Shut out potential disaster by
Providing Reliable
Switchgear Control Power
By RONALD J. KASPER, Executive Engineer-National, Industrial Risk Insurers, Hartford, CT
W
HEN a 350 MCM cable
faulted in a manhole of a
13.8 kv distribution system in a
cable manufacturing plant, the 15
kv switchgear failed to operate to
clear the fault. Passing the fault
through the system damaged some
oil circuit breakers and a 600 kva
voltage-regulating transformer; the
cost of repairs exceeded $200,000.
The 48v lead-acid storage battery
supplying switchgear trip power
was found to be discharged. The
battery, in a steel cabinet in the
switchyard, was out of sight and,
apparently, out of mind.
A 6000 hp, 13.8 kv synchronous motor powering a log
grinder in a paper mill was seriously damaged when battery power to the motor control system
and motor power circuit switchgear was lost. When dc control
power was lost, the motor reverted
to the “start” mode, but this condition would have been sensed, and
the motor breaker would have
tripped, if trip power to operate
the breaker had been available.
Continued operation in the start
mode caused overheating and
motor failure, upon which the
plant main breaker-which drew
its control power from another
battery--tripped to clear the fault
imposed on the power system by
the failed motor. The cost for
motor repairs and lost production
was $250,000. The debacle would
have been prevented if the switchgear dc control power system for
the motor had been properly maintained.
In a cement plant, an electrical
fault and ensuing tire damaged
three 1500 hp, 4160 v synchronous motors, three cubicles of
5 kv switchgear, and hundreds of
feet of 1000 MCM 5 kv power
cable. Damage exceeded $300,000.
Almost all of this damage would
have been prevented if either the
5 kv breakers or switchgear in
the 69 kv main substation had
operated; the switchgear failed
to operate because both the 5 kv
switchgear and main substation
were without control power. All
switchgear was served from a 12.5
v stationary battery system that
had one lug burned off because of
a ground fault; no one knows how
long the situation had existed.
Loss of dc excitation to two 300
hp synchronous motors in a cement plant resulted in destruction
of the motors and ensuing fire;
damage was $500,000. The motor
control circuitry sensed the condition, but breakers failed to trip
because of lack of control power.
The 20 ampere circuit breaker in
the ac supply to the battery
“A battery is the preferred, and by far the most commonly
used, method of providing reliable switchgear control power.”
charger was found to be tripped,
explaining why the battery was
charged to only 30 volts. At least
100 volts were needed to trip the
breakers.
Electrical fault and ensuing fire
and explosion caused $1,000,000
in damage to a paper mill’s main
transformers and other substation
equipment, cable trays, and raceways. This loss would not have
been sustained if the 48 v battery
serving the plant’s switchgear had
been maintained. The breakers
failed to trip because battery voltage was only 9.5 volts; the operating range for the breaker trip coils
was 28 to 60 volts. Eleven of the
36 cells of the nickel-cadmium
battery were found to be dry.
All of these disasters have a
common denominator; failure to
maintain the dc power supply
system to switchgear prevented
breakers from operating. Loss of
dc control power can render tens
of thousands of dollars worth of
switchgear useless, and leave millions of dollars worth of electrical
equipment without protection.
Medium-voltage power switchgear (and low-voltage switchgear
employing shunt-trip relaying)
requires a source of electric power to operate the breaker trip
mechanism when a fault is sensed
by the breaker’s associated protective relaying. Two methods are
commonly used to provide power
to the breaker trip coil: charged
capacitor (charged through a rectifier supplied by an ac source),
and a stationary battery. Of these
two methods, the battery is the
preferred, and by far the most
commonly used, method.
A switchgear battery is composed of a number of individual
cells, or a group of multicell bat-
An arcing fault initiated this burndown
of a piece of 1200 amp switchgear.
Inadequate dc control power prevented
the circuit breaker from operating to
clear the fault.
teries, connected in series or seriesparallel to provide the proper
breaker operating voltage. Some
breaker control circuits are designed for 48 v dc operation, but
voltages of 125 and 250 volts dc
are recommended, especially if the
battery serves a number of breakers. In addition to providing
breaker trip power, the battery also provides power to the closing
coil for electrical closure.
Proper battery performance will
be difficult to maintain if the battery is not installed in a manner
conducive to long battery life and
to the performance of maintenance operations. The location of
the battery and its charger should
be selected carefully, and the battery system should be installed in
accordance with recognized standards. Standards for battery installation and maintenance are
available from the Institute of Electrical and Electronics Engineers,
American National Standards Institute, Underwriters Laboratories,
National Fire Protection Association, and the National Electrical
Manufacturers Association.
The battery should be located in
a protected room that is clean,
dry, well ventilated, and relatively
cool. Battery room temperature
should be maintained at 72 to 77
F. As a general rule, battery life
will be cut in half for every 18 deg
rise in temperature. On the other
hand, battery capacity will be reduced if the operating environment is too cold.
The battery room should be
well ventilated, because lead-acid
batteries liberate hydrogen gas as
a normal part of their charging
process; five to six air changes per
hour is recommended. Smoking
must be prohibited in the battery
room, and as a precaution against
failure of the ventilating system,
electrical equipment should be approved for Class I, Group B, Division II locations. The battery
should be located in a sprinklered
masonry room that contains no
combustibles to minimize exposure to fire. If the battery is in an
open area, rather than in a separate room, there should be no
combustibles within 25 feet of the
battery.
Battery cells should be arranged
in a manner that will provide easy
access to all cells and that will
permit battery heat to be readily
dissipated. Three-high battery
racks should not be used because
the top tier of batteries might be
subjected to excessive temperatures that could reduce battery
life.
Although the battery charger
should be located in a clean, dry
cool environment comparable to
the environment of the battery
room, the charger should not be
located in the same room as the
battery; a number of major fires
have been caused by faulty battery
chargers. The charger should be
selected with care to ensure its
compatibility with the battery and
should be certified for its intended
use by a qualified electrical testing
laboraory.
Maximum reliability and best
battery life are realized if the
charger is automatic. The ac supply to the battery charger should
be planned for reliability, and precautions should be taken to ensure
that it cannot be inadvertently
turned off. A means for monitoring the charger ac supply should
be provided.
The battery system should be
installed with a load-test circuit
that permits testing the battery to
ensure that proper voltage and
current are available for breaker
trip-coil operation, and an alarm
circuit should be installed to signal
if the proper voltage for breaker
operation is not available.
If breakers are equipped with
red (closed) and green (open) indicating lights, the lights should be
monitored regularly; an extinguished red light that coincides
with an extinguished green light
should never be ignored. The red
light monitors the continuity of,
and the dc supply to, the trip circuit, in addition to signifying that
the breaker is closed. An extinguished red indicating light with
the breaker closed can mean a discontinuous trip circuit because of
a burned-out trip coil or other reasons, loss of dc voltage, or simply
a burned-out lamp.
A backup supply of dc power
for switchgear operation should be
considered because the plant is exposed to the possibility of severe
losses if a fault should occur in the
electrical system when the normal
dc supply is not available. The
best backup supply is a redundant
battery: as an alternative to a redundant battery, an engine-driven
generator can be installed to provide switchgear control power
when the normal (battery) supply
is not available.
The most reliable arrangement
is to have several batteries interconnected with transfer switching
arrangements. With such a system,
selective portions of the power
system are served from different
batteries, rather than a single battery serving all of the plant’s
switchgear. Batteries should be
sized to permit each battery to
carry its normal load plus the load
of other batteries whose load it
might be called upon to assume.
Whether only one battery or
several are used, switchgear batteries should be specified explicitly
for switchgear service. Batteries
designed for switchgear service are
built to deliver very high discharge
currents (but only for brief periods), because when a fault occurs,
several breakers are likely to trip
simultaneously. It is not uncommon, however, to use the switchgear battery to provide dc control
power to instrumentation and
other equipment in the plant. If
this is done, care must be taken
not to burden the battery to the
point at which it will be unable to
perform its primary function.
Both the battery and the
charger should be serviced on a
regular basis; service of a switchgear control power supply system
should “ever be relegated to “job
of convenience” status. Both battery and charger should be cleaned
Failure of a circuit breaker to trip
caused extensive damage to this
93,000 kva. 138/69 kv transformer;
repairs to the damaged windings, core,
and bushings took 4 months to complete. Failure of the breaker to trip
was traced to inadequate maintenance
of the switchgear dc control power
supply system.
“The integrity of the ac supply to
the charger should be checked periodically.”
periodically, and battery terminals
and intercell connections should
be checked for tightness when the
battery is cleaned. Battery posts,
studs, and cable ends should be
cleaned with a solution of bicarbonate of soda and left with a
liberal coating of antioxidant grease
to retard corrosion.
The charger should be kept free
of dirt and dust to permit proper
heat transfer. The preferred method of cleaning is to use a soft-
bristled brush in conjunction with
low-pressure compressed air to
loosen dust and dirt deposits and
then to remove the dislodged contaminants with a vacuum cleaner.
The integrity of the ac supply to
the charger should be checked periodically to ensure that fuses are
not blown or the breaker is not
tripped. If the charger is served
through fuses, fuses and fuse clips
should be inspected to see if there
is any sign of overheating.
Batteries must be inspected
regularly for proper electrolyte
level, and hydrometer readings
should be taken regularly to ensure that the electrolyte specific
gravity is at the recommended
value for the charged state. The
hydrometer readings must be correlated with temperature according
to the battery manufacurer’s instruction manual, because specific
gravity varies with temperature for
a given state of charge.
The charge rate should be set at
a value that will maintain the battery at full charge without overcharging; the charge rate might require adjustment from time to
time. Overcharging will cause liberation of excessive amounts of
hydrogen, increasing water consumption, generating high temperatures that reduce battery life,
and increasing explosion potential
because of increased hydrogen liberation. Undercharging, on the
other hand, leads to a condition
known as “sulfation,” in which the
active plate area of the battery is
reduced, thereby reducing battery
capacity.
A rubber apron, gloves, boots,
and eye protection should be used
when batteries are serviced. As an
added precaution against electrolyte spillage, and to prevent contaminants from entering the battery, vent plugs should be left in
place unless hydrometer readings
are being taken or water is being
added to the cell.
Smoking and open flames must
not be permitted in the battery
room, and precautions must be
taken to ensure that no sparks will
be produced in the room. Before
workers enter the battery room,
tool pouches should be removed,
and tools should be removed from
trouser pockets to prevent metal
objects from contacting battery
posts and causing sparks. Metal
tools should never be rested on
batteries.