Niche Market Testing
by Lynn Hamrick, ESCO Energy Services
A Primer for Testing Medium-Voltage
Breakers in the Industrial Environment
D
uring its 20-40 year service life, a circuit breaker must be constantly
prepared to operate. Typically, long periods of inactivity often elapse
during which the breaker’s mechanical and electrical components never
move. The circuit breaker is the active link in a fault clearance situation. When a
fault occurs on the electrical system, the associated fault current must be interrupted quickly and reliably. This action is referred to as fault clearing. If a breaker
fails to clear a faulted circuit, the resulting damage can be very serious in terms
of both personnel injury and equipment damage. Even though circuit breakers
are comparatively reliable, circuit breaker failures can and do occur. Therefore,
circuit breakers must be tested and maintained to ensure proper operation during these faults.
Many circuit breakers provide longer service lives than expected. If you can
ascertain that a breaker is in good condition, you can continue to use it rather
than replace it at its estimated end-of-life. For the remainder of this discussion,
we will concentrate on medium-voltage (1 kV up to 35 kV) circuit breakers
utilized within an industrial environment.
Different Medium-Voltage Breaker Designs
There are three basic designs of medium-voltage breakers:
Air Circuit Breaker – Air circuit breakers are the prevalent type of medium voltage
breaker in an industrial environment due to the typical age of the equipment and
their lower cost; however, very few of this type of breaker are still being manufactured. Air circuit breakers are commonly used in medium-voltage electrical
distribution systems and include operating mechanisms that are mainly exposed
to the environment. Since the air circuit breakers are rated to carry current continuously, the exposure to convection cooling air assists in keeping the operating
components within reasonable temperature limits. A typical air circuit breaker
includes a main contact assembly, which is either opened (“tripped”), interrupting
a path for power to travel from the source to the load, or “closed”, providing a
path for power to travel from the source to the load. In many air circuit breakers,
the force necessary to open or close the main contact assembly is provided by an
arrangement of compression springs. In many air circuit breakers, the mechanism
for controlling the compression springs comprised of a configuration of mechanical
linkages between a latching shaft and an actuation device. The actuation device may
be manually operated, or electrically operated with trip and close coils. In a comwww.netaworld.org mon construction of a medium-voltage
air circuit breaker, the movable contact
is mounted on a contact arm that is
pivoted to open or close the contacts by
a spring powered operating mechanism
triggered through the trip or closing
coils by a control switch or closing relay
(i.e., protective relay). Smaller opening springs are put in tension during
closing and assist in opening when
the trip coil is given tripping energy
from the control switch or protective
relays. These protective relays may also
be used to provide auxiliary functions
along with over-current protection.
The current interruption capability
of air circuit breakers is dependent in
part upon their ability to extinguish
the arc that is generated when the
breaker main contacts open or close. A
medium-voltage air circuit breaker in
an industrial environment is typically
a draw-out type in which a breaker is
movable so as to be separated (“racked
out”) from the power source in order
to facilitate maintenance or connected
(“racked in”) to the power source using
either a manual or motor-assisted racking capability.
Vacuum Circuit Breaker – Vacuum
circuit breakers are the most popular
type of medium-voltage breaker in an
industrial environment due to their
smaller size and improved reliability
over air circuit breakers. Vacuum circuit breakers are similar to air circuit
breakers with the difference being
that the main contacts are housed in a
vacuum chamber (“bottle”). One of the
contacts is fixed relative to the bottle
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and to an external electrical conductor which is interconnected with the circuit to be controlled by the circuit breaker.
The other separable main contact is movable. The moveable
contact assembly is usually comprised of a stem of circular
cross-section having the contact at one end enclosed within
a vacuum bottle and a driving mechanism at the other end
which is external to the vacuum bottle. An operating rod
assembly comprised of a push rod, which is fastened to
the end of the stem opposite the moveable contact, and a
driving mechanism provide the motive force to move the
moveable contact into or out of engagement with the fixed
contact. Compression springs are provided in connection
with the operating rod assembly in order to be able to
separate the moveable contact from the fixed contact and
to assure the necessary force so that the contacts will not
accidentally open under inappropriate conditions. As with
the air circuit breaker, the actuation device may be manually
or electrically operated by trip and close coils triggered by
a protective relay. These protective relays may also be used
to provide auxiliary functions along with over-current protection. Vacuum circuit breakers offer many advantages as
compared to other types, including: relatively long life due
to controlled contact erosion; maintenance-free operation
provided by enclosure of the contacts within the hermetically sealed housing; excellent operational reliability due to
the use of hard contact materials; little or no atmospheric
contact contamination, which contamination can detrimentally form oxides and corrosion layers on the contacts;
little or no noise during operation due to containment of
arcing within the hermetically sealed housing; relatively few
environmental effects; and very low current chop, resulting
in a minimal induced transient voltage spike during circuit
interruption so that surge suppressors are typically not
required. A medium-voltage vacuum circuit breaker in an
industrial environment is also typically a draw-out type.
Oil Circuit Breaker – Oil circuit breakers, which are
typically located outdoors, were the past standard but are
becoming more rare in the industrial environment as they
are being replaced with air or vacuum circuit breakers. Oil
circuit breakers have all contacts and mechanism housed
within an oil-filled tank, which provides for both an insulation and arc-quenching capability. A typical oil circuit
breaker includes the same operational characteristics as an
air circuit breaker. A medium-voltage oil circuit breaker in
an industrial environment is typically a stand-alone type
and requires that oil be drained and the breaker de-tanked
to perform maintenance.
Suggested Electrical Tests
There are three basic electrical tests that should be performed on medium-voltage circuit breakers as part of a
preventive maintenance program.
Contact resistance – This test should be performed from
line-to-load terminals for each phase with the breaker
closed. The values should be within 50% of each other and
comparable to similar circuit breakers.
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Insulation resistance – This test should be performed
phase-to-phase and phase-to-ground (i.e., case, if applicable) with the breaker closed, as well as across the open
contacts for each phase. For medium-voltage breakers, the
manufacturer should provide the testing voltages and acceptance values for this test. When not available or where
no recommendations are provided, the following abbreviated
table from ANSI/NETA MTS-2007 may be used.
Insulation Resistance Test Values –
MV Breakers
Nominal Rating of
Breaker in Volts
Minimum Test
Voltage, DC
2,500
1,000
8,000
2,500
5,000
Recommended
Minimum
Insulation Resistance
in Megohms
500
2,500
1,000
15,000
2,500
5,000
35,000 and above
15,000
25,000
5,000
2,000
20,000
100,000
Dielectric withstand voltage – This basically an ac or dc
high potential test. This test should be performed from
the line side of each phase with the breaker open and
the load side of that phase and line and load sides of the
other phases tied together and tied to ground. For vacuum
circuit breakers, this test will also provide an indication of
the vacuum bottle integrity. For medium voltage breakers,
the manufacturer should provide the testing voltages for
this vacuum bottle integrity. It should be noted that for
a medium voltage vacuum breaker, only use of an ac high
potential is recommended. When not available or where no
recommendations are provided, the following abbreviated
table from ANSI/NETA MTS-2007 may be used
Dielectric Withstand Test Voltages –
MV Breakers
Nominal System
(Line) Voltage
Maximum Field
Applied AC Test
(kV)
Maximum Field
Applied DC Test
(kV)
1.2
6.0
8.5
4.8
11.4
14.4
20.4
2.4
8.3
18.0
25.0
34.5
9.0
12.7
15.6
22.1
24.0
30.0
42.0
16.1
28.8
33.9
42.4
59.4
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In addition to these basic tests, the protective relays
should be tested for functionality with all implemented protective functions being tested. In an industrial environment,
over-current protection is typically the main, and possibly
only, function provided by the protective relay; however, if
there are other protective functions being provided, they
should also be tested. It is also important that the breaker
actually be tripped and closed by the manual controls, and
tripped by the protective relay to ensure that the relay and
breaker are operating correctly. This will functionally test the
trip and close coils as well as the protective circuit.
Power-factor or dissipation-factor testing should be
considered as an additional test of each breaker’s insulation
system. For oil circuit breakers, a tank loss index should be
determined and, where individual mechanisms are provided
by phase for operation, time-travel testing should be considered for each phase. Oil sampling is also recommended
annually with oil sample quality analysis being provided for
dielectric breakdown, acid neutralization number, interfacial
tension, moisture, color, and power factor. Where cubicle
heaters are provided, functional testing of the heaters should
be verified.
In conclusion, even though circuit breakers are reliable
when compared to other electrical devices, circuit breaker
failures occur. When a breaker fails to operate, the resulting damage can be very serious in terms of both personnel
injury and equipment damage. Therefore, circuit breakers
must be tested and maintained to ensure proper operation
during these faults.
As Operations Manager of ESCO Energy Services
Company, Lynn brings over 25 years of working
knowledge in design, permitting, construction, and
startup of mechanical, electrical, and instrumentation and controls projects as well as experience in
the operation and maintenance of facilities.
Lynn is a Professional Engineer, Certified Energy Manager and has a BS in Nuclear Engineering
from the University of Tennessee.
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