powered to be put in a position where stored energy
and status of electric power.
can be released to close or open the contacts very
e. Protects the generation and distribution sys-
quickly. Closing or tripping action is applied man-
tems.
ually (by hand or foot power) or electrically (where
a solenoid provides mechanical force). The me-
5-2. Types of switchgear.
chanical force may be applied magnetically. Air
Voltage classification. Low voltage and medium
circuit breakers contain power sensor overcurrent
voltage switchgear equipment are used in auxiliary
trip devices that detect an overcurrent to the load
power generation systems. Switchgear at military
and initiate tripping or opening of the circuit
installations is usually in a grounded, metal enclo-
breaker.
sure (see fig 5-l). Per the Institute of Electrical and
(a) Manual circuit breakers employ spring-
Electronics Engineers (IEEE), equipment rated up
operated, stored-energy mechanisms for operation.
to 1000 volts AC is classed as low voltage. Equip-
Release of the energy results in quick operation of
ment equal to or greater than 1000 volts but less
the mechanism to open or close the contacts. Oper-
than 100,000 volts AC is classed as medium voltage.
ating speed is not dependent on the speed or force
a. Low voltage. Major elements of low voltage used by the operator to store the energy.
switchgear are circuit breakers, potential trans-
(b) Fast and positive action prevents unnec-
formers, current transformers, and control circuits,
essary arcing between the movable and stationary
refer to paragraph 5-3. Related elements of the
contacts. This results in longer contact and breaker
switchgear include the service entrance conductor,
life.
main box, switches, indicator lights, and instru-
(c) Manual stored-energy circuit breakers
ments . The service entrance conductor and main
have springs which are charged (refer to the glos-
bus (sized as required) are typical heavy duty con-
sary) by operation of the insulated handle. The
ductors used to carry heavy current loads.
charging action energizes the spring prior to closing
b. Medium voltage. Medium voltage switchgear
or opening of the circuit breaker. The spring, when
consists of major and related elements as in low
fully charged, contains enough stored energy to pro-
voltage switchgear. Refer to paragraph 5-4 for de-
vide at least one closing and one opening of the
tails. Construction of circuit breakers employed in
circuit breaker. The charged spring provides quick
the two types of switchgear and the methods to
and positive operation of the circuit breaker. Part of
accomplish breaker tripping are the primary differ-
the stored energy, which is released during closing,
ences. The service entrance conductors and main
may be used to charge the opening springs.
5-1
TM 5-685/NAVFAC MO-912
Figure 5-l. Typical arrangement of metal enclosed switchgear.
(d) Some manual breakers require several
ate. Arcing and intermediate contacts are adjusted
up-down strokes to fully charge. The springs are
to open after the main contacts open to reduce burn-
released on the final downward stroke. In either of
ing or pitting of the main contacts.
the manual units, there is no motion of the contacts
(i) A typical power sensor for an air circuit
until the springs are released.
breaker precisely controls the breaker opening time
(e) Electrical quick-make/quick-break break-
in response to a specified level of fault current. Most
ers are operated by a motor or solenoid. In small
units function as overcurrent trip devices and con-
units, a solenoid is used to conserve space. In large
sist of a solenoid tripper and solid-state compo-
sizes, an AC/DC motor is used to keep control-power
nents. The solid-state components are part of the
requirements low (4 amps at 230 volts).
power sensor and provide precise and sensitive trip
(f) When the solenoid is energized, the sole-
signals.
noid charges the closing springs and drives the
(2) Molded-case circuit breakers. Low current
mechanism past the central/neutral point in one
and low energy power circuits are usually controlled
continuous motion. Motor-operated mechanisms au-
by molded-case circuit breakers. The trip elements
tomatically charge the closing springs to a predeter-
act directly to release the breaker latch when the
mined level. When a signal to close is delivered, the
current exceeds the calibrated current magnitude.
springs are released and the breaker contacts are
Typical time-current characteristic curves for
closed. The motor or solenoid does not aid in the
molded-case circuit breakers are shown in figure
closing stroke; the springs supply all the closing
5-3.
power. There is sufficient stored-energy to close the
(a) Thermal-magnetic circuit breakers have
contacts under short-circuit conditions. Energy for
a thermal bi-metallic element for an inverse time-
opening the contacts is stored during the closing
current relationship to protect against sustained
action.
overloads. This type also has an instantaneous mag-
(g) A second set of springs opens the contacts
netic trip element for short-circuit protection.
when the breaker receives a trip impulse or signal.
(b) Magnetic trip-only circuit breakers have
The breaker can be operated manually for mainte-
no thermal elements. This type has a magnetic trip-
nance by a detachable handle.
ping arrangement to trip instantaneously, with no
(h) Circuit breakers usually have two or
purposely introduced time delay, at currents equal
three sets of contacts: main; arcing; and intermedi-
to, or above, the trip setting. These are used only for
5-2
TM 5-685/NAVFAC MO-912
4 5 0 V O L T S , 3 P H 6 0 C P S
GENERATOR BUS
LEGEND
- AMMETER
-
V R
VOLTAGE REGULATOR
- WATTMETER
- SYNCHROSCOPE
VM
VOLTMETER
TEMPERATURE METER
P T
POTENTIAL TRANSFORMER
GEN. CKT BREAKER
C T
CURRENT TRANSFORMER
GE. CKT BREAKER
F U S E
GOVERNOR
Figure 5-2. Typical switchgear control circuitry, one-line diagram.
short-circuit protection of motor branch circuits (1) Ratings. A PT is rated for the primary volt-where motor overload or running protection is pro- age along with the turns (step down) ratio to secure vided by other elements. 120 VAC across the secondary.
(c) Non-automatic circuit interrupters have
(2) Application. The primary of potential trans-
no automatic overload or short circuit trip elements.
formers is connected either line-to-line or line-to-
These are used for manual switching and isolation.
neutral, and the current that flows through this
Other devices must be provided for short circuit and
winding produces a flux in the core. Since the core
overload protection.
links the primary and secondary windings, a volt-
b. Potential transformers. A potential trans-
age is induced in the secondary circuit (see fig 5-4).
former (PT) is an accurately wound, low voltage loss
The ratio of primary to secondary voltage is in pro-
instrument transformer having a fixed primary to
portion to the number of turns in the primary and
secondary “step down” voltage ratio. The PT is
secondary windings. This proportion produces 120
mounted in the high voltage enclosure and only the
volts at the secondary terminals when rated voltage
low voltage leads from the secondary winding are
is applied to the primary.
brought out to the metering and control panel. The
(3) Dot convention. A dot convention is used in
PT isolates the high voltage primary from the me-
figure 5-5. The dot convention makes use of a large
tering and control panel and from personnel. The
dot placed at one end of each of the two coils which
step down ratio produces about 120 VAC across the
are mutually coupled. A current entering the dotted
secondary when rated voltage is applied to the pri-
terminal of one coil produces an open-circuit voltage
mary. This permits the use of standard low voltage
between the terminals of the second coil. The volt-
meters (120 VAC full scale) for all high voltage cir-
age measured with a positive voltage reference at
cuit metering and control.
the dotted terminal of the second coil.
5-3
TM 5-685/NAVFAC MO-912
CURRENT IN AMPERES AT-
a
0 7
0 6
0 1
CURRENT IN AMPERES AT 13.8K VOLTS
Figure 5-3. Typical time-current characteristic curve.
c. Current transformers. A current transformer
either a toroid (doughnut) winding with a primary
(CT) is an instrument transformer having low
conductor wire passing through the “hole”, or a sec-
losses whose purpose is to provide a f’ixed primary
tion of bus bar (primary) around which is wound the
to secondary step down current ratio. The primary
secondary. The bus bar CT is inserted into the bus
to secondary current ratio is in inverse proportion to
being measured. The CT ratio is selected to result in
the primary to secondary turns ratio. The secondary
a five ampere secondary current when primary
winding thus has multiple turns. The CT is usually
rated current is flowing (see fig 5-4).
5 - 4
TM 5-685/NAVFAC MO-912
POTENT I AL CURRENT
TRANSFORMER TRANSFORMER
L I
D
V - V O L T M E T E R W-WATTMETER A-AMMETER
Figure 5-4. Instrument transformers, typical applications.
(1) Ratings. Toroidal CTs are rated for the size
cuit. Never open a CT secondary while the primary
of the primary conductor diameter to be surrounded
circuit is energized.
and the primary to secondary current (5A) ratio.
d. Polarities. When connection secondaries of PTs
Bus bar type CTs are rated for the size of bus bar,
and Cts to metering circuits the correct polarities of
primary voltage and the primary to secondary cur-
all leads and connections must be in accordance
rent 5A) ratio.
with the metering circuit design and the devices
(2) Application. The primary of a CT is either
connected. Wrong polarity connections will give
the line conductor or a section of the line bus. The
false readings and result in inaccurate data, dam-
secondary current, up to 5A, is directly proportional
age and injury. All conductors and terminations
to the line current. The ratio of the primary to
should carry identification that matches schemat-
secondary current is inversely proportional to the
ics, diagrams and plans used for construction and
ratio of the primary turns to secondary turns.
maintenance.
(3) Safety. A CT, in stepping down the current,
e. Control circuits. Switchgear control circuits also steps up voltage. The voltage across the second-provide control power for the starting circuit of the
ary is at a dangerously high level when the primary
prime movers and the closing and tripping of the
is energized. The secondary of a CT must either be
switchgear circuit breakers. Additionally, the con-
shorted or connected into the closed metering cir-
trol circuits provide control power to operate the
5-5
TM 5-685/NAVFAC MO-912
various relays and indicating lights associated with
the control circuitry. The control circuits are classi-
fied as either AC or DC.
(1) AC control circuits. AC control circuits usu-
_
PRIMARY
ally derive their power from the source side of the
circuit breaker being controlled. This procedure ap-
plies to main incoming line circuit breakers, genera-
tor circuit breakers, and feeder circuit breakers (see
fig 5-6). Depending on the system voltage, the con-
trol power can be taken directly from the main bus
since it can be connected through a control power
transformer.
(2) Tie break er control circuits. In systems us-
ing a tie breaker, the control power for the tie
breaker and the feeder breakers is supplied through
a throw-over scheme so control power is available if
either side of the tie breaker is energized (see fig
5-7). In applications that require synchronizing cir-
cuitry, the running and incoming control buses are
usually supplied via the potential transformers. The
transformer primaries are connected to both the
Figure 5-5. Current flow in instrument transformers. ‘Polarity”
line side and the load side of the circuit breakers
marks show instantaneous flows.
that are used for synchronizing. The transformer
M A I N I N C O M I N G L I N E GENERATOR
CONTROL
POWER
TRANSFORMER
F U S E CONTROL POWER
TRANSFORMER
M A I N
CONTROL
POWER
GE N E R A T O R
BUS
B R E A K E R I
F U S E ,
FEEDER
BREAKER
L O A D
LOAD
Figure 5-6. AC control circuits.
TM 5-685/NAVFAC MO-912
LOAD
Figure 5-7. AC control circuits with tie circuit breaker.
secondaries are connected to the proper control bus
program that is built around equipment and system
through contacts on the synchronizing switch, or
records and visual inspections. The program is de-
through contacts on certain auxiliary relays. The
scribed in the manufacturer’s literature furnished
synchronizing switch would be used for manual op-
with the components. If a problem develops, the
eration and the auxiliary relay would be used when
user should perform general troubleshooting proce-
automatic synchronizing is provided.
dures. The program includes appropriate analysis of
(3) DC control circuits. DC control circuits de-
the records.
rive their power from a battery source consisting of
(1) Record keeping. Equipment and system log
a bank of batteries and a battery charger that main-
sheets are important and necessary functions of
tains the batteries at the proper charge. The battery
record keeping. The log sheets must be specifically
bank can be rated at various levels ranging between
developed to suit individual application (i.e., auxil-
24 volts and 125 volts DC. Those circuits that re-
iary use).
quire a source of control power completely indepen-
(2) Troubleshoo ing
t
. Perform troubleshooting
dent of the power system are connected to the DC
procedures when abnormal operation of the system
control bus. Examples of these are the prime mover
or equipment is observed. Maintenance personnel
starting circuits, and in some cases, the trip circuits
must then refer to records for interpretation and
for the circuit breakers when devices, other than
comparison of performance data (i.e., log sheets).
the direct-acting overcurrent trip devices, are used.
Comparisons of operation should be made under
Also, the closing circuits for the circuit breakers are
equal or closely similar conditions of load and ambi-
sometimes connected to the DC control bus.
ent temperature. The general scheme for trouble-
f. Service practices. Service practices for low volt-shooting is outlined in the following paragraphs and
age switchgear consist of a complete maintenance
troubleshooting table.
5-7
TM 5-685/NAVFAC MO-912
(a) Use recognized industrial practices as the
several times during the period, preferably under
general guide for servicing and refer to manufactur-
load. If the breaker is operated by a relay or a
er’s literature.
switch, it should be so operated at this time.
(b) The user should refer to manufacturer’s
( e ) Service for molded-case circuit breakers
literature for specific information on individual cir-
consists of the following procedures. Inspect connec-
cuit breakers.
tions for signs of arcing or overheating. Replace
(c) General service information for circuit
faulty connectors and tighten all connections. Clean
breakers includes the following safety require-
the connecting surfaces. Perform overload tripping
ments. Do not work on an energized breaker. Do not
tests. Verify automatic opening of breaker. Verify
work on any part of a breaker with test couplers
that the magnetic tripping feature is operating. Per-
engaged. Test couplers connect the breaker to the
form circuit breaker overload tripping tests. Proper
control circuit during testing. Spring-charged
action of the breaker tripping components is veri-
breaker mechanisms shall be serviced only by per-
fied by selecting a percentage of breaker current
sonnel experienced in releasing the spring load in a
rating (such as 300%) for testing. This overload is
controlled manner. Make operational tests and
applied separately to each pole of the breaker to
checks on a breaker after maintenance, before it is
determine how it will affect automatic opening of
returned to service. Do not work on a spring-
the breaker. Refer to manufacturer’s test informa-
charged circuit breaker when it is in the charged
tion. Turn the breaker on and off several times to
position.
verify satisfactory mechanical operation.
(d) Switchgear needs exercise. If the circuit
( f ) Service for air circuit breakers consists of
breaker remains idle, either open or closed, for six
the following procedure (see fig 5-8). Install the
months or more, it should be opened and closed
safety pin to restrain the closing spring force. With
CONNECTED
ALL POWER
(PR I MA R Y
CONTROL)
CONTROL POWER
STILL CONNECTED
--DISCONNECTED
ALL POWER DISCON-
NECTED
WITHDRAWN
BREAKER WITHDRAWN
READY FOR REMOVAL
Figure 5-8. Maintenance for typical low voltage switchgear with air circuit breakers.
5-8
TM 5-685/NAVFAC MO-912
the pin in place, the contacts will close slowly when
Table 5-l. Low voltage circuit breaker
the breaker is manually operated. Inspect connec-
troubleshooting-Continued
tions for signs of arcing or overheating. Replace
Note
faulty connectors and tighten all connections. Clean
the connecting surfaces. An infrared (IR) survey is a
Refer to manufacturer’s literature for specific information on recommended inspection procedure. The IR survey
individual circuit breakers.
should be performed when the circuit breaker is
Cause
Remedy
under load and closed to detect overheating of con-
FALSE TRIPPING
nections. Perform general troubleshooting of the
Overcurrent pick-up too low
Check application of overcurrent
breaker (refer to the following table) if a problem
trip device
develops. If the trouble cannot be corrected, refer to
Check application of overcurrent
the manufacturer’s literature for specific informa-
Overcurrent time setting too short
trip device
tion on individual breakers. Instrument transform-
ers require no care other than keeping them dry
Mechanical binding in over-
Correct binding or replace over-
current trip device
and clean. Refer to manufacturer’s literature if spe-
condition current trip device
cific information is required. Information related to
Captive thumbscrew on power sen-
Adjust power sensor. Tighten
control circuit components is provided in paragraph
sor loose. Fail safe circuitry reverts
thumbscrew on desired setting
characteristics to minimum setting
5-3 e of this chapter.
and maximum time delay
Ground sensor coil improperly con- Check polarity of connections to nec ted
coil. Check continuity of shield
Table 5-l. Low voltage circuit breaker troubleshooting.
and conductors connecting the ex-
ternal ground sensor coil
Note
FAILURE TO CLOSE AND LATCH
Refer to manufacturer’s literature for specific information on Binding in attachments preventing Realign and adjust attachments individual circuit breakers.
resetting of latch
Cause Remedy
Latch out of adjustment
Adjust latch
OVERHEATING