CHAPTER LVIII
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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
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.
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.
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.
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.
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 Inverse Time Element principle.
Fig. 2,255.—Condit 600 volt, 1,200 ampere, single pole, type K, circuit breaker with pull down handle.
Fig. 2,256.—Condit 600 volt, 6,000 ampere, single pole, switch board mounting, circuit breaker, with pull down handle.
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.
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:
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.
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.
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.
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.
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.
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.
Fig. 2,270.—Diagram of General Electric shunt trip with auxiliary circuit opening switch to throw coil out of circuit after tripping.
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.
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.
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.
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.
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.
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.
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.