SWITCHES, FUSES, AND CIRCUIT-BREAKERS.
Electrical transmission has worked a revolution in the art of switching.
As long as the distances to be covered by distribution lines required
pressures of only a few hundred volts, the switch contacts for
generators and feeders could well be exposed in a row on the surface of
vertical marble slabs and separated from each other by distances of only
a few inches. These switches were capable of manual operation even at
times of heavy overload without danger of personal injury to the
operator or of destructive arcing between the parts of a single switch
or from one switch to another near-by. On the back of these marble slabs
one or more sets of bare bus-bars could be located without much
probability that an accidental contact between them would start an arc
capable of destroying the entire switchboard structure and shutting down
the station.
The rise of electric pressures to thousands and tens of thousands of
volts in distribution and transmission systems has vastly increased the
difficulty of safe and effective control with open-air switches. The
higher the voltage of the circuit to be operated under load the greater
must be the distance between the contact parts of each switch and also
between adjacent switches. Such switches must also be farther removed
from the operators as the voltages of their circuits go up, as a person
cannot safely stand very close to an electric arc of several feet or
even yards in length. In the West, where long transmissions are most
common, long break-stick switches have been much used with high
voltages. These switches depend on the length of the break to open the
circuit and on the length of the stick that moves the switch-jaw or plug
to insure the safety of the operator. Where switches of this sort are
used it is highly important to have ample distances between the contact
points of each switch and also between the several switches. On circuits
of not more than 10,000 volts an arc as much as a yard long will in some
cases follow the opening switch blade and hold on for several seconds.
On the 33,000-volt transmission line at Los Angeles a peculiar form of
switch is used which makes a break between a pair of curved wire horns
that are ten inches apart at their nearest points. When the contact
between these horns is broken the arc travels up between portions of the
horns that curve apart and is thus finally ruptured. Besides the very
large space required for open switches on circuits of 5,000 to 10,000
volts or more, there is a further objection that the arcs developed by
opening such switches under heavy loads rapidly destroy the contact
parts and produce large quantities of metallic vapor that is
objectionable in a central station. In some experiments performed at
Kalamazoo (A. I. E. E., vol. xviii., p. 407) with open-air switches the
voltages ranged from 25,000 to 40,000. The loads on circuits broken by
the switches were highly inductive and mounted from 1,200 to 1,300
kilovolt-amperes. At 25,000 volts the arc produced by the open-air
switch held on for several seconds. At 40,000 volts the arc following
the opening of this switch was over thirty feet long, and being out of
doors near the pole line the arc struck the line wires and
short-circuited the system. It has been shown that the oscillations of
voltage occurring when a circuit under heavy load is opened by an
open-air switch may be very dangerous to insulation (A. I. E. E., vol.
xviii., p. 383). In the Kalamazoo test the oscillations of this sort
were reported to have reached two or three times the normal voltage of
the system when the open-air switch was used.
[Illustration: FIG. 55.--Connections between Power-houses 1 and 2 at
Niagara Falls.]
Facts of the nature just outlined have led to the development of oil
switches. The general characteristic of oil switches is that the contact
parts are immersed in, and the break between these contacts takes place
under, oil. Two types of the oil switch are made, one having all of its
contact parts in the same bath of oil and the other having a separate
oil-bath for each contact. Compared with those of the open-air type, oil
switches effect a great saving of space, develop no exposed arcs or
metallic vapors, cause little if any oscillation or rise of voltage in
an alternating circuit, and can be depended on to open circuits of any
voltage and capacity now in use. In the tests above mentioned at
Kalamazoo, a three-phase oil switch making two breaks in each phase and
with all the six contacts in a single oil-bath was used to open circuits
of 25,000 volts and 1,200 to 1,300 kilovolt-arcs with satisfactory
results. At 40,000 volts, however, this type of switch spat fire and
emitted smoke, indicating that it was working near its ultimate
capacity. A three-phase switch with each of its six contacts in a
separate cylindrical oil-chamber was used to open the 40,000-volt 1,300
kilovolt-arc circuit at Kalamazoo with perfect success even under
conditions of short-circuit and without the appearance of fire or smoke
at the switch. The three-phase switch used in the tests at Kalamazoo
and having each of its contacts in a separate oil-chamber was similar in
construction to the switches used in the Metropolitan and Manhattan
railway stations in New York City. In each of these switches the two
leads of each phase terminate in two upright brass cylinders. These
cylinders have fibre linings to prevent side-jumping of the arcs when
the switch is opened, and each cylinder is filled with oil. Into the two
brass cylinders of each phase dips a ∩-shaped contact piece through
insulating bushings, and the ends of this contact piece fit into
terminals at the bottom of the oil pots. A wooden rod joins the centre
or upper part of the ∩-contact piece, and the three rods of a
three-phase switch pass up through the switch compartment to the
operating mechanism outside. The six brass cylinders and their three
∩-contact pieces are usually mounted on a switch cell built entirely of
brickwork and stone slabs. For a three-phase switch the brick and stone
cell has three entirely separate compartments, and each compartment
contains the two brass cylinders that form the terminals of a single
phase. On top of and outside the cell the mechanism for moving the
wooden switch rods is mounted. In the Metropolitan station, where the
voltage is 6,000, the vertical movement of the ∩-shaped contact piece
with its rod is twelve inches. At the Manhattan station, where the
operating voltage is 12,000, the vertical movement of the ∩-contacts in
opening a switch is seventeen inches. The total break in each phase in a
switch at the Metropolitan station is thus twenty-four inches, or four
inches per 1,000 volts, and the total break per phase in switches at
the Manhattan station is thirty-four inches, or 2.66 inches per 1,000
volts total pressure.
Oil switches are now very generally employed on alternating circuits
that operate at 2,000 volts or more for purposes of general
distribution. On circuits of moderate voltage like that just named, and
even higher, it is common practice to use oil switches that have only a
single reservoir of oil each, the entire six contacts in the case of a
three-phase switch being immersed in this single reservoir. Such
switches are usually operated directly by hand and are located on the
backs of or close to the slate or marble boards on which the handles
that actuate the switch mechanism are located. A good example of this
sort of work may be seen at the sub-station in Manchester, N. H., where
energy from four water-power stations is delivered over seven
transmission lines and then distributed by an even larger number of
local circuits at 2,000 volts three-phase. At the Garvin’s Falls
station, one of the water-power plants that delivers energy to the
sub-station in Manchester, the generators operate at 12,000 volts
three-phase, and these generators connect directly with the bus-bars
through hand-operated oil switches on the back of the marble
switchboard. These last-named switches, like those at the Manchester
sub-station, have all the contacts of each in a single reservoir of oil.
With very high voltages, where only a few hundred kilowatts are
concerned, and also with powers running into thousands of kilowatts at
as low a pressure as 2,000 volts, it is very desirable to remove even
oil switches from the switchboard and the vicinity of the bus-bars.
Great powers as well as very high voltages not only increase the element
of personal danger to an attendant who must stand close to a switch
while operating it, but also render the damage to other apparatus that
may result from any failure of or short-circuit in a switch much more
serious.
[Illustration: FIG. 56.--Wire-room Back of Switchboard in Power-station
on French Broad River, North Carolina.]
As soon as the switches are removed to a distance from the operating
board the necessity for some method of power control becomes evident,
since the operator at the switchboard should be able to make or break
connections of any part of the apparatus quickly. The necessity for the
removal of switches for very large powers to a distance from the
operating boards and for the application of mechanical power to make and
break connections was met before the development of oil switches. Thus
at the first Niagara (A. I. E. E., vol. xviii., p. 489) power-house, in
1893, the switches for the 3,750-kilowatt, 2,200-volt generators, though
of the open-air type, were located in a special switch compartment
erected in the generator room and over a cable subway at some distance
from the operating board. These switches were actuated through
compressed-air cylinders into which air was admitted by the movement of
levers near the switchboard. Evidently a switch of this capacity--1,000
amperes per pole and 2,200 volts, two-phase--could not well be operated
by hand-power wherever located, because of the large effort required. In
the second generating station at Niagara Falls oil switches similar to
those used at the Manhattan Elevated Railway plant in New York, but
two-phase, were employed. Each of these oil switches at Niagara Falls
has a capacity of 5,000 horse-power, like the previous open-air
switches, and is electrically actuated.
[Illustration: FIG. 57.--Section through Cable Subway under Oil Switches
in Niagara Power-house No. 2.]
In these electrically operated oil switches a small motor is located on
top of the brick cell that contains the contact parts, and this motor
releases and compresses springs that open and close the switch. While it
is not desirable to employ open-air switches to open circuits of several
thousand or even hundreds of kilowatts at voltages of 2,000 or more, it
is nevertheless possible to do so. This is shown by the experience of
the first Niagara Falls station, where the 2,200-volt two-phase switches
are reported to have opened repeatedly currents of more than 600 amperes
per phase without injurious sparking. The great rise of voltage that was
shown by the experiments at Kalamazoo to follow the opening of a simple
open-air switch was avoided at the first Niagara switches by a simple
expedient. In these 5,000 horse-power open-air switches a shunt of high
resistance was so connected between each pair of contacts that the
blades and jaws that carried the main body of the current never
completely opened the circuit. When the main jaws of one of these
switches were opened the shunt resistance continued in circuit until
subsequently broken at auxiliary terminals. That no excessive rise of
voltage took place when one of these switches was open was shown by
connecting two sharp terminals in parallel with the switch and by
adjusting these terminals to a certain distance apart. Had the voltage
risen on opening the switch above the predetermined amount there would
have been an arc formed by a spark jumping the distance between the
pointed terminals.
[Illustration: FIG. 58.--Schenectady Switch-house on Spier Falls Line.]
[Illustration: FIG. 59.--Second-floor Plan of Saratoga Switch-house on
Spier Falls Line.]
Safety and reliability of operation at high voltages, say of 5,000 or
more, require that each element of the equipment be so isolated as well
as insulated from every other element that the failure or even
destruction of one element will not seriously endanger the others. With
this end in view the cables from each generator to its switch should be
laid in a conduit of brick or concrete that contains no other cables.
The brick or stone compartment for each phase of each switch should be
so substantial that the contacts of that phase may arc to destruction
without injury to the contacts of another phase. Bus-bars, like
switches, should be removed from the operating switchboard, because an
arc between them might destroy other apparatus thereon, and even the
board itself. It is not enough to remove bus-bars from the switchboard
where very high voltages are to be controlled, but each bar should be
located in a separate brick compartment so that an arc cannot be started
by accidental contact between two or more of the bars. It is convenient
to have the brick and stone compartments for bus-bars built horizontally
one above the other. The top and bottom of each compartment may
conveniently be formed of stone slabs with brick piers on one side and a
continuous brick wall on the other to hold the stone slabs in position.
Connections to the bus-bars should pass through the continuous brick
wall that forms what may be termed the back of the compartments. To
close the openings between the brick piers at the front of the
compartments movable slabs of stone may be used. Feeders passing away
from the bus-bars, like dynamo cables running to these bars, should not
be grouped close together in a single compartment, but each cable or
circuit should be laid in a separate fireproof conduit to the point
where it passes out of the station.
[Illustration: FIG. 60.--Ground Floor of Saratoga Switch-house.]
The folly of grouping a large number of feeders that transmit great
powers together in a single combustible compartment was well illustrated
by the accident that destroyed the cables that connected the first
Niagara power-station with the transformer-house on January 29th, 1903.
On the evening of that day lightning short-circuited one of the cables
in the short bridge that connects No. 1 station with the
transformer-house, and all the cables in this bridge, supplying local
consumers as well as railways and lighting in Buffalo, were destroyed.
This bridge contained probably more than thirty-six cables, as that
number of new cables was put in position within twenty-four hours after
the accident, and these cables, covered with inflammable insulation,
were close together. The result was not only the loss of the cables, but
also the damage to power users. If these cables had been located in
separate fire-proof conduits, it is highly probable that only the one
directly affected by lightning would have been destroyed.
The brick and stone compartments for bus-bars may be located in the
basement underneath the switchboard, as at the Portsmouth station of the
New Hampshire Traction Company, or at any other place in a station where
they are sufficiently removed from the other apparatus. In power-house
No. 2 at Niagara Falls a cable subway beneath the floor level runs the
entire length, parallel with the row of generators (A. I. E. E., vol.
xix., p. 537). In this subway, which is thirteen feet nine and
three-quarter inches wide and ten feet six inches high, the two
structures for bus-bar compartments are located. Each of these
structures measures about 6.6 feet high and 1.8 feet wide, and contains
four bus-bar compartments. In each compartment is a single bar, and the
four bars form two sets for two-phase working. Above the bus-bar
compartments and rising from the floor level are the oil switches. A
space over the cable subway midway of its length and between the two
groups of oil switches is occupied by the switchboard gallery which is
raised to some elevation above the floor and carries eleven generator,
twenty-two feeder, two interconnecting, and one exciter panels. In
power-house No. 1 the bus-bars are located in a common space above the
5,000 horse-power open-air switches already mentioned, and each bar has
an insulation of vulcanized rubber covered with braid and outside of
this a wrapping of twine. Of course; an insulation of this sort would
amount to nothing if by any accident an arc were started between the
bars. Where each bus-bar is located in a separate fireproof compartment,
as at Niagara power-house No. 2, the application of insulation directly
to each bar is neither necessary nor desirable. Consequently the general
practice where each bar has its own fireproof compartment is to
construct the bars of bare copper rods.
With main switches for generators and feeders removed from the operating
board and actuated by electric motors or magnets, the small switches at
the board with which the operator is directly concerned must of course
control these magnets or motors. The small switches at the operating
board are called relay switches, and the current in the circuits opened
and closed by these switches and used to operate the magnets or motors
of the oil switches may be conveniently obtained from a storage battery
or from one of the exciting dynamos.
Probably the best arrangement of the relay switches is in connection
with dummy bus-bars on the face of the switchboard, so that the
connections on the face of the board constitute at all times a diagram
of the actual connections of the generator and feeder circuits. It is
also desirable for quick and correct changes in the connections of the
main apparatus that all the relay switches and instruments necessary for
the control of any one generator or any one feeder be brought together
on a single panel of the switchboard. If this plan is followed, the
operator at any time will have before him on a single panel all of the
switches and instruments involved in the connections then to be made,
and the chance for mistakes is thus reduced to a minimum. The plan just
outlined was that adopted at the Niagara power plant No. 2, where a
separate panel is provided for each of eleven generators and each of
twenty-two feeders. On each of the eleven generator panels there are two
selector relay switches, one generator relay switch, and one relay
generator field switch. On each of the twenty-two feeder panels there
are two relay selector switches. The relay switches on the two
interconnecting panels serve to make connections between the two groups
of five and six generators respectively in power-house No. 2 and the ten
generators of power-house No. 1. On each panel there are relay
indicators to show whether the oil switches that carry the main current
respond to the movements of their relay switches.
Where the electric generators operate at the maximum voltage of the
system, as at Garvin’s Falls and in the power-house of the Manhattan
Elevated Railway, there may be said to be only one general plan of
connections possible. That is, the generators must connect directly with
the main bus-bars at the voltage of the system, and the feeders or
transmission lines must also connect to these same bars. Of course there
may be several sets of bus-bars for different circuits or classes of
work, but this does not change the general plan of through connections
from generators to lines. So, too, the arrangement of switches is
subject to variations, as by placing two switches in series with each
other in each dynamo or feeder cable, or by connecting a group of
feeders through their several switches to a particular set of bus-bars
and then supplying this set of bars from the generator bus-bars through
a single switch.
[Illustration: FIG. 61.--Switchboard Wiring, Glens Falls Sub-station on
Spier Falls Line.]
Where the voltage of transmission is obtained by the use of step-up
transformers, the connections of these transformers may be such as to
require nearly all switching to be done on either the high- or
low-tension circuits. The more general practice was formerly to do all
switching in the generator circuits and on the low-tension side of
transformers, except in the connection and disconnection of transformers
and transmission lines with the high-tension bus-bars, when not in
operation. Where generators operate at the maximum voltage of the system
only two main groups of switches are necessary, one group connecting
generators to bus-bars, and the other group connecting bus-bars to the
transmission lines. As soon as step-up transformers are introduced the
number of switch groups must be increased to four if the usual method of
connection is followed, and there must be both a high voltage and a low
voltage set of bus-bars. That is, one set of switches must connect
generators with low-tension bus-bars, another group must connect
low-tension bars with the primary coils of transformers, a third group
joins the secondary coils of transformers with the high-tension bars,
and the fourth group of switches joins the transmission lines to the
high-tension bus-bars. Switches connecting the secondary coils of
step-up transformers to the high-tension bus-bars, and also the
transmission lines to these same bars, have often been of the simple
open-air type with short knife-blade construction. These switches have
been used to disconnect the secondary coils of transformers and also the
transmission lines from the high-tension bus-bars when no current was
flowing, and switches of the simple knife-blade construction with short
breaks could of course be used for no other purpose. With switches of
this sort on the high-tension side of apparatus the practice is to do
all switching of line circuits on the low-tension side.
It is possible to avoid some of this multiplication of switches if each
generator with its transformers is treated for switching purposes as a
unit and the switching for this unit is done on the secondary or
high-voltage side of the step-up transformers. The adoption of this
plan, of course, implies the use of switches that are competent to break
the secondary circuit of any group of transformers under overload
conditions and at the maximum voltage of the system, but oil switches as
now made are competent to meet this requirement. When all switching of
live circuits is confined to those of high voltage there is also the
incidental advantage that heavy contact parts carrying very large
currents are avoided in the operating switches. Where each generator is
connected directly to its own group of transformers the secondary coils
of these transformers will pass through oil switches to high-tension
bus-bars, and the use of low-tension bus-bars may be avoided. From these
high-tension bus-bars the transmission lines will pass through oil
switches, so that on this plan there are only two sets of oil switches,
namely, those connecting the secondary coils of transformers to the
high-tension bus-bars, and those connecting the transmission lines to
the same bars. Each group of two or three transformers, according as
two or three are used with each generator, should be connected to its
generator through short-break, open-air knife switches for convenience
in disconnecting and changing transformers that are not in operation,
but these switches are not intended or required to open the circuit of
the generators and primary coils when in operation.
[Illustration: FIG. 62.--Distributing Switchboard, Central Sub-station,
Montreal.]
A plan similar to that just outlined was followed at the station of the
Independent Electric Light and Power Company, San Francisco, where each
of the 550-volt generators is ordinarily connected directly to the
primary coils of two transformers that change the current from two-phase
to three-phase and then deliver it through oil switches to the
high-tension bus-bars at 11,000 volts. To these bus-bars the 11,000-volt
feeders for five sub-stations are connected through switches. At this
station there is a set of 550-volt bus-bars to which any of the
generators may be connected, but to which no generator is connected in
ordinary operation. The generators alone have switches connecting with
these bars. When it is desirable to operate any particular generator on
some pair of transformers other than its own, that generator is
disconnected from its own transformers and connected to the 550-volt
bus-bars. The generator whose transformers are to be operated by the
generator before mentioned next has its switch connected to the 550-volt
bus-bars, while the brushes of the contact rings of the former generator
are raised. As the leads from each generator to its two switches are
permanently joined, the switching operations just named connect the
transformers of one generator with the other generator that has its
switch closed on the 550-volt bars.
[Illustration: FIG. 63.--Switchboard at Chambly Power-station.]
Where it is desired that a single reserve transformer may be readily
substituted for any one of a number of transformers in regular use, the
connections to each of these latter transformers may be provided with
double-pole double-throw knife switches on both the primary and
secondary sides, so that when these switches are thrown one way at any
transformer in regular use the reserve transformer will be connected in
its place.
Fuses and automatic circuit-breakers alike are intended to break
connections without the intervention of human agency under certain
predetermined conditions. In the fuse the heat generated by a certain
current is sufficient to melt or vaporize a short length of special
conductor. In the circuit-breaker a certain current gives a magnet or
motor sufficient strength to overcome the pressure of a spring, and
contact pieces through which the current is passing are pulled apart.
The primary object of both the fuse and the circuit-breaker is thus to
open connections and stop the flow of energy when more than a certain
current passes. When any current passes through a circuit in the reverse
of its regular direction the circuit-breaker can be arranged to break
the connections, though the fuse cannot. A fuse must carry the current
at which it is designed to melt during some seconds before enough heat
is developed to destroy it, and the exact number of seconds for any
particular case is made a little uncertain by the possibility of loose
connections at the fuse tips which develop additional heat and also by
the heat-conducting power of its connecting terminals. A circuit-breaker
may be set so as to open its connections in one or more seconds after a
certain current begins to flow. When connections are broken by a fuse
the molten or vaporized metal forms a path that an arc may easily
follow. A circuit-breaker with its contacts under oil offers a much
smaller opportunity than a fuse for the maintenance of an arc. These
qualities of fuses and circuit-breakers form the basis of their general
availability and comparative advantages in transmission circuits.
Much variation exists in practice as to the use of fuses and
circuit-breakers on transmission circuits. One view often followed is
that fuses and circuit-breakers should be entirely omitted from the
generator and transmission lines. The argument in favor of this practice
is that temporary short circuits due to birds that fly against the lines
or to sticks and loose wires that are thrown onto them will interrupt
all or a large part of the transmission service if fuses or
circuit-breakers that operate instantly are employed. On the other hand,
it may be said that if fuses and circuit-breakers are omitted from the
generator and transmission circuits a lasting short circuit will make it
necessary to shut down an entire plant in some cases until it can be
removed. Electric transmission at high voltages became important before
magnetic circuit-breakers competent to open overloaded circuits at such
voltages were developed. Consequently the early question was whether a
transmission line and the generators that fed it should be provided with
fuses or be solidly connected from generators to the distribution
circuits of sub-stations. The original tendency was strong to use fuses
in accord with the practice at low voltages. The great importance of
continuous service from transmission systems and the many interruptions
caused by temporary short circuits where fuses were used led to their
abandonment in some cases. An example of this sort may be seen at the
first Niagara station. In 1893, when this station was equipped, no
magnetic circuit-breaker was available for circuits of either 11,000 or
2,200 volts, carrying currents of several thousand horse-power, and
fuses were employed in lines at both these pressures (A. I. E. E., vol.
xviii., pp. 495, 497). The fuses adopted in this case were the same for
both the 2,200 and the 11,000-volt lines and were of the explosive type.
Each complete fuse consisted of two lignum-vitæ blocks that were hinged
together at one end and were secured when closed at the other. In these
blocks three parallel grooves for fuses were cut and in each groove a
strip of aluminum was laid and connected to suitable terminals at each
end. Vents were provided for the grooves in which the aluminum strips
were placed so that the expanding gas when a fuse was blown would
escape. When these fuse blocks were new and the blocks of lignum vitæ
made tight joints the metallic vapor produced when a fuse was blown was
forced out at the vents and the connections of the line were thus
broken. After a time, however, when the joints between the blocks were
no longer tight because of shrinkage, the expanding gas of the fuse
would reach the terminals and an arc would continue after the fuse had
blown. These aluminum fuses, which were adopted about 1893, were
abandoned at the Niagara plant in 1898. Since this later date the
2,200-volt feeders from the No. 1 power-house to the local consumers
have had no fuses at the power-house, nor have circuit-breakers been
installed there in the place of the fuses that were removed. At the
large manufacturing plants supplied through these local Niagara feeders,
the feeders formerly terminated in fuses, but these have since been
displaced by circuit-breakers. In the second Niagara power-station,
completed in 1902, the local 2,200-volt feeders are provided with
circuit-breakers, but no fuses. Between the generators and bus-bars of
the first Niagara plant the circuits were provided with neither fuses
nor automatic circuit-breakers, and this practice continues there to the
present time.
Besides the aluminum fuses in the 11,000-volt transmission line at the
first Niagara station, there were lead fuses in the 2,200-volt primary
circuits of the step-up transformers that supplied these lines. At the
other end of these lines, in the Buffalo sub-station, another set of
aluminum fuses was inserted before connection was made with the
step-down transformers. Between the secondary coils of these
transformers and the 550-volt converters there were no fuses, but these
converters were connected to the railway bus-bars through direct current
circuit-breakers. These lead fuses, which contained much more metal than
those of aluminum, when blown set up arcs that lasted until power was
cut off by opening a switch, and usually destroyed their terminals. An
effort was made to so adjust the sizes of the fuses in this transmission
system that in case of a short circuit in distribution lines at Buffalo
only the fuses in the sub-station would be blown, leaving those at
Niagara entire. This plan did not prove effective, however, and a severe
overload on the distribution lines in Buffalo would blow out fuses clear
back to the generator bus-bars at the Niagara station.
In order to accomplish the opening of overloaded circuits with greater
certainty, to delay such opening where the overload might be of only a
momentary nature, and to confine the open circuit to the lines where the
overload existed, automatic circuit-breakers were substituted for the
fuses named in the Niagara and Buffalo transmission system. This system
was also changed from 11,000 to 22,000 volts on the transmission lines,
thus rendering the requirements as to circuit-opening devices more
severe. These circuit-breakers were fitted with time-limit attachments
so that any breaker could be set to open at the end of any number of
seconds after the current flowing through it reached a certain amount. A
circuit-breaker with such a time-limit attachment will not open until
the time for which it is set, after the amperes flowing through it reach
a certain figure, has elapsed, no matter how great the current may be.
Moreover, if the overload is removed from a line before the number of
seconds for which its time-limit circuit-breaker is set have elapsed,
the circuit-breaker resets itself automatically and does not open the
connections. If a circuit-breaker is set to open a line after an
interval of say three seconds from the time when its current reaches the
limit, the line will not be opened by a mere momentary overload such as
would blow out a fuse. By setting the time-limit relays of
circuit-breakers in transmission lines to actuate the opening mechanism
after three seconds from the time that an overload comes on, and then
leaving the breakers on distribution lines to operate without a
time-limit, it seems that the opening of breakers on the distribution
lines should free the system from an overload there before the breakers
on the transmission lines have time to act. Such a result is very
desirable in order that the entire service of a transmission system may
not be interrupted every time there is a fault or short circuit on one
of its distribution lines. This plan was followed in the Niagara and
Buffalo system. In the 22,000-volt lines at the Niagara station the time
relays were set to actuate the breakers after three seconds, at the
terminal house in Buffalo, where the transformers step down from 22,000
to 11,000 volts, the circuit-breakers in the 11,000 volt lines to
sub-stations had their relays set to open in one second. Finally the
circuit-breakers in the distribution lines from the several sub-stations
were left to operate without any time limit. By these means it was
expected that a short circuit in one of the distribution circuits from a
sub-station would not cause the connections of the underground cable
between that sub-station and the terminal house to be broken, because of
the instant action of the circuit-breaker at the sub-station.
Furthermore, it was expected that a short circuit in one of the
underground cables between the terminal house and a sub-station would be
disconnected from the transmission line at that house and would not
cause the circuit-breakers at the Niagara station to operate. It is
reported that the foregoing arrangement of circuit-breakers with time
relays failed of its object because the breakers did not clear their
circuits quick enough and that the time-limit attachments on the 22,000
and 11,000 volt lines are no longer in use (A. I. E. E., vol. xviii., p.
500). As the circuits under consideration convey thousands of
horse-power at 11,000 and 22,000 volts it may be that time-limit devices
with circuit-breakers would give good results under less exacting
conditions. Time-limit relays are perhaps an important aid toward
reliable operation of transmission systems, but they are subject to the
objection that no matter how great the overload they will not open the
circuit until the time for which they are set has run. In the case of a
short circuit the time-limit relay may lead to a prolonged drop in
voltage throughout the system, which is very undesirable for the
lighting service and also allows all synchronous apparatus to fall out
of step. With a mere momentary drop in voltage the inertia of the
rotating parts of synchronous apparatus will keep them in step. For
these reasons it is desirable to have circuit-breakers that will act
immediately to open a line on which there is a short circuit or very
great overload, but will open the line only after an interval of one or
more seconds when the overload is not of a very extreme nature. This
action on the part of circuit-breakers at the second Niagara
power-station was obtained by the attachment of a dash-pot to the
tripping plunger of each circuit-breaker (A. I. E. E., vol. xviii., p.
543). With moderate overloads of a very temporary nature this dash-pot
so retards the action of a tripping plunger that the circuit-breaker
does not open. When a short circuit or great overload comes onto a line
the pull on the tripping plunger or the circuit-breaker on that line is
so great that the resistance of the dash-pot to the movement is overcome
at once and the line is disconnected from the remainder of the system.
The fact that a circuit-breaker may be designed to open the line which
it connects, whenever the direction from which the flow of energy takes
place is reversed, is taken advantage of at some sub-stations to guard
against a flow of energy from a sub-station back toward the generating
station. By this means a flow of energy from a sub-station to a short
circuit in one of the lines or cables connecting it with the generating
plant is prevented.