
geometry. This dispersal will generate numerous low energy
arcs (macroscopically one diffused arc) which will spread
out across the entire contact surface by contact part plus 3
ms. As the contacts continue to separate, the density of the
diffuse arc will continue to decrease so that at contact part
plus 5 ms interruption will occur when the next current zero
is encountered. By dispersing the arc energy into numerous
low energy arcs spread across the entire contact surface,
the amount of contact erosion and thus metal vapor in the
contact area can be controlled. This will result in greater
interrupting ratings for the contacts and longer contact life.

• 2.4 Short Circuit Fault Interruption

Fig. 6 shows a typical oscillograph during short-circuit
current interruption. When the contacts separate during
fault interruption an arc appears between the contacts and
is maintained until the next current zero crossing is encountered.
At the same time, an arc voltage is present across the
contacts. Once the current flow is interrupted a TRV will
appear across the contacts. If at the time the TRV appears,
the dielectric withstand of the contact gap is greater than the
TRV value, the current will be successfully interrupted. If the
TRV is greater than the dielectric withstand of the contact
gap, the arc will reappear and will be successfully interrupted
at the next current zero crossing. This condition will occur
when the time to the first zero crossing event is less than
the designed minimum arcing time of the interrupter.

• 3. Design Test

The capability of a medium-voltage circuit breaker is
verified by design tests specified in the IEEE/ANSI standards,
C37 series. In the design tests, a series of capabilities
(dielectric, current carrying, short-circuit current interruption,
and other current switching) are tested to meet the
required ratings. The following items, based on the design
test requirements, need to be taken into consideration
when demonstrating the circuit breaker’s capability and
application.

• 3.1 Asymmetrical Short-Circuit Current

Interruption and the Effect of Various X/R
Ratios
3.1.1 The rated interrupting time of a circuit breaker is
defined as the maximum permissible time interval
between the energization of the trip circuit (at rated
control voltage), and the interruption of the current
in the main circuit in all three phases. The standards
allow for ½ cycle of relay time prior to the trip circuit
energizing. This ½ cycle of relaying time is not
allowed to be counted as part of the interrupting
time of the circuit breaker. A circuit breaker rated
for three cycles must completely interrupt all current
flowing in the main circuit by the time three cycles
of 60 Hz current has passed (50 ms) after the trip
circuit is energized. If the current continues to flow
beyond the 50 ms, the breaker cannot be rated as a
three-cycle breaker. A five-cycle breaker is one that
interrupts the current between three and five cycles
(50-83 ms) after the trip circuit is energized. If there
is any current flow after 83 ms, the breaker cannot be
rated as a five-cycle breaker. The interrupter must be
able to interrupt current flow on the first or second
current zero after contact part. See Fig. 7.

• 3.1.2. A typical short-circuit asymmetrical wave is shown in

Fig. 8. For a three-cycle circuit breaker, it is necessary
to complete the interruption where the interrupting
time is equal to or less than three cycles. Moreover,
the test procedure in the IEEE standards requires
demonstrating that an interrupter can reliably
interrupt current under the most severe switching
conditions with the maximum arcing energy (longest
arcing time and a major loop of asymmetrical current).
Symmetrical interruptions must be performed
to meet the ANSI/IEEE standards. The test duties
included in Table 1 of IEEE C37.09-1999 demonstrate
the interrupter’s capability to interrupt current
flow, both symmetrical and asymmetrical.

• 3.1.3 The current peak value and major loop duration

are controlled by the value of X/R. The X/R ratio
is the ratio of the main circuit’s inductance divided
by the resistance. The higher the ratio is, the slower
the decay of the dc component of the asymmetrical
fault current. Fig. 8 shows the decay characteristic
of an asymmetrical current. Notice the peak current
decreasing with time. A standard value of X/R is
given as 17 at 60 Hz in IEEE Std C37.09-1999.
However, the purchaser of the breaker needs to evaluate
his system and verify that the X/R of the system
is less than 17. When the circuit has an inherently
high X/R, the interrupting time must still meet
the interruption rating of the breaker (three or five
cycles); however, the arcing time will increase due to
a larger major current loop. The major loops will not
only have an increased time between current zeros, it
will also have a higher peak current due to a higher
X/R ratio. Very high X/R ratios may not even have
a current zero for the first couple of cycles. For example,
if you compared an X/R=30 with an X/R=17
(see Fig. 8), the arcing time increases by a factor of
1.1, and the current peak value increases by a factor
of 1.4.

• 3.1.4 Symmetrical current interruptions must also be performed

according to IEEE C37.09-1999, Table 1.
The symmetrical interruptions are required for design
testing and to verify that the contact serviceability
as stated in IEEE C37.04-1999 section 5.8.2.5 is
achieved. The standards require a minimum of 800
percent of the required asymmetrical interrupting
capability of the circuit breaker be accumulated on
a single interrupter. Contact serviceability is the
amount of contact erosion that the interrupter must
be able to withstand and still be able to perform a
successful interruption.

1. 3.1.5 There are several design features that cause a vacuum

interrupter to be superior to other types of interrupters
for this application. The AMF style of contact
inside the interrupter allows for a longer arcing time
due to the low current density of the diffused arc. This
reduced energy will keep the surface of the contacts
from overheating and thus reduce the possibility of
restrikes. The reduction of restrikes is due to reduced
hot spots and reduced metal vapor in the contact gap. For circuit breakers rated below 100 kV, in IEEE Std
C37.04-1999, the rated TRV is represented by a 1-cosine
wave, while on the other hand, in IEC 62271-100, 2003
the TRV is represented by an exponential-cosine wave as
shown in Table 2 and Fig. 9. These ratings are for shortcircuit
tests, duty 4 and duty 5 in IEEE C37.09 and for
terminal fault test, duty T100a, in IEC. It is worth noting
that in the IEEE standards the TRV specifications are
different for outdoor and indoor applications. The outdoor
specification is more severe than the indoor. Breakers tested
and rated for indoor use, may not be suitable for outdoor
applications. Also, the differences between the IEC and
the IEEE TRV requirements are significant enough that
breakers tested to the IEC standard may not pass testing
for IEEE. The steeper slope of the recovery voltage will
change the requirements of the contact design to achieve a
greater rate of dielectric recovery inside the interrupter. The
capability of the interrupter to successfully interrupt this
high rate of rise of recovery voltages (RRRV) is dependent
upon the internal geometry of the interrupter. The type of
contact and the surrounding dielectric fields produced by
the internal shield all have a large impact on the capability
of the interrupter. The AMF style of contact is well suited
for this purpose due to reduced hot spots and metal vapor
in the contact gap. The smooth contact surface allows for
a very rapid dielectric recovery of the gap which allows
the vacuum interrupter to withstand the TRV better than
other types of medium. The rapid recovery allows for small
contact gaps, which translates to a very short stroke of the
interrupter and operating mechanisms.

• 3.3 Continuous Current Carrying Capability of

Vacuum Interrupters
3.3.1 The rated continuous current of a breaker is determined
by the current carrying capability without
exceeding the temperature limitations as listed in
Table 2 of IEEE C37.04-1999. This temperature
limit takes into consideration the types of materials,
plating, and atmosphere that surround the point
where the temperature is being measured.The TRV characteristics depend on the system in which a
circuit breaker is installed. The TRV is controlled mainly by
the amount of inductance and/or capacitance on each side
of the breaker. However, to provide a general application
guide, the TRV values under most system circuit conditions
are specified in the following standards. The most common spot in a VCB where temperature
rise is a concern is the moving stem of the vacuum
interrupter. It is necessary to monitor the temperature
very closely at this critical connection point where the
current path must allow for the motion of conductors.
A typical temperature rise graph is shown in Fig. 10.
In a medium-voltage VCB, the thermal time constant
is usually small. If a VCB is required to carry overload
current for several hours, the temperature rise on a
VCB will approach the saturation values. This temperature
may be over the thermal limits of materials.
Whether or not the breaker will exceed any critical
temperature limits will need to be evaluated carefully.
The IEEE standards cover standard overload
requirements; refer to IEEE C37.10-1999 section
5.4.4. If the breaker will be required to exceed these
limits, the purchaser and manufacturer will need to
review the continuous current test data to verify the
breaker’s capability for higher overload ratings.temperature rise associated with the
higher resistance. The spiral or butt
type contact is generally better suited
for high continuous currents than the
AMF style of contact. Consideration
must also be given to the location
where the circuit breaker will be installed.
If the installation site has an
ambient temperature rating of 50°C,
as compared to the 40°C used in
the IEEE standards, then the total
temperature rise must be decreased accordingly (by
10°C in this case). The overall performance of the
interrupter needs to be considered to determine the
best contact design, material, and size for any given
application. To ensure proper application of the circuit
breaker, the purchaser and manufacturer should
review the test reports.

• 3.4 Dielectric Capabilities of Vacuum

Interrupters.
3.4.1 Vacuum interrupters are well suited to withstand
voltage surges due to the rapid recovery of the dielectric
strength of the contact gap caused by the high
level vacuum. Other types of interrupter medium
take much longer to recover due to the need to remove
hot gas (in SF6 for example) from the gap. The
smooth contact surfaces and surrounding geometry
(primarily the internal shield), as well as the high
level of vacuum, all contribute to the interrupter’s
voltage withstand capability.
3.4.2 Lightning Impulse Withstand Voltage (BIL)
Vacuum is very reliable during lightning impulse
withstand voltage. This is demonstrated by testing
performed per section 4.4. of IEEE C37.09-1999.
The flat design of contacts and the voltage profiling
of the surrounding components (for example,
the internal shield, bellows, moving contact stem,
etc.) make this a very impulse friendly interrupter.
Another factor that allows the small contact gap to
withstand voltage surges is the high level of vacuum
inside the interrupter. A high level vacuum eliminates
all foreign particles within the interrupter. This results
in having no impurities and a very low molecular
count across the contact gap. The absence of foreign
particles and molecules will not allow the voltage to
jump across the internal gap.
There are several ways of improving the external
dielectric capabilities of a vacuum interrupter. Many
manufacturers will use various types of minor modifications
to improve the dielectric capabilities, or
margins, within a particular interrupter design. Some
examples are the use of an externally contoured, or
shedded, porcelain body to increase the creepage
distance. The increase in creepage distance allows for
more margin in a high contamination environment 3.3.2 The size and design of the main contacts and conductors
contribute to the continuous current rating
of the interrupter. The larger the contact diameter,
the better the current carrying capability. The contact
design and material also play a part in the continuous
current rating. The different contact materials may
have different resistance values. A higher resistance
value may limit the amount of current due to the
Another example would be to use a heat shrink material
on the ends to increase creep and strike. The use
a potting material to completely cover the outside of
the interrupter to increase the dielectric strength has
also been used for increased margins or capabilities
of a given interrupter design.
Vacuum interrupters are ideal for use in low-, medium-,
and even the lower end of high-voltage applications. Vacuum
will eventually be used in high-voltage applications
as technology keeps improving the capability of vacuum
interrupters. The internal design of the vacuum interrupter,
especially the main contacts, greatly impacts the capability of
the interrupter. There are different types of contact designs
that will utilize different materials to achieve different ratings
and capabilities. The internal design of the interrupter
will depend on the rating of the circuit breaker where the
interrupter will be installed. Some of the items to consider
on the internal design are not only the contact shape but also
the internal shield shape, the moving stem shape, blending
radii, and even the mounting configuration of the vacuum
interrupter. Whether the breaker will be primarily used for
capacitor switching duty, transformer magnetizing, or just
fault interruptions, will affect the design. It is the responsibility
of the end user of the circuit breaker to verify that
the breaker (ultimately the interrupter) is suited to meet the
specific needs of the system. The ability of the circuit breaker
to function properly on a given system will ultimately come
down to the review of the test data to ensure that the breaker
has been tested and verified to function correctly under the
duties in which it will be asked to perform.

in the power distribution market as replacements
for oil circuit breakers has exposed this technology
to people who have not been previously exposed to it. This
article covers the features of the vacuum interrupter and how
they correspond to the ability of the interrupter to perform
according to its ratings.
Vacuum interruption technology has been used for
many years and has proven itself to be a reliable means for
interrupting fault currents in distribution switchgear. The
application of vacuum circuit breakers as replacements for
oil circuit breakers offers many advantages to the user in
areas such as maintenance, performance, and environmental
concerns. From a maintenance point of view, with a vacuum
breaker there is no oil handling required with the associated
clean up as well as long contact life and lower mechanism
operating forces due to the small size and stroke required
by a vacuum interrupter. A typical vacuum interrupter as
applied in a distribution breaker will easily handle 10,000
operations at rated continuous current and more than 20 full
fault operations without the need for contact replacement.
Also, as a result of the lower mechanical requirements the
mechanisms in today’s vacuum breakers are much simpler
in design with fewer moving parts and lighter loads being
applied.