CURRENT AND PRESSURE LIMITING DEVICES
In any electric installation there must be provided a number of
automatic devices to secure proper control. The great multiplicity
of devices designed for this purpose may be divided into two general
classes, as
1. Current limiting;
2. Pressure limiting.
Because of the heating effect of the current which increases in
proportion to the square of the strength of the current, it is
necessary to protect circuits with devices which do not allow the
current to exceed a predetermined value.
Accordingly fuses, circuit breakers, reactances, etc., are used, each
possessing certain characteristics, which render it suitable for
particular conditions of service.
For instance, just as in analogy, steam boilers must be
protected against abnormal pressures by safety valves, electric
circuits must be guarded against excessive voltages by pressure
limiting devices, otherwise much damage would occur, such as
the burning out of incandescent lamps, grounding of cables, etc.
The control of steam is simple as compared to the electric
current, the latter being the more difficult to manage because
of its peculiar behaviour in certain respects, especially in
the case of alternating current which necessitates numerous
devices of more or less delicate construction for safety both
to the apparatus and the operator.
~Fuses.~--A fuse is "an electrical safety valve", or more specifically,
_the actual wire or strip of metal in a cut out, which may be fused by
an excessive current_, that is to say, by a current which exceeds a
predetermined value. A fuse, thus serves to protect a circuit from any
harm resulting from an undue overload.
Fuses have been treated at such length in Guide No. 2, Chapter
XXV, that very little can be said here, without repetition.
[Illustration: FIG. 2,238.--Sectional view of Noark 250 volt, 400
ampere ~enclosed fuse~. The ~fusible element~ _is divided into strips_
A, B, C, and D. This parallel link construction results, upon the
operation of the fuse, in the formation of a number of small arcs, thus
facilitating the absorption of the metal vapor formed when the fuse
blows. ~The fusible strips~, of which there are two or four in number,
according to the ampere capacity of the fuse, are entirely surrounded
by a granular material which is chemically inactive with respect to the
fusible link and whose function is to absorb the metallic vapor formed
upon the blowing of the fuse. ~The contact blades~ T and L are made of
round edge copper, the round edges facilitating the insertion of the
fuses in the circuit terminals. R and S are the end ferrules, attached
to cover E, by the pin M.]
~Ques. What effect have the terminals on a fuse?~
Ans. The current at which a fuse melts may be greatly changed by the
size and shape of the terminals.
If near together and large, they may conduct considerable heat
from the fuse thus increasing the current required to blow the
fuse.
~Ques. What is the objection to large fuses?~
Ans. The discharge of molten metal when the fuse blows is a source of
danger.
~Ques. What should be used in place of large fuses?~
Ans. Circuit breakers.
~Ques. What are the objections to fuses in general?~
Ans. The uncertainty as to the current required to blow them; the
constant expansion and contraction is liable to loosen the terminal
screws when screws are used.
~Ques. What is the advantage of fuses?~
Ans. They form an inexpensive means of protecting small circuits.
[Illustration: FIG. 2,239.--Cross section through plug fuse. With this
type of fuse it is impossible to place any except the correct size of
plug in the socket.]
~Ques. Describe a plug fuse.~
Ans. It is constructed as shown in fig. 2,239, the fuse wire being
visible and stretching between the two metal portions of the plug.
~Ques. What is a cut out fuse?~
Ans. One similar to a simple fuse, but provided with clip contacts as
used for knife switch contacts.
The fuse wire is usually contained in a china or porcelain
tube, which also serves the purpose of a handle for withdrawing
the fuse.
~Ques. What is an expulsion fuse?~
Ans. One in which the fuse is placed in an enclosed chamber with a vent
hole.
In operation, when the fuse blows, the hot air and molten metal
are expelled through the vent.
~Ques. What is a no arc fuse?~
Ans. A cartridge type fuse, in which the space surrounding the fuse
wire is filled with powdered material.
[Illustration: FIG. 2,240.--Inside view of end ferrule of Noark
enclosed fuse. Two prongs O and V, which are a part of the knife blade
K, pass through the square holes in the ends of the ferrule R, and
are riveted to the anchor plate T. ~The object of this plate~ _is to
stiffen the structure and to increase the current carrying capacity
of the metal between the holes, also to permit of proper alignment
of the plates_. In each ferrule is placed a ~vent screen~, composed
of reticulate material, such as cheese cloth. The fuzz between the
threads of the cheese cloth prevents the escape of the granular
material through the vent holes A, but when the fuse operates, allows
free egress of the air, thereby permitting the vapor formed upon the
operation of the fusible element to quickly and freely pass through the
interstices of the filling material and become cooled, eliminating any
possibility of flame issuing from the ends of the tube.]
The object of the powdered material is to assist in
extinguishing the arc formed when the fuse blows.
~Ques. What is a magnetic blow out fuse?~
Ans. An enclosed fuse which is subject to the action of a magnetic
field produced by the current, the magnetic field tending to blow out
the arc when fusing occurs.
~Ques. What is a quick break fuse?~
Ans. One having a weight suspended from its center, or springs attached
to its ends so that the arc formed at fusing is quickly attenuated and
extinguished.
~Ques. What is the disadvantage of a fuse as compared to an oil switch
circuit breaker?~
Ans. When a fuse blows, the arc causes oscillations in the line, which
cause excessive rise of pressure under certain capacity conditions,
whereas this disturbance is reduced to a minimum with an oil switch.
[Illustration: FIG. 2,241.--~Quick break fuse.~ The fuse wire is
connected between the fixed terminal A and the movable arm B, and is
held under tension by the spring which exerts pressure on the movable
arm in a direction tending to separate A and B. ~In operation~, when
the fuse blows, the movable arm quickly moves to the position B´, thus
attenuating the arc and accelerating its extinguishment.]
~Ques. What metal is used for fuse wires?~
Ans. Various metals. Ordinary fuse wire is made of lead or an alloy of
lead and tin.
~Ques. What is the objection to aluminum?~
Ans. It becomes coated with oxide or sulphide, which acts as a tube
tending to retain the metal inside and prevent rupture.
~Ques. What is the objection to copper?~
Ans. Its high fusing point.
~Current Limiting Inductances.~--The great increase in capacity of
power stations, for supplying the demands of densely populated centers
and large manufacturing districts, together with the decrease in the
reactance of modern alternators and transformers due to improvement
in design to obtain better regulation, has presented a problem
in apparatus protection not contemplated in the earlier days of
alternating current distribution. This problem is entirely separate and
distinct from that of eliminating the tendency toward short circuit,
incident to the high voltages now common in transmission lines. It
accepts that all short circuits must occasionally occur and considers
only the protection of the connected apparatus against the mechanical
forces due to the magnetic stresses of such enormous currents.
[Illustration: FIG. 2,242.--~Notched end fuse.~ This is a simple form
of fuse ~consisting of~ _a strip of metal (or wire) fixed between
two end pieces to fit around the terminals_. This type is often
proportioned so that it is only possible to place the correct size of
fuse in the terminals. Sometimes, in place of the end pieces as shown,
the fuse metal is fixed between two clamping screws.]
~Ques. What means are employed to limit the value of a short circuit
current?~
Ans. A current limiting inductance coil (called a _reactance_) is
placed in series with the alternators or transformers.
[Illustration: FIG. 2,243.--General Electric current limiting
reactance; view showing details of construction. The core consists
of a hollow concrete cylinder, alloy anchor plates or sockets being
embedded in the core near the ends to receive the radial brass bolts.
An extension at each end of the core provides for clamping and bracing
the reactance in installation. The supports for the winding are made of
resin treated maple and are located upon the core by radial brass studs
screwed into the alloy sockets, and insulated by mica tubes. The nuts
by which the structure is tightened, rest upon heavy fibre washers.
Wooden barriers fitted and shellacked into the supports add to the
creepage surface between layers of the winding and between the winding
and the core. The supports of the layer next to the core are separated
from the core by strips of treated pressboard. The coil consists of
bare stranded cable in several layers, usually three in number. It is
wound into grooves in the treated wood supports, which are protected
from contact with the cable by heat shields of asbestos shellacked
into the grooves. The winding is usually in the form of two back turn
sections, thereby allowing the terminals of the coil to be brought out
at the ends of the outside layer. This assures accessibility and ease
of connection, and the removal of the leads from proximity to the core.
Two turns at each end of the winding are given extra spacing for the
purpose of additional insulation. The final turn at each end of the
coil is securely held in place by alloy clamps bolted to the supports.
The wood is protected from contact with the clamps by shields of
asbestos. The ends of the cable between the two sections are welded by
the oxyacetylene process.]
~Ques. What are its essential features of construction?~
Ans. It consists of bare stranded cable wound around a concrete core
and held in place by wooden supports as shown in fig. 2,243.
In order to avoid the prohibitive expense of high voltage
insulation, the reactance coil is designed for the low tension
circuit. This requirement prohibits the use of a magnetic core
which, if economically designed for normal operation, would
become saturated at higher densities, or, if designed large
enough to avoid saturation at short circuit conditions, would
become prohibitive in cost and dimensions.
The elimination of all magnetic material from the construction
of the concrete core reactance permits of no saturation, and
assures a straight line voltage characteristic at all current
loads.
[Illustration: FIG. 2,244.--Westinghouse ~magnetic blow out circuit
breaker~, designed for the protection of street railway and electric
locomotive equipments; it serves the combined purpose of fuse block
and canopy switch. ~The contact tips~ _are surrounded by a moulded
arc chute which confines and directs the arc until the magnetic blow
out extinguishes it_. The current carrying contacts consist of copper
strips separated by air spaces. An auxiliary contact or "arcing tip"
at the end of the switch lever takes the burning of the arc when the
breaker opens, and thus confines the burning to a very small piece
which can be easily removed and replaced at small cost. The hand
tripping lever and the resetting lever have insulated handles, so that
they can be safely handled, even in the dark.]
~Ques. Where is the proper location for a current limiting reactance?~
Ans. As near the alternator as possible.
~Ques. Why?~
Ans. To lessen the possibility of a short circuit occurring between the
reactance and the alternator.
~Ques. Beside limiting the current, what other service is performed by
the reactance?~
Ans. It protects the alternator from high frequency surges coming in
from the outside, and limits the current from other machines on the
same bus.
[Illustration: FIG. 2,245.--General Electric magnetic blow out circuit
breaker. This type may be used in air or water tight boxes and is
peculiarly adapted for service where the arc must be confined.]
~Circuit Breakers.~--The importance of circuit protective devices,
commonly called circuit breakers, is fully recognized. The duty
of a circuit breaker is to protect the apparatus in an electrical
circuit from undesirable effects arising from abnormal conditions,
by automatically breaking the circuit. Accordingly a circuit breaker
must comprise a switch in combination with electrical control devices
designed to act under abnormal conditions in the circuit.
A circuit breaker is a _device which_ ~automatically~ _opens the
circuit in event of abnormal conditions, in the circuit_.
[Illustration: FIG. 2,246.--Magnetic blow out circuit breaker. This is
a direct current breaker in which the final break occurs in a magnetic
field. ~It is a principle in electromagnetics~ _that a conductor
carrying a current in a magnetic field will tend to move in a direction
at right angles to the field_. The arc set up on breaking a circuit
constitutes a conductor, and in magnetic blow out circuit breakers, as
generally manufactured, there is an electromagnet, energized by the
current to be broken, which produces a field in the neighborhood of
the arc, with the result that the arc moves outward, and so becomes
attenuated and is finally extinguished. The form shown in the figure is
used on cars equipped with heavy motors. When so used, it is in many
cases mounted in a box with the handle H projecting at one end. A and K
are the terminals of the breaker and B is the tripping coil, which also
serves to set up the magnetic field necessary for blowing out the arc.
X is the armature of coil B and is pulled down against the action of
the spring S whenever the current exceeds that for which the breaker is
set. The tripping current is adjusted by means of nut T. The iron plate
P and a similar one back of it are magnetized by the current in coil
B, and as the break takes place between these two poles, the arc is
promptly extinguished by the field that exists there. ~In operation~,
A and K are the terminals, D D is a contact that is forced up against
F, F when the breaker is set. The current then takes the path A-B-F-D
D-F-K. ~When the breaker trips~, the contact piece D D flies down
and the tendency is for an arc to form between F, F; the magnetic
field blows the arc upwards, and whatever burning takes place is on
the contacts E, E, which are so constructed that they may be readily
renewed. ~To trip the breaker by hand~, the knob N is pressed.]
In the design of circuit breakers, there are several methods used to
effect the rupturing of the arc between contacts when opened on heavy
overload, such as:
1. Magnetic blow out; 2. Thermal break; 3. Carbon break.
In the magnetic blow out type, the arc is extinguished between
auxiliary contacts confined by a chute in which the arc is rapidly
blown out due to a powerful magnetic field from one or more
electromagnets. This type may be used in air or watertight boxes and is
peculiarly adapted for service where the arc must be confined.
[Illustration: FIG. 2,247.--~Thermal overload circuit breaker.~ ~In
construction~ two contact blocks are fixed rigidly to, but insulated
from, the switch arm. They are connected electrically by two parallel
strips of suitable metal, each fitted with a steel catch piece. ~When
the switch is closed~ the strips are sprung apart over a fixed catch,
and the full rated current does not release the catch. Overload causes
the strips to move apart, and the circuit breaker flies off under the
action of a spring.]
In a carbon break type, the arc is finally ruptured between carbon
break contacts. The breaking of the circuit is accomplished
progressively, that is to say, it is done in three stages, by several
sets of contact, known respectively as
1. The main contacts;
2. The intermediate contacts;
3. The carbon contacts.
_In operation_, as the circuit breaker acts to break the circuit, first
the ~main~ _contacts_, separate, then the ~intermediate~ _contacts_,
and finally the ~carbon~ _contacts_ between which the arc is ruptured.
~Ques. What is the object of the intermediate contacts?~
Ans. To prevent the forming of an arc on the main contacts.
[Illustration: FIG. 2,248.--Carbon break discs of Condit circuit
breaker. The two pairs of similar discs which slide past each other
are so arranged that these surfaces coincide at the instant the
intermediate contacts separate after which, as the contact arm opens
further, they gradually disengage.]
~Ques. What is the object of the carbon contacts?~
Ans. First to protect the intermediate contacts by providing a path
for the current after the intermediate contacts separate, and 2, to
"slow down" the current by means of the considerable resistance of the
carbon, thus reducing to a minimum the arc which is formed when the
carbon contacts separate.
~Ques. How is the automatic operation of a circuit breaker usually
accomplished?~
Ans. Usually through the medium of a solenoid, or electromagnet
energized by current from the circuit controlled by the breaker.
[Illustration: FIG. 2,249.--Mechanically connected insulated latches
used on Condit circuit breakers to produce inter-locking tripping.]
The essential features of construction and operation of a
circuit breaker is shown in the elementary diagrams, figs.
2,250 to 2,253. ~In construction~ as shown in fig. 2,250 it
consists essentially of three sets of contacts, a swinging
contact arm which is set in the closed position by the handle
operating through the toggle joint, the movement of which is
limited in the closing direction by the stop. The latter is
made adjustable by an eccentric pin or equivalent. Connected
to the toggle is the plunger of the solenoid whose winding is
energized by current from the circuit which the circuit breaker
is to control.
[Illustration: FIGS. 2,250 to 2,253.--Elementary diagrams illustrating
the operation of a carbon circuit breaker of the overload type, showing
the progressive opening of such device. Fig. 2,250, closed position;
fig. 2,251, main contacts open; fig. 2,252, intermediate contacts open;
fig. 2,253, carbon contacts open, circuit broken.]
~In operation~, the circuit is closed by hand by turning the
handle downward to the position shown in fig. 2,250, that is as
far as it will go.
Since the toggle has passed the center line the arm will be
held normally in this position because of the spring action of
the contacts. Now, if the current rise above a predetermined
limit, the pull exerted by the solenoid will overbalance the
tendency of the toggle to remain in the closed position, and
pull the two toggle links downward below the center line,
drawing the contact arm back and breaking the circuit.
[Illustration: FIG. 2,254.--I-T-E overload circuit breaker. ~In
operation~: the current from one side of the circuit enters the circuit
breaker at A, passing through the laminated bridge B to contact block
C, thence through coil D and terminal E to the motor. The coil D
surrounds a magnetic core, having pole pieces F and G and armature H.
The effect of the current in the coil is to energize the magnet, thus
tending to lift the armature against the force of gravitation. The
volume of current required to trip the circuit breaker is determined
by the position of the armature, which is subject to ready adjustment,
and is indicated on the calibration plate P. From the opposite side
of the line, the current enters at I, passing downward through the
laminated bridge member J, into terminal K, whence it passes out to the
motor. When the current passing through the circuit breaker attains
sufficient volume, the force generated by the magnetic coil overcomes
the weight of the armature H; and the latter is drawn upward toward
the pole pieces with constantly increasing force, until the insulated
projections L and M strike against the respective restraining latches
N and O, thereby releasing the two switch members, which at once open
in response to the force supplied by the spring of the contact members
and auxiliary springs provided for the purpose. Positiveness in opening
is further assured by the blow of the armature, which is added to the
other opening forces; hence, the heavier the overload, the more violent
the blow and the quicker the circuit breaker opens; or the greater the
current the more promptly it is interrupted. This is the ~I-T-E~ or
~I~nverse ~T~ime ~E~lement principle.]
[Illustration: FIG. 2,255.--Condit 600 volt, 1,200 ampere, single pole,
type K, circuit breaker with pull down handle.]
[Illustration: FIG. 2,256.--Condit 600 volt, 6,000 ampere, single pole,
switch board mounting, circuit breaker, with pull down handle.]
[Illustration: FIG. 2,257.--General Electric triple pole, overload,
circuit breaker, with two overload coils, capacity 300 amperes, 480
volts.]
The progressive action which takes place during this operation
is shown in figs. 2,250 to 2,253 in which the main contacts
separate first, then the intermediate, and finally the carbon
contacts as mentioned before.
~Ques. What name is given to this type of circuit breaker?~
Ans. It is called an overload circuit breaker.
[Illustration: FIG. 2,258.--Parts of General Electric 2,000 ampere 650
volt circuit breaker. A, cover for secondary contact bracket; B, spring
washer for Ea.; C, pin for links and G; D, spring for carbon support;
E, plate for F; F, carbon support; G, secondary contact bracket; H,
contact plate; I, screw for H; J, nut for K and W; K, contact stud,
upper; L, laminated brush, complete with support; M, leather buffer for
L; N, main link; O, pin for Na and La left hand and Cb and Na right and
left hand; P, screw for N and magnet frame shaft; Q, washer for N and
magnet frame shaft; R, screw for S and V; S, index plate; T, plate for
Gb; U, screw for T; V, magnet frame; W, contact stud, lower; X, pin for
Cb, Na and V; Y, washer for X and O; Z, calibrating screw with thumb
nut; Aa, armature with contact plate; Ba, catch lever complete with
catch Ca, button handle for Ba; Da, spring cotter for Ea; Ea, pin for
F and Fa; Fa, operating link for G; Ga, pin for D; Ha, carbon holder
with copper and carbon contacts; Ia, flexible connections for G and F;
Ja, screw for G and flexible connection plate; Ka, screw for Na and Ha;
La, copper secondary contact; Ma, screw for La; Na, secondary contact
lever; Oa, cross bar for Na; Pa, screw for L and M; Qa, secondary
toggle link (left hand); Ra, spring cotter for Wa and O; Sa, brush
lever; Ta, buffer for Cb and Sa; Ua, secondary toggle link (right
hand); Va, washer for Wa; Wa, pin for Cb, Qa, Ua and N; Xa, pin for Sa
and Cv; Ya, spring cotter for all pins, except Wa, catch lever pin and
buffer; Za, secondary contact link; Ab, washer for Fb; Bb, guard for
Fb; Cb, handle lever; Db, catch for Cb; Eb, screw for Db; Fb, handle
with stud; Gb, secondary connection.]
~Automatic Features.~--There are three methods of connecting the
winding of the solenoid, or _trip coil_ as it is called:
[Illustration: FIGS. 2,259 to 2,262.--Elementary diagrams illustrating
the various methods of electromagnetic control for circuit breakers.
Fig. 2,259, overload trip; fig. 2,260, underload trip, fig. 2,261, low
voltage trip; fig. 2,262, control from auxiliary circuit by means of a
"relay."]
1. In series with the main circuit;
2. In shunt with the main circuit;
3. In shunt with an auxiliary circuit.
[Illustration: FIG. 2,263.--Diagram of General Electric low voltage
trip with tripping switch _normally_ ~open~.]
The automatic controls arising from these connections give various
kinds of protection to the circuit and are known as
1. Overload trip;
2. Underload trip;
3. Low voltage trip;
4. Auxiliary circuit trip.
[Illustration: FIG. 2,264.--Diagram of General Electric low voltage
trip, with tripping switch _normally_ ~closed~.]
~Ques. What is the object of the overload trip?~
Ans. It is intended to open the circuit when the current exceeds a
predetermined value.
~Ques. What modifications are made in the mechanism shown in the
elementary diagrams?~
Ans. Sometimes a latch is used in place of the toggle and a magnet in
place of the solenoid as in figs. 2,265 and 2,266.
~Ques. Why is a magnet used in combination with a latch?~
Ans. Because with this arrangement very little movement is required to
trip the breaker, and for such conditions, a magnet is more efficient
than a solenoid.
[Illustration: FIGS. 2,265 and 2,266.--Circuit breaker with automatic
control mechanism consisting of ~magnet and latch~; views showing
breaker in open and closed positions, and essential features. The
toggle is used to obtain sufficient leverage to easily close switch
against the pressure of the brush contacts but not to lock switch,
this being done by the latch as shown, the latter closing by the
action of a spring, there being a roller R at the end which engages
the arm to reduce friction. ~In operation~, when the current exceeds
a predetermined limit the magnet attracts the latch and releases the
contact arm. The brush contacts which are exerting pressure against the
contact arm, rapidly push it away, and assisted by gravity, the arm
flies open to the position shown in fig. 2,266.]
~Ques. How does the latch arrangement work?~
Ans. When the proper current is reached, the magnet pulls open the
latch and the contact arm of the breaker moves by the force of gravity
or other means and opens the circuit.
~Ques. How does the underload trip operate?~
Ans. The same as the overload type except that they operate on a
diminution of current instead of an excess.
[Illustration: FIGS. 2,267 and 2,268.--Positions in circuit of
_current_ and _pressure_ coils of circuit breakers.]
~Ques. Describe the no voltage trip.~
Ans. The energy for the trip of this breaker is derived from a high
resistance or fine wire coil which is arranged to be placed directly
across the line, in operation, when the current flowing through the
circuit falls below a predetermined value, the energy of the coil is
insufficient to counteract the force of a spring, which then trips the
breaker.
[Illustration: FIG. 2,269.--Diagram of General Electric shunt trip
with coil connected beyond breaker and thrown out of circuit after
tripping.]
~Ques. Describe the auxiliary circuit trip.~
Ans. A pressure coil is used which is energized by current from an
auxiliary circuit. The coil is only _momentarily_ energized, by push
button, relay or other control, as distinguished from the preceding
types, in which the coil is _constantly_ energized.
[Illustration: FIG. 2,270.--Diagram of General Electric shunt trip with
auxiliary circuit opening switch to throw coil out of circuit after
tripping.]
[Illustration: FIG. 2,271.--General Electric shunt trip attachment.
The shunt trip attachment has been designed to provide for conditions
under which the low voltage attachment cannot be successfully applied.
It resembles the low voltage attachment in construction, but differs in
that it trips the circuit breaker when energized. The shunt trip should
be allowed to remain only momentarily in circuit; hence it should
be so connected that the opening of the circuit breaker immediately
disconnects it from the circuit. Whenever it is impossible to connect
the shunt trip in this manner, the circuit opening auxiliary switch
should be used in connection with it.]
[Illustration: FIG. 2,272.--General Electric low voltage attachment
for circuit breakers. This low voltage trip is designed to operate the
circuit breaker when the line voltage drops to approximately 50 per
cent or less of the normal voltage. It should be noted that the coil
is always in circuit, as is the case with the overload and underload
coils, and that it operates with the _releasing_ of its armature. It is
always necessary to use a fixed amount of resistance (depending upon
the voltage of the system) in series with the low voltage release.
The low voltage release performs the functions of a shunt trip coil
when used in conjunction with a push button, auxiliary switch or
speed limiting device, and is generally preferred to the shunt trip
attachment.]
[Illustration: FIG. 2,273.--General Electric ~circuit opening~
_auxiliary switch_. This switch opens an auxiliary circuit when the
circuit breaker opens, and is intended to be used in connection with
a shunt trip attachment to insure the immediate disconnection of the
shunt coil from the circuit. It may also be employed to serve other
purposes, such as tripping another circuit breaker having a low voltage
attachment, and permitting another circuit breaker to remain closed
only when the circuit breaker equipped with the auxiliary switch is
open.]
~Ques. What other name is given to the auxiliary circuit trip?~
Ans. It is sometimes called the shunt trip, though ill advisedly so.
[Illustration: FIG. 2,274.--General Electric ~circuit closing~
_auxiliary switch_. This switch closes when the circuit breaker opens,
and may be used to announce the automatic opening of the circuit
breaker through the means of an indicating lamp or an alarm bell. It
is often necessary to arrange one circuit breaker so that, in opening,
it will trip others. This may be accomplished by using a circuit
closing auxiliary switch in connection with a low voltage or shunt trip
attachment on the circuit breakers to be tripped. The construction of
this type of switch is such that it may be opened by hand after the
circuit breaker opens, but it is automatically reset when the circuit
breaker is closed.]
~Relays.~--Oil break switches and carbon break circuit breakers are
commonly used to open electrical circuits at some given overload and
on short circuit. To secure additional protection under a variety of
abnormal conditions or to provide for a certain predetermined operation
or sequence of operations, relays may be employed.
[Illustration: FIG. 2,275.--General Electric type C circuit breaker.
Specially adapted to motor driven machine tool applications. For use in
mills, machine shops, factories, foundries and office buildings. For
general motor work, automobile charging outfits, storage batteries,
rectifier sets, cranes, etc. List of parts: A, calibrating post; B,
laminated contact; C, secondary contact spring; D, contact blade; E,
cotter pin for G; F, toggle link; G, pin for D and F; H, stop for Aa;
I, hinge frame; J, operating lever; K, pin for I and J; L, toggle
link; M, connection; N, screw for M, O and P; O, nut for N and P;
P, terminal; Q, tripping coil; R, calibrating screw; S, laminated
contact; T, calibrating scale; U, calibrating spring; V, connection
post; W, knob; X, washer for Y; Y, handle; Z, buffer; Aa, armature; Ba,
laminated connection; Ca, connection; Da, base.]
A relay is defined as: _A device which_ ~opens~ _or_ ~closes~ _an_
~auxiliary circuit~ _under predetermined electrical conditions in the
main circuit_.
The object of a relay is generally to act as a sort of electrical
multiplier, that is to say, _it enables a comparatively weak current to
bring into operation a much stronger current_.
[Illustration: FIG. 2,276.--Diagram of connections of General Electric
shunt trip coil ~with~ and ~without~ circuit opening auxiliary switch.]
~Ques. For what service are relays largely used?~
Ans. They are employed in connection with high voltage switches
where the small amount of energy derived from an ordinary instrument
transformer is insufficient for tripping.
The connections between relays and circuit opening devices are
usually electrical. Combinations of this nature are extremely
flexible since they permit the use of a number of devices, each
having a different function, with a single circuit breaker or
oil switch as well as with two or more switches, to secure the
desired operation and protection.
~Selection.~--In all electrical installations protection of apparatus
is important, but in some large central stations this is secondary to
continuity of service.
To combine maximum protection without interruptions of service is
not always possible, but these requirements can be approximated very
closely by the use of reliable and simple controlling or protecting
devices if proper care be taken to select the relays suited to the
special conditions of the installation. To do this intelligently, a
knowledge of the various types of relay is necessary.
[Illustration: FIG. 3,073.--Diagram of connections of General Electric
low voltage release coil when used with speed limiting device on rotary
converter.]
There is a multiplicity of types and a classification to be
comprehensive, should, as in numerous other cases, be made from several
points of view. Accordingly relays may be classified:
1. With respect to the nature of the service performed, as
_a._ Protective;
_b._ Regulative;
_c._ Communicative.
2. With respect to the operating current, as
_a._ Alternating current;
_b._ Direct current.
3. With respect to the manner of performing their function, as
_a._ Circuit opening;
_b._ Circuit closing.
4. With respect to the operating current circuit, as
_a._ Primary;
_b._ Secondary.
5. With respect to the abnormal conditions which caused them to
operate, as
_a._ Overload;
_b._ Underload;
_c._ Over voltage;
_d._ Low voltage;
_e._ Reverse energy;
_f._ Reverse phase.
6. With respect to the time consumed in performing their function, as
_a._ Instantaneous (so called);
_b._ Definite time limit;
_c._ Inverse time limit.
7. With respect to the character of its action, as
_a._ Selective;
_b._ Differential.
8. With respect to whether it acts directly or indirectly on the
circuit breaker, as
_a._ Main;
_b._ Auxiliary.
[Illustration: FIG. 2,278.--General Electric overload and low voltage
type C circuit breaker for 600 volts or less. It has one overload, and
one low voltage coil as shown. Screens are provided between contacts.]
~Protective Relays.~--These are used to protect circuits from abnormal
conditions of voltage, or current, which would be undesirable or
dangerous to the circuit and apparatus contained therein.
~Ques. How do protective relays operate?~
Ans. They act in combination with automatic circuit breakers, operating
when their predetermined setting has been reached, energizing the trip
coil of the circuit breaker and opening the circuit.
Fig. 2,279 shows the principles of relay operation. When
the current or pressure in the main circuit reaches the
predetermined value at which the protective system should
operate, the relay magnet attracts the pivoted contact arm and
closes the auxiliary circuit; this permits current to flow from
the current source in that circuit and energize the trip coil
thus opening the main circuit.
[Illustration: FIG. 2,279.--Diagram illustrating the operation of a
~circuit closing relay~. When the predetermined abnormal condition is
reached in the main circuit, the relay closes the auxiliary circuit,
thus energizing the trip coil and opening the breaker.]
~Regulative Relays.~--This class of relay is used to control the
condition of a main circuit through control devices operated by a
secondary circuit.
~Ques. For what service are relays of this class employed?~
Ans. They are used as feeder circuit or generator regulators.
~Ques. How do they differ from protective relays?~
Ans. They have differentially arranged contacts, that is to say,
arranged for contact on either side of a central or normal position.
[Illustration: FIG. 2,280.--Diagram showing a railway synchronous
converter protected by a single pole overload circuit breaker with
low voltage release attachment and bell alarm switch. The low voltage
attachment trips the breaker on failure of direct current voltage also
when speed limit device closes. Internal troubles are taken care of by
the alternating current automatic devices (not shown).]
~Communicative Relays.~--These are used for signalling in a great
variety of ways for indicating the position of switching apparatus or
predetermining the condition of electric circuits.
~A. C. and D. C. Relays.~--As here used, the classification refers to
the kind of current used on the auxiliary circuit. In some cases direct
current is used to energize the trip gear of the circuit breaker or oil
switch, and in others, alternating current.
[Illustration: FIG. 2,281.--Diagram showing three phase motors
protected by triple pole overload circuit breakers, with two overload
coils, also one overload coil and low voltage release coil. The use
of the low voltage release allows the breaker to be tripped from a
distance by means of a short circuiting switch or push button.]
A. C. and D. C. relays are respectively known as _circuit opening_ and
_circuit closing_ relays, being later fully described.
~Circuit Opening Relays.~--The duty of a circuit opening relay is
_to open the_ ~auxiliary circuit~, _usually alternating current, nd
thereby cause the oil switch or circuit breaker to be opened by the use
of a trip coil in the secondary of a current transformer, or by low
voltage release coil_.
The trip coil of the breaker is generally shunted by the relay
contacts and when the moving contact of the relay disengages
from the stationary contact, the current from the transformer
which supplies the relay, flows through the trip coil thus
opening the breaker. These features of operation are shown in
fig. 2,282.
[Illustration: FIG. 2,282.--Diagram illustrating the operation of a
~circuit opening relay~. When the relay contacts are in the normal
closed position, as shown, the coil is short circuited. When the
predetermined abnormal condition is reached in the main circuit, the
relay contacts are opened with a quick break, sending the current
through the trip coil momentarily, and opening the breaker.]
~Ques. Where are circuit opening relays chiefly employed?~
Ans. In places where direct current is not available for energizing the
trip coil.
~Ques. What is the objection to alternating current trip coils?~
Ans. They have relatively high impedance and impose a heavy volt ampere
load on the transformers.
~Circuit Closing Relays.~--The duty of a circuit closing relay is to
close the auxiliary circuit at the time when the predetermined abnormal
condition is reached in the primary circuit. The closing of the
auxiliary circuit energizes the trip coil and opens the breaker.
[Illustration: FIGS. 2,283 to 2,291.--General Electric instantaneous
overload ~circuit opening~ relays, covers removed. Circuit opening
relays are used chiefly in those cases where direct current for the
tripping circuit is not available. Alternating current trip coils
have relatively high impedance and impose a heavy volt ampere load on
the current transformers. To reduce this load during normal operation
the circuit opening relay is frequently used and is usually necessary
where instruments and meters are to be operated on the same current
transformers as the trip coils if the greatest accuracy be required.
The relay contacts in the normal, closed position, short circuits
the trip coil. When the relay operates on overload or other abnormal
condition the contacts are opened with a quick break, sending the
current through the trip coil circuit momentarily and tripping the
switch. With circuit opening relays, the trip coils of the oil switch
must be set to trip somewhat lower than the setting of the relay. ~In
construction~ the relay consists of a solenoid with iron frame forming
the support for the relay; a central plunger or armature of special
construction which is picked up or released by the magnetic action of
the solenoid; a plunger rod which actuates the relay contacts, which
are mounted on an insulated base usually above the solenoid; a tube or
plate for the calibration marking and adjustment; covers of glass or
metal to keep out dust; terminal boards with points corresponding to
tagged leads from relay coils and external wiring diagrams. The relay
contacts are of two kinds, circuit opening, as shown above, and circuit
closing, as shown in figs. 2,292 to 2,300.]
~Ques. What kind of current is generally used for the auxiliary circuit
of a circuit closing relay?~
Ans. Direct current.
~Ques. At what pressure?~
Ans. From 125 to 250 volts.
~Ques. Where is this current usually obtained?~
Ans. From a storage battery, or from the exciter.
~Ques. For what current are the contacts ordinarily designed?~
Ans. About 10 amperes.
[Illustration: FIGS. 2,292 to 2,300.--General Electric alternating
current, instantaneous overload ~circuit closing~ relays, covers
removed. The function of a circuit closing relay is to close an
electrical circuit, usually direct current, through a trip coil on an
oil switch or circuit breaker, or it may short circuit a low voltage
release coil, and thereby open the oil switch or circuit breaker
on occurrence of the condition upon which the relay is designed to
operate. Direct current at 125 or 250 volts taken from exciter bus
bars or storage battery system is generally used for the tripping
circuit. Circuit closing contacts have a cone shaped central element
of carbon or metal which makes contact with flexible contact fingers
symmetrically arranged above the cone. These contacts will make and
break a circuit of 10 amperes at 125 volts without the use of auxiliary
circuit opening switches. Relays are made with two or three contacts
for connecting one side of a direct current circuit through one or
two separate circuits, or trip coils respectively, to the side of
opposite polarity. Usually only two contacts are required. Where two
or more trip coils are used, which may not be connected permanently in
parallel, the three contact relays are selected and in some cases four
contacts furnished.]
~Primary and Secondary Relays.~--Primary relays are sometimes called
series relays as they have the current coils connected directly in
series with the line, both on high and low tension circuits.
Secondary relays receive their current supply from the secondary
circuits of current transformers. Alternating current relays connected
to secondary of pressure transformers and relays with both current and
pressure windings are included in this class.
~Ques. What is the usual winding of the coils?~
Ans. The current coils are usually wound for 5 amperes and the pressure
coils for 110 volts.
[Illustration: FIG. 2,301.--Alternating current low voltage circuit
closing low voltage relay, for 600 volts or less. The contacts are
similar to those of the circuit closing overload type except that they
are inverted. As long as the pressure is normal the contact cone is
held above the contacts. When the pressure falls below one half normal,
the cone and plunger rod drop and close the contact. This relay does
not pick up its own plunger. The plunger rod is pushed up by hand after
the pressure circuit is established. Low voltage relays are generally
used in connection with a low voltage release or shunt trip coil on
an oil switch or a circuit breaker. They are used in connection with
motor booster sets to prevent a disastrous speed of the booster which
might result from the loss of alternating current power. They are also
sometimes used for indicating purposes.]
~Ques. What refinement is made in the design of relays and why?~
Ans. Care is exercised to reduce to a minimum the volt ampere load
imposed by the relay on the current transformer to permit the use of
un-stranded meters and relays upon the same transformer.
The use of circuit opening relays to cut out the trip coil of
an oil switch during normal operation, has been described,
and in the short time that the trip coil is in circuit, it
does not affect the accuracy of the instrument readings. This
practice, however, does not apply in the case of curve drawing
meters, voltage compensators or other devices which have in
themselves sufficient load for separate current transformers.
In this connection it should be noted that to obtain accurate
instrument and meter readings; the current transformers should
not be loaded beyond certain limits which depend upon the volt
ampere load and power factor of each of the connected devices.
[Illustration: FIG. 2,302.--Condit type K circuit breaker with shunt
trip and no voltage attachment. ~The shunt trip~ is usually applied
as an auxiliary to other types of trip. It consists of a fine wire
coil which is mounted as a self-contained part of the breaker and
which when energized, trips the circuit breaker. It is used to open
the breaker from some distant point, and the coil is arranged to be
connected across the line. The coils are so arranged that the circuit
breakers will operate on a voltage 25% above or 25% below normal. The
shunt trip coil is not intended to remain across the line and should
be only momentarily energized. ~The no voltage trip~, receives energy
from a high resistance or fine wire coil which is arranged to be placed
directly across the line, but in contradistinction to the shunt trip
type, in which the coil is momentarily energized to trip the breaker,
the no voltage coil is _constantly_ energized and a _decrease_ or
failure of pressure trips the breaker. It can be used as a remote
control device the same as the shunt trip. Its general use, however, is
to cause the circuit breaker to open when the voltage of the line fails
from any cause. Its use is recommended on all motor circuits, as it
affords an additional protection against accidents, for if the voltage
should fail, the breaker immediately opens, and before the machine can
start again the attendant must close the breaker. It will not work for
the protection of storage batteries or of motor generator sets charging
storage batteries, as, when the voltage of the generator fails, the
voltage of the battery still maintains its full value. The action of
the coil is independent of the direction of flow of current; it simply
allows the breaker to stay closed as long as the voltage is on the line
and opens the breaker when the voltage on the line ceases. No voltage
circuit breakers are normally so adjusted that they will not release
until the voltage approaches 50% of normal.]
So great is the variety of combination used and the variations of these
factors in their several combinations at different loads and settings,
that special consideration of each arrangement is advisable.
[Illustration: FIG. 2,303.--General Electric alternating current high
pressure series overload relays controlling 45,000 volt oil switches.
These relays are connected in series with the line. If current
transformers are to be used on the same circuit for other purposes, and
have sufficient capacity to supply energy for operating relay coils,
then secondary relays would be more economical, otherwise the series
relays are much less expensive. By means of a specially treated wooden
rod, the relay operates a tripping switch, closing a separate tripping
circuit, usually 125 or 250 volts direct current. Relays and switches
are for mounting on flat surfaces. Series relays are essentially the
same as secondary relays except in the coil winding and insulation.
The corrugated horizontal arms which carry the relays, as shown, are
insulated posts, insulating the relays from the ground. The wood rod
from each relay is connected directly to a tripping shaft on the oil
switch which buckles an auxiliary toggle, thereby opening the main
toggle and tripping the oil switch.]
~Overload Relays.~--Series relays are connected directly in series with
the line and are chiefly used with high pressure oil break switches
for overload protection. If current transformers are to be used on the
same circuits for other purposes, and have sufficient capacity to admit
of adding a relay coil, secondary relays would be more economical;
otherwise, the series relays are less expensive.
By means of a specially treated wooden rod, the relay operates a
tripping switch, closing a separate tripping circuit, usually 125 or
250 volts direct current. Series relays are essentially the same as
secondary relays except in the coil winding and insulation.
~Underload Relays.~--These are similar in construction to low voltage
relays but have current instead of pressure windings.
~Over Voltage Relays.~--These are usually of the circuit closing type
and are similar to secondary overload relays, but have pressure instead
of current windings.
~Low Voltage Relays.~--Relays of this class are in most cases used
[Illustration: FIG. 2,304.--Condit 600 volt, 1,500 ampere single pole
back connected type K circuit breaker, ~motor operated~. The mechanical
and electrical features of the circuit breakers are no different than
when hand operated, the only difference being that the motor is used
for the operating means. This motor is so arranged that even should it
over travel, due to an accident to the controlling circuit, it cannot
produce more than a predetermined strain on the circuit breaker. In
other words, after the motor has closed the circuit breaker, further
travel of the motor will not result in putting a strain on the
operating parts. Suitable motors are supplied for this service, the
type of motor varying in accordance with the character of the operating
current supplied. The advantage of this type of electrical operation
is that it puts very little strain on the switch mechanism, takes very
little operating current, allows the use of standard parts, and makes
an extremely substantial and flexible structure. Its disadvantage is
that it closes slowly, and it must not, therefore, be used in places
where quick closing is essential.] for the protection of motors in the
event of a temporary weakening or failure of the pressure. They are
also used in connection with a low voltage release or shunt trip coil
on an oil switch or a circuit breaker.
~Reverse Energy Relays.~--The chief object of this species of relay is
to protect the generator. When so used, the overload adjustment is set
at the maximum value to give overload protection only at the maximum
carrying capacity of the generator and a sensitive reverse protection
to prevent a return of energy from the line.
[Illustration: FIG. 2,305.--General Electric direct current solenoid
control relay. Solenoids for operating large switches, etc., frequently
require comparatively large operating currents in the "closing" coils.
This necessitates the use of relatively heavy leads between the control
switch and the solenoid and is the cause of severe arcing at the
control switch, especially with solenoids of high inductance. These
objectionable features can best be eliminated by the use of a suitable
control relay located near the solenoids. The control relay consists
of a solenoid plunger and switch, the latter insulated from the frame
of the relay. It operates satisfactorily on one-half the rated voltage
and requires only a very small operating current. The terminals of the
switch and the relay coils are independent. The relay can be wound for
operation on 125, 250, or 600 volt circuits.]
~Reverse Phase Relays.~--This type of relay is used chiefly to prevent
damage in case of reversal of leads in reconnecting wiring to two or
three phase motors.
~Time Element.~--It is often inconvenient that a circuit breaker should
be opened immediately on the occurrence of what may prove to be merely
a momentary overload, so that time lag attachments are frequently
provided, particularly with relays. These devices, which may form part
of the relay or may be quite distinct from it, retard its action until
the overload has lasted for a predetermined time--several seconds or
more.
[Illustration: FIG. 2,306.--Alternating current series reverse phase
single pole, circuit closing, two contact relay for 600 volts or less.
This type of relay is used chiefly to open motor circuits for elevators
to prevent damage in case of reversal of leads in reconnecting wiring
to two or three phase motors. The relay is provided with a dust proof
metal cover.]
~Ques. What should preferably govern the time lag?~
Ans. It should depend on the extent to which the overload is reduced as
the time elapses.
~Instantaneous Relays.~--The so called instantaneous relays operate
almost instantly on the occurrence of the abnormal condition that they
are to control.
There is of course a slight time element comparable with that
of an overload circuit breaker, but for practical purposes, the
operation may be considered as instantaneous.
[Illustration: FIG. 2,307.--Electric circuits of Condit type "A" relay.
The construction is described in fig. 2,309. As here shown, the relay
is not in operation, but should the current passing through the coil
be of sufficient value to cause the lower movable half of the magnetic
circuit to approach the upper stationary half of the circuit, _the
relay will be transformed from an ordinary_ ~electromagnet~ _into a_
~repulsion motor~. The contact will short circuit the brushes of the
armature and thus cause it to revolve, the speed of rotation being
dependent on the amount of current flowing to a predetermined point,
and thereafter the speed of rotation of the motor remains constant
irrespective of the current value. ~Time adjustment:~ This is obtained
by varying the distance through which the contact travels, provision
being made whereby adjustment can be made as close as .1 of a second.
Current adjustment: This is obtained by means of a calibrated spring.
Standard relays are calibrated at 6, 8, 10, and 12 amperes, the coils
being designed to carry five amperes continuously, with a temperature
rise not exceeding 86° Fahr. ~Power to operate relay:~ The relay
requires twenty volt amperes for its operation at full load; the
influence of this type of relay on the ratio and phase angle of current
transformers is small.]
~Time Limit Relays.~--Under this classification there are two
sub-divisions.
1. Definite time limit;
2. Inverse time limit.
[Illustration: FIG. 2,308.--Characteristic curves of Condit type A
selective relay. Curves 1, 2, 3, and 4 show the time variation of
this relay with different settings at the various current values. The
relay may be adjusted to trip the switch at any point represented
between curves 1 and 4. This relay is a combination of an _inverse time
limit_ relay and a _definite time limit_ relay. The combination of the
characteristics of the two types are seen in the curve, the first part
of which is ~inverse~, and the latter part ~definite~ from a point of
three or four times full load current. This combination of features
being desirable as, for instance, in transmission work, particularly
where it is necessary to use circuit breakers set selectively, as, due
to the inverse feature of the curve, the relays can be set so that on
a moderate overload, they will require the proper length of time to
operate, and at the same time will operate quickly enough on heavy
short circuits to prevent damage to the distribution system or its
apparatus. Due to the definite feature of the latter part of the curve,
the relays of the varying circuit breakers when once set to operate at
different time values will never operate simultaneously irrespective of
the value of the short circuit current, thus tending toward continuity
of service.]
~Ques. Describe the time mechanism of a definite time limit relay.~
Ans. It consists of an air dash pot, and an air diaphragm or equivalent
retarding device connected to the contact mechanism.
~Ques. How does it operate?~
Ans. In some designs, when the contacts are released, they descend
by gravity against the action of the retarding device thereby making
contact a definite interval after the occurrence of the abnormal
condition.
[Illustration: FIG. 2,309.--Condit type "A" ~selective relay~, designed
for use with circuit breakers where selective or discriminating
action is required. The circuits and connections of this relay are
illustrated in fig. 2,307, and its characteristics in fig. 2,308. ~In
construction~, the relay consists of _a special motor with a short
circuited armature and a split field_. ~Under normal conditions~, the
fields are separated from each other and the motor armature does not
revolve. The force tending to pull the two faces of the field together
is opposed by a spring, the compression of which determines the number
of amperes necessary to cause the relay to begin operation. The motor
structure performs the whole work and the motor itself un-meshes and
meshes the gears without the aid of any external device.]
~Ques. How does the inverse time limit type operate?~
Ans. The actuating and contact mechanism is attached directly to an
air bellows and in operation tends to compress the bellows against the
action of a specially constructed escape valve in the latter.
[Illustration: FIG. 2,310.--Condit type "B" ~time limit attachment~,
designed to give sufficient time to allow an induction motor to start
without opening the circuit breaker, and not have the circuit breaker
trip on the momentary rush of current. ~Its action is inverse~; that
is, _the greater the current the less time it takes to operate_ and
is so arranged that four to five times full load current or a short
circuit will trip the circuit breaker instantly. The time limit
attachment is applied directly to the armature which trips the circuit
breaker and is adapted for the so called primary trip. It consists
of an air vacuum dash pot with a graphite piston, the dash pot being
fastened to the stationary calibrating ring of the trip coil and the
moving outside cylinder is fastened to the armature of the circuit
breaker. When the current reaches a point where it overcomes the weight
of the armature and lifts the same, the magnetic force tending to raise
the armature is opposed by the vacuum created in the interior of the
cylinder. As the magnetic force continues the vacuum is overcome due to
the leakage of air past the plunger and the armature gradually moves up
until it reaches the point where it trips the circuit breaker. If at
any point of the armature travel, the current drop back to normal, the
armature immediately resets itself by means of a ball valve in the top
of the brass cylinder.]
~Ques. Why is the arrangement called _inverse_ time limit?~
Ans. Because the retardation varies inversely with the pressure on the
bellows, and therefore inversely with the magnitude of the abnormal
condition.
~Ques. What other device may be used to retard the operation?~
Ans. A damping magnet is sometimes used which acts on a disc or drum
and which may be adjustable.
[Illustration: FIGS. 2,311 and 2,312.--General Electric alternating
current ~low pressure~ series overload relays. Fig. 2,311,
instantaneous time limit relay; fig. 2,312, inverse time limit relay.
These relays have carbon contacts and will make or break a direct
current circuit of 10 amperes at 125 volts without auxiliary circuit
opening switch. They are used where several circuits are controlled
by one automatic oil break switch or one shunt trip, overload and
shunt trip or low voltage release carbon break circuit breaker. These
relays may be used for signal purposes; they are back connected, the
connections can be seen in the illustrations.]
~Ques. How is the inverse time element introduced by this arrangement?~
Ans. The retardation is due to eddy currents induced by moving the disc
or drum through the magnetic field. The reaction thus induced varies
inversely with the magnitude of the force with which the disc or
drum is urged through the field and hence inversely with the abnormal
condition.
~Ques. What are the ordinary limits of adjustment for inverse time
limit relays?~
Ans. From one-half second to 30 seconds, depending upon the time
setting and magnitude of the overload current.
[Illustration: FIGS. 2,313 to 2,321.--General Electric time limit
overload circuit opening relays with covers removed. The construction
of this relay is similar to that of the inverse time limit relay,
except that it has a compression spring interposed between the plunger
and diaphragm. The plunger compresses the spring and further motion
is prevented by a stop, making the relay practically independent of
the amount of the overload, only the stored energy of the spring, if
the overload continue, applies power, dependent on its own mechanical
strength, to the diaphragm. The time limit therefore becomes
practically a constant for any given setting under ordinary conditions
of overload or short circuit. If, however, the overload come on slowly
so that the spring is not fully compressed at once, the time limit will
vary slightly. If the scheme of selective operation make it necessary
to take care of a creeping load of this character, two relays may
be used and definite time limit positively secured. In this case,
an instantaneous circuit closing, overload relay would be used and
a definite time limit relay, provided with a direct current coil in
circuit with the closing contacts of the first relay. The time limit
relay would be of the circuit closing type and control a direct current
trip coil on the oil switch.]
A setting of from two to six seconds is ordinarily used,
depending upon the requirements. Where selective operation is
desired a minimum setting of two seconds is recommended.
~Differential Relays.~--In this type of relay there are two
electromagnets. In normal working these oppose and neutralize each
other. Should, however, either winding become stronger or weaker than
the other, the balance is upset, the magnet energized, and the relay
comes into operation.
[Illustration: FIG. 2,322.--Differential relay transformer and reverse
current circuit breaker discriminating device. A differential relay is
_one whose electromagnet has two windings_. ~In normal working~ _these
oppose and neutralize one another_. Should however, either winding
become stronger or weaker than the other, the balance is upset, the
magnet is energized, and the relay comes into operation. A modification
of such a relay for alternating current is here shown, from which it
will be seen that when the currents are as indicated, the circuit A has
the larger pressure induced in it, whereas, should the main current
reverse with reference to the shunt current, the circuit B would have
the larger induced pressure.]
A modification of such a relay for alternating current is shown in
fig. 2,322, from which it will be seen that when the currents are
as indicated, the circuit A has the larger pressure induced in it,
whereas, should the main current reverse with reference to the shunt
current, the circuit B would have the larger induced pressure.
[Illustration: FIG. 2,323.--Diagram of modern power house wiring and
busses showing location of relays.]
~[1]How to Select Relays.~--The following general information on
relays, together with reference to the one line diagram, fig. 2,323,
will be of interest and assistance in making a selection from the
various relays previously described to meet the requirements of modern
power house and sub-station layouts.
[1] NOTE.--As suggested by the General Electric Co.
~Single pole relays~ are used on single phase and on balanced
three phase circuits.
~Double pole relays~ are used on ungrounded three phase and on
quarter phase.
[Illustration: FIG. 2,324 to 2,329.--General Electric inverse time
limit overload circuit closing relays. In this type of relay its
mechanism is so designed that a delay or lapse of time in opening the
circuit breaker after a predetermined condition of the circuit has
been reached, depends on the flow of current, that is, if the current
be great, the time will be small, and if the current be of a moderate
value, the time will be correspondingly longer.]
~Triple pole relays~ are used on three phase grounded neutral
and interconnected quarter-phase.
~Circuit closing relays~ are recommended in all cases where a
constant source of direct current is available for operating
trip coils.
The conditions for which relays have been designed for power
circuits may perhaps be best described, by considering a
one line diagram from the generator end to the sub-station
auxiliary machines and feeders.
Considering first alternating current circuits, the
prevailing practice is to make the circuit breakers by
which the alternators are connected to the low tension bus
_non-automatic_, in order to insure minimum interruption of
alternator service. The chance of trouble in this part of the
circuit is remote, but should it occur, the station attendant
could generally open the circuit breaker before the machines
would be injured.
~Reverse current relays~ of instantaneous or time limit types
are often connected to the secondaries of current and of
pressure transformers to indicate by lamp or bell any trouble
that may occur in the generator circuit.
These relays operate with a low current reversal at full
pressure and conversely with a proportionally greater current
at voltages less than normal. At zero pressure, the relay would
act as an overload one, set for high overload. At zero current,
a voltage considerably in excess of normal would be required to
operate it.
[Illustration: FIG. 2,330.--Diagram showing two phase motor or feeder
circuit protected by double pole double coil, overload circuit breaker
(or two single pole breakers interlocked) with bell alarm switch.]
Specifications sometimes call for automatic generator circuit breakers:
in this case _definite time limit overload relays_ are used. They are
connected in the secondaries of current transformers and are designed
to give the same time delay for all trouble conditions; they allow the
defective circuit to be opened, if possible, at a point more remote
from the generator than the generator circuit breaker.
When the total generator capacity exceeds the rated rupturing capacity
of the circuit breakers, one or more sectionalizing circuit breakers
are placed in each bus.
If operating conditions admit, these devices are made
non-automatic and are left disconnected except in case of
emergency; but if it be necessary for them to be continually in
service, they may be made automatic by _means of instantaneous
overload relays_ connected to current transformers in the low
voltage bus; the relays being adjusted to trip the circuit
breaker under short circuit conditions, confining the trouble
to one section and preventing the circuit breakers rupturing
more than their rated capacity.
Installations with but _one bank of power transformers_, and without
high voltage bus, are provided with automatic circuit breakers operated
by an _inverse time limit relay_.
The relay is connected to the secondaries of current
transformers, which in turn are connected in the low voltage
side of the power transformer.
Stations with _more than one bank of power transformers_, a
high voltage bus, and high and low voltage circuit breakers,
may have both circuit breakers arranged to trip at the same
time or one after the other. As in the former case, they are
operated from the inverse time limit relay connected in the low
voltage side.
[Illustration: FIG. 2,331.--Condit 600 volt, 1,500 ampere, single pole
type K circuit breaker ~pneumatically operated~. It is the same as the
electrically operated circuit breaker, except that a pneumatic cylinder
mechanism is supplied in place of either the electromagnet or the
motor. This cylinder mechanism is so arranged that the air pressure is
only on the cylinder at the instant of operation. At all other times
the air pressure is shut off by means of a control valve. The kind of
remote control to be used depends on local conditions. In general, the
hand operated remote control device is preferable where conditions are
such that it can be used, and where it is necessary to use electrically
operated, the motor operated type is recommended if conditions be such
that slow closing is not objectionable.]
[Illustration: FIGS. 2,332 and 2,333.--Diagram showing two phase
four wire no voltage connections for I-T-E circuit breaker. The two
no voltage coils for two phase four wire circuits are connected
respectively to binding posts B, C and A, D on the face of the base. B
and D are connected to lower spring contacts 2 and 1 respectively, of
the small disconnecting switch. (In instruments supplied on individual
bases, these connections are made in the factory, let into channels
in back of base and covered with wax.) Each of the upper contacts _a_
and _b_ of the disconnecting switch is connected respectively through
resistance R2 and R1 to one main in each phase at _aa_ and _bb_. C
and A are respectively connected to the other main in each phase at
3 and 4. Thus each of the no voltage coils operates across one phase
independent of the other. The terminals 3, _aa_, _bb_ and 4, must,
in all cases, be so connected that they will be subject to the full
voltage of the circuit, irrespective of the position of the starting
switch.]
[Illustration: FIGS. 2,334 and 2,335.--Diagram showing two phase
three wire no voltage connections for I-T-E circuit breaker. The two
no voltage coils for two phase, three wire circuits are connected
respectively to binding posts B, C and A D on the face of the base,
and from A and C connections are made to lower contacts 2 and 1
respectively of the disconnecting switch. Binding posts B and D are
connected together on the back of the board. (In instruments supplied
on individual bases, these connections are made in the factory, let
into channels in back of base and covered with wax.) Each of the upper
contacts _a_ and _b_ is connected respectively through resistance R2
and R1 to one of the mains at _aa_ and _bb_ as shown. D is connected
through resistance R3 to the common wire of both phases at 3 B and D
being connected as aforesaid, thus forming a common connection for
both no voltage coils. Terminals _aa_ and _bb_ of the resistances must
be connected to the outside main across the two phases, terminal 3 to
the main common to both phases, the connections being so made that
these terminals will be subject to the full voltage of the circuit
irrespective of the position of the starting switch.]
[Illustration: FIG. 2,336 and 2,337.--Diagram showing three phase no
voltage connections for I-T-E circuit breaker. The no voltage coils
for three phase circuits are connected in Δ by means of binding posts
A, B, C and D on the face of the base, and from the A and B of the no
voltage coils, connections are made respectively to spring contacts
1 and 2 of the small disconnecting switch. Each of the contacts
_a_ and _b_ of the disconnecting switch is connected respectively
through resistance R2 and R1 to one of the mains at _aa_ and _bb_.
The terminal C is connected through resistance R3 on the back of
the board to the middle main as shown at point 3. The terminal D is
linked on the back of the board to terminal B to complete the [Greek:
D] connection. The terminals _aa_, _bb_ and 3 of the circuit breaker
must, in all cases, be so connected that they will be _subject to full
voltage of the circuit_ irrespective of the position of the starting
switch. Each no voltage coil is supplied with two terminal wires, one
covered with green and one with black insulation. In replacing these
coils particular care should be taken to see that the terminal wires
connected to any one binding post are of unlike color.]
In plants in which two or more banks of transformers are operated in
parallel between high and low voltage busses, it is desirable to have
for each transformer bank, an automatic circuit breaker equipment which
will act selectively and disconnect only the bank in which trouble
may occur. With a circuit breaker on each side of transformer bank,
selective action may be secured in two ways as follows:
1. By means of an instantaneous differential relay connected in the
secondaries of current transformers installed on both the high and low
voltage sides of each transformer bank.
The relay operates on a low current, reversal on either side of
the bank.
2. By means of one inverse time limit, secondary or series relay
installed on that side of the transformer bank which is opposite the
source of power, the relay being arranged to trip both the high and low
voltage circuit breakers.
_The first method_ has the disadvantage of high first cost
due to the high voltage current transformers required, but is
more positive than the second method and is independent of the
number of transformer banks in parallel.
_The second method_ is the less expensive of the two and
protects against overloads as well as short circuits in the
transformers, but it is less positive and introduces delay
in the disconnection of the transformer when trouble occurs.
Furthermore, it is not selective when less than three banks are
operating in parallel.
The automatic circuit breakers in the outgoing line may be operated
from inverse time limit relays connected in the secondaries of current
transformers; or in case transformers are not necessary for use with
instruments, series high voltage inverse time limit relays connected
directly in the line may be used.
Whether to select current transformers with relays insulated for low
voltage, or to choose series relays, is a question of first cost and
adaptability to service conditions. Below 33,000 volts, the commercial
advantages in favor of the series relay are slight, and since it
is somewhat difficult to design this device for the large current
capacities met with at the lower voltage, it is generally the practice
to use the relay with current transformer, because of its operating
advantage. This practice, however, is not entirely followed, since some
service conditions (described later) make the use of series relays very
desirable and practical.
[Illustration: FIGS. 2,338 and 2,339.--General Electric instantaneous
direct current reverse current or ~"discriminating" relays~. Fig.
2,238, for 500 amperes; fig. 2,339 for 2,000 amperes. These relays are
designed for mounting directly on circuit breaker studs. ~These relays
consist~ _of a horseshoe magnet with a shunt wound armature pivoted
between its poles_. ~The magnet~ is mounted on the current carrying
stud of the circuit breaker between the back of the panel and the
first contact or supporting nut, and is placed in a vertical position.
~The contacts~ are insulated from the magnet permitting the use of an
auxiliary circuit for the tripping device, independent of the circuit
controlled by the circuit breaker. The magnet is excited by the current
flowing through the stud, and the armature is connected across the
line in series with suitable resistance. Rotation of the armature in
the normal direction is prevented by a stop. ~Reversal of the current~
flowing through the stud _changes the direction in which the armature
tends to rotate_, causing it to move away from the stop and close the
circuit through an auxiliary trip coil and trip the circuit breaker.
~These relays are used~ to protect dynamos, storage batteries, or main
station busses from damage on reversal of current due to short circuit,
or from the grounding of machines or connection. Relay contacts must
not be used to open the shunt trip coil circuits. An auxiliary switch
should be provided for this purpose in all cases where the opening of
the circuit breaker does not disconnect the trip coil from the source
of supply.]
_Inverse time limit_ relays are satisfactory for one, or more than two
outgoing lines in parallel as they act selectively to disconnect the
defective line only, but installations with only two outgoing lines
in parallel have the same load conditions in both lines and selective
tripping of the circuit breakers in the defective line is obtained by
means of a selective relay acting instantaneously under short circuit
conditions only.
[Illustration: FIG. 2,340.--General Electric ~direct current, reverse
current relay,~ _used to protect dynamos, storage batteries, or main
station busses from damage on reversal of current due to short circuits
or from the grounding of machine or connections._ It is mounted on
vertical bus bars as in the case of cables, on the side wall, or other
flat surface, and the cables threaded through the frame. ~When used
to trip a circuit breaker,~ the breaker is provided with a shunt trip
connected across the circuit, the tripping circuit being closed through
the relay contacts on the occurrence of sufficient reverse current to
lift the relay armature. The relay is either instantaneous or time
limit as desired. ~In the time limit relay,~ the time interval is
obtained by the leather bellows shown in the illustration. The time
setting can be varied within certain limits by means of a valve on the
bellows outlet. ~The operation of the relay~ _depends on the relative
value and direction of magnetic flux set up by a pressure coil, shown
in the illustration, and the current in the vertical bars._ Under
normal conditions these fluxes are in the same direction and circulate
around a closed magnetic circuit. ~When the current in the bars
reverses,~ _the two fluxes oppose each other and force flux through
the normally open leg of the magnetic circuit._ When the reversal of
current is of predetermined value, the relay armature is lifted and the
purpose of the relay accomplished.]
The relay design and action is similar to the reverse current
relay previously mentioned, and is connected to the secondaries
of current transformers in each high voltage line and pressure
transformers in the low voltage bus.
In the sub-station, the conditions are the reverse of those in
the main station, the incoming lines becoming the source of
power.
If there be only one incoming line and no high voltage bus,
the line circuit breaker is generally non-automatic. With
one incoming line and high voltage bus, the circuits from the
service side of the bus are equipped with automatic circuit
breakers and relays. These relays and those used for other
arrangements of two or more incoming lines in parallel, as
well as high and low voltage circuit breakers, are of the same
design and are applied in the same manner as for the generating
station.
Regarding the relay equipments for auxiliary machines, the same
practice is recommended with the generator end of alternating
current motor generator sets as with the main generators, the
outgoing feeder circuit breakers being tripped from inverse
time limit or instantaneous relays.
[Illustration: FIG. 2,341.--General Electric ~direct current
differential relay~ for balancer set; instantaneous, 500 (or less) volt
type for mounting on panel. In many power plants direct current, three
wire, power service is furnished by "high voltage" two wire dynamos
operating in connection with balancer sets consisting of two "low
voltage" machines on a common shaft. With this combination of machine,
a short circuit or heavy overload on one side of the system will shift
the neutral considerably, and the lamps on the opposite side may "burn
out". To protect the lamps, a differential relay operating on 15 volts
unbalancing, is commonly used; it is connected to trip either the
dynamo's circuit breakers (or a circuit breaker connected in the bus
between the balancer set and the other dynamos).]
With several synchronous machines in parallel, the relays
are arranged to operate with the least time delay with which
it is possible to get selective action, in order to prevent
the machines being thrown out of step in event of trouble
conditions causing a decrease of voltage.
The various types of _induction motor_ and various conditions
under which they are employed, have brought about the
development of several types of relay to protect the motors and
the apparatus with which they are used.
It is desirable to disconnect a _large motor_ in case of voltage
failure, and with conditions requiring either a motor operated, or
a solenoid operated circuit breaker, a _low voltage relay_ is used
to close the tripping circuit whenever the voltage decreases to,
approximately, 50 per cent. below normal.
[Illustration: FIG. 2,342.--Condit ~time limit relay~, designed
primarily for use in connection with feeder circuits, where close
selection or discrimination of circuit breakers is not required.
~It may be used~ satisfactorily on lighting and power circuits and
also where there are sudden, momentary fluctuations of current. This
relay is used in connection with series transformers. The contact
arrangements are provided so that the relays may be used as circuit
closing or circuit opening relays. ~The delayed action~ is produced
by an air vacuum dash pot with a graphite piston. The piston of the
dash pot is connected to an arm arranged to be moved by the armature.
When the current reaches a point where it overcomes the weight of the
armature and lifts the same, the magnetic force tending to lift the
armature is opposed by the pull of the vacuum created in the interior
of the shell into which fits the graphite piston. As the magnetic pull
continues the vacuum is overcome due to the leakage of air past the
piston, and the armature gradually moves until it reaches a point where
it causes the circuit breaker to trip, either by closing the contacts
in the circuit closing type, or by opening the contacts in the circuit
opening type. If, at any portion of its travel, the current drop to
normal, the armature immediately resets. ~The time adjustment~ consists
of an arrangement whereby the distance through which the armature moves
before tripping the breaker, may be changed, thus altering the time of
tripping. ~The current adjustment~ is made _by changing the effective
turns of the actuating coil_, the travel of the armature and the force
exerted by it being the same for all current adjustment. The winding is
designed to carry 5 amperes continuously with a temperature rise not
exceeding 68° Fahr. standard calibration is provided so that the relay
will start to operate at 5, 6, 8 and 12 amperes.]
Up to 550 volts, these relays may be connected across the line,
but for higher voltages they are connected to secondaries
of pressure transformers. _Smaller motors_ with which hand
operated circuit breakers are used, are generally provided with
low voltage release attachments that perform the same function
as the relay.
Induction motors are sometimes subjected to _high voltage conditions_
and to protect them from injury, high or excess voltage relays are
employed to trip the automatic circuit breaker. These relays are of
similar design and wired in the same manner as the low voltage relays.
[Illustration: FIG. 2,343.--Characteristic curves of Condit time limit
relay as illustrated in fig. 2,342. Settings: curve A, 5 amperes; curve
B, 6 amperes; curve C, 8 amperes; curve D, 12 amperes.]
_Reverse phase relays_ have been developed for operating conditions
under which a _reversal of phase_ would cause trouble, as for example,
in the case of _elevator motors_.
These are so designed that any phase reversal that would
reverse an induction motor, would operate the relay and
disconnect the automatic circuit breaker.
The design is based on the principle of the induction motor,
and in the case of low voltage motors of limited capacity,
the relay may be connected in series in the motor leads. If
the voltage or capacity of the motor make this arrangement
inexpedient, the relay may be placed in the secondaries of
current or pressure transformers connected in the motor leads.
_Underload relays_ are often used to trip the automatic circuit breaker
that is placed in the primaries of _arc lighting circuits_ to prevent
an abnormal rise of secondary voltage in case of a break in the
secondary circuit.
[Illustration: FIG. 2,344.--Diagram showing _storage battery and
charging dynamo protected by_ ~double pole single coil underload
circuit breaker. In operation,~ _the circuit breaker disconnects the
battery when fully charged, and protects the dynamo from reverse
current_.]
The underload relay is similar in design to the low voltage
relay excepting that it acts on a decrease of current.
The problem of _protecting induction motors_, from injury, that may
result from running on single phase, or from an overload, and at the
same time permit the motor to be started with the necessarily high
starting current that may be greatly in excess of the overload current,
has caused the development of the _series relay_.
[Illustration: FIG. 2,345.--Diagram showing direct current motors
protected by overload circuit breakers with bell alarm switches: _a_,
double pole single coil breaker no switch required. Low voltage device
is on the starting rheostat; _b_, single pole breaker in series with
lever switch. Low voltage attachment on the breaker.]
[Illustration: FIG. 2,346.--Diagram showing two wire dynamo, protected
by a single pole overload circuit breaker with bell alarm switch.
Breakers must be on opposite side from the series field.]
[Illustration: FIG. 2,347.--Diagram showing dynamo protected by a
single pole overload circuit breaker with reverse current relay and
combined circuit opening and bell alarm switch.]
This device may be connected in series with the motor leads for
voltages up to 2,500; it is designed with an inverse time limit
device which may be adjusted to give the desired protection.
The field for relays is more extensive for alternating current than
for direct current power circuits, the latter being generally confined
to much smaller and simpler systems and areas of distribution, and
generally sufficient selective action can be obtained by the use of
fuses or circuit breakers arranged with instantaneous trip.
[Illustration: FIG. 2,348.--Diagram showing three wire dynamo protected
by double pole double coil overload circuit breaker (or two single pole
breakers with interlock) with bell alarm switches. Complete protection
is secured as breaker is connected between armature and series field.]
Operating conditions sometimes make it advisable for the generator
circuit breakers to open only after the auxiliary and feeder circuit
breakers have failed to isolate the trouble.
This is accomplished by using direct current _series inverse
time limit relays_ to trip the generator circuit breakers.
_Instantaneous reverse current relays_ are used to trip the machine
circuit breaker of battery charging sets, rotaries and motor generator
sets to prevent their running as a motor on the charging or direct
current end. These relays can act only in case of current reversal.
To prevent serious unbalancing of voltages in Edison three-wire systems
causing trouble, _differential balance relays_ are used to trip the
circuit breakers on a small percentage of unbalancing.