Hawkins Electrical Guide v. 02 (of 10) / Questions, Answers, & Illustrations, A progressive course of study for engineers, electricians, students and those desiring to acquire a working knowledge of electricity and its applications

CHAPTER XXV AUXILIARY APPARATUS

There are numerous devices that must be used in connection with dynamos and motors for proper control and safe operation. Among these may be mentioned:

1. Switches;
2. Fuses;
3. Circuit breakers;
4. Rheostats;
5. Switchboards.

Switches.—A switch is a device by means of which an electric circuit may be opened or closed. There are numerous types of switch; they may be either single or multi-pole, single or double-throw and either of the "snap" or knife form.

Ques. What is the difference between a single and double-pole switch?

Ans. A single-pole switch controls only one of the wires of the circuit, while a double-pole switch controls both.

Ques. What is the difference between a single-break and a double-break switch?

Ans. The distinction is that the one breaks the circuit at one point only, while the other breaks it at two points.

Ques. What is the advantage of a double-break?

Ans. If the circuit be opened at two points in series at the same instant, the electromotive force is divided between the two breaks and the length to which the current will maintain an arc at either break is reduced to one-half; thus there is less chance of burning the metal of the switch. Another reason for providing two breaks is to avoid using the blade pivot as a conductor, the contact at this point being too poor for good conductivity.

Figs. 455 to 457.—Adam's single-throw knife switches without fuse connections. Fig. 455, single-pole switch; fig. 456, double-pole switch; fig. 457, three-pole switch.

Figs. 458 to 460.—Adam's single-throw knife switches with fuse connections at the handle end. Fig. 458, single-pole switch; fig. 459, double-pole switch; fig. 460, three-pole switch.

Ques. When should a knife switch be used?

Ans. When the capacity of the circuit in which it is to be placed exceeds 10 amperes.

Ques. Describe a knife switch.

Ans. Fig. 461 illustrates a knife switch of the double-pole, single-throw type. It consists of the following parts: base, hinges, blades, contact jaws, insulating cross bar, and handle, as shown.

Ques. How should knife switches be installed?

Ans. They should be placed so that gravity tends to open them.

Otherwise if the hinges become loose, the weight of the blades and handle would tend to close the switch, thus closing the circuit and possibly resulting in considerable damage.

Fig. 461.—A single-throw, two pole knife switch. As usually constructed it is made of hard-drawn copper with cast terminal lugs and fibre cross bar.

Ques. How should switches be proportioned?

Ans. The minimum area of the contact surfaces should not be less than .01 square inch per ampere, and in those used on arc lighting or other high voltage circuits where the current is usually small, the area of the contact surfaces are usually from .02 to .05 inch per ampere. Since dirt or oxidation would prevent good contact under a simple pressure between the contact surfaces, the mechanism of a switch provides a sliding contact.

In the general design of switches, all parts which carry current are given a cross sectional area of at least one square inch per 1,000 amperes if they be made of copper, and about three times as much if made of brass, as the conductivity of the latter is only one-third that of the former. Furthermore, the current should never be permitted to pass through springs, as the heat generated will destroy their elasticity.

Fig. 462.—Triple-pole, double-break double-throw knife switch for very heavy current. The blades are made up of numerous strips to give adequate contact area. A double-throw switch is used when it is desirable to open one circuit and immediately close another, or to transfer one or more connections from one circuit to another in the least practical interval of time, also, when one connection is to be broken and another closed and it is undesirable to allow both to be closed at the same time.

Ques. What difficulty is experienced in opening a circuit in which a heavy current is flowing?

Ans. It is impossible to instantly stop the current by opening the switch, consequently the current continues to flow and momentarily jumps the air gap, resulting in a more or less intense arc which tends to burn the metal of the switch.

Ques. How is this remedied to some extent?

Ans. The contact pieces are so shaped that they open along their whole length at the same time, so as to prevent the concentration of the arc at the last point of contact. This feature is clearly shown in fig. 461.

Fig. 463.—A "quick break" knife switch of the single-throw, single-break, one pole, type. The contact blade is held between the jaws by their clamping friction until the handle compresses the spring sufficiently to force the blade out. As soon as it breaks contact with the jaws, the spring expands and drives the blade away from the jaws with greater rapidity than could be done by hand. The object of this action is to break the arc as quickly as possible to prevent burning the metal of the switch.

Figs. 464 and 465.—Snap switch; views showing switch with cover on, and exposed to show mechanism. The switch is provided with indicating dial which registers "on" and "off" positions.

Fig. 466.—Gas Engine snap switch. The first snap makes connection so that igniter is run from storage battery; second snap connections are changed so that igniter is supplied from dynamo; third snap makes connections so that dynamo supplies igniter and charges storage battery; fourth snap, all off.

Ques. For what service are "snap" switches suitable?

Ans. They are used on circuits containing lamps in comparatively small groups, and other light duty service.

Ques. What is a quick break switch?

Ans. A form of switch in which the contact pieces are snapped apart by the action of the springs, as shown in fig. 463, so as to make the duration of the arc as short as possible.

The current allowed in each branch circuit of an electric lighting system is limited by the insurance rules to a maximum of 660 watts equivalent to 12 lamps of 16 c.p. each at 110 volts. They are also employed to control lamps in groups in theatres and other places where many lamps are turned on or off at about the same time.

Fig. 467.—Spool of fuse wire. The wire is usually made of an alloy of tin and lead, such as half and half solder. Bismuth is frequently added to the alloy to lower the melting point For half and half solder the melting point is 370° Fahr. The quickness with which a fuse will melt after the current has reached the limit depends upon the specific heat and latent heat of the metal. The current required to "blow" a fuse increases somewhat with the age of the fuse owing to oxidation and molecular changes. Fuses are sometimes rated according to the number of amperes to be taken normally by the circuit they are to protect. Thus, a 10 ampere fuse is supposed to protect a circuit whose regular current should not exceed 10 amperes, and to blow if the current rise to say 12 amperes. The Underwriters' rule requires that the rating be about 80% of the maximum current it can carry indefinitely, thus allowing about 25% overload before the fuse melts. The fusing current varies considerably according to circumstances. The temperature of the surrounding air or other substances affects the melting current greatly, because the rate at which heat from the fuse will be transferred to the surroundings depends upon the difference of temperance between them and the fuse. Hence a fuse in a warm place will be melted by a smaller current than a similar fuse in a cold place. For a similar reason, a fuse in an enclosed place where there is little chance for the heat to be dissipated, will melt with a smaller current than the same in an open place. If the current increase gradually to that which would ordinarily melt the fuse, the high temperature makes the fuse wire oxidize rapidly; this sometimes makes a sort of tube of oxide which will not break even after the fuse wire inside has melted, and so the fuse carries more than its rated current. Open fuses are so unreliable that circuit breakers are preferable for large currents; when fuses are used, the enclosed type as shown in figs. 468 to 470, is usually the more desirable.

Fuses.—All circuits subject to abnormal increase of current which might overheat the system, should be protected by fuses which will melt and thus open the circuit. A fuse is simply a strip of fusible metal, often consisting of lead with a small percentage of tin, connected in series in the circuit.

Experiments have shown that for large fuses, a multiple fuse is more sensitive than a single one. A one hundred ampere fuse may be made by taking four wires of twenty-five amperes capacity. A fuse block may be overloaded, not because the metal of the terminals is not of sufficient cross section to carry the current, but because of insufficient area of, contact, or loose contact of fuse and wires; the overload thus caused results in heating and frequently melts the fuse.

Figs. 468 to 470.—D & W, enclosed "cartridge" fuses. Fig. 468, type for 3 to 60 amperes; fig. 469, type for 61 to 100 amperes; fig. 470, type for 101 to 1,000 amperes.

Ques. Where should fuses be placed?

Ans. They should be inserted wherever the size of wire changes or wherever there is a branch of smaller size wire connected, unless the next fuse on the main or larger wire is small enough to protect the branch or small wire.

Figs. 471 to 478.—Interior construction of D. & W. fuses. In the manufacture of these fuses, four types of fuse link are used according to capacity of fuse, and classified as: 1, air drum link; 2, flat link; 3, multiple link; 4, cylinder link. In the air drum link, figs. 471 and 472, a capsule provides an air space about the center of the link, the rate of heat conduction through the confined air being very slow, the temperature of that portion of the link rises rapidly with increasing current, rendering the blowing point practically constant; fig. 473 shows a section through the complete fuse. In the flat link, fig. 474, the section is reduced in the center, cutting down as far as possible the volume of metal to be fused. Figs. 475 to 478 show various form of multiple link construction. By subdividing the metal, increased radiating surface is obtained which permits a reduction in the volume of fusible metal necessary, and the metal vapor formed when the fuse blows on heavy overload is more readily dissipated. Figs. 477 and 478 show two forms of the cylinder link, the plain cylinder fig. 477, being used for low voltage and large current, and fig. 478, for certain high tension service. The corrugated cylinder presents more surface to the fuse filling than the plain type and secures a maximum radiating surface with resulting minimum volume of metal for a given current.

Ques. How should fuses be mounted?

Ans. They should be placed on a base of slate, porcelain, marble, or other incombustible material.

Ques. What is the objection to copper fuses?

Ans. They heat perceptibly soon after their rated capacity is passed. The melting temperature is higher than lead alloy.

Ques. Upon what consideration does the choice between switches and circuit breakers depend?

Ans. Simple knife switches are suitable for use when the circuit is not liable to be opened while carrying large current. A circuit breaker, operated automatically or by hand should be used for interrupting heavy currents.

Figs. 479 and 480.—D & W fuse indicator. The operation is illustrated in the figures which show appearance of the label before the blowing of the fuse, fig. 479, and the same fuse blown, as indicated by the appearance of the black spot within the circle fig. 480.

Circuit Breakers.—A circuit breaker is a switch which is opened automatically when the current or the pressure exceeds or falls below a certain limit, or which can be tripped by hand.

Ques. What is the construction of a circuit breaker?

Ans. It is composed of a switch and a solenoid in the main circuit. When the current, flowing through the circuit, exceeds a certain value, the core of the solenoid is drawn in and trips a trigger which allows the switch to fly open under the action of a spring.

Figs. 481 to 486.—Various open fuses. Fig. 481, fuse for main and branch blocks; fig. 482, standard railway fuse; fig. 483, Edison main style; fig. 484, W. U. pattern; fig. 485, Bell telephone style; fig. 486, sneak current fuse. When an open fuse "blows" as a result of overloading, the rupture is accompanied by a flash, and by spattering of the fused material. With large currents this phenomenon is a source of danger, and the use of enclosed fuses is accordingly recommended whenever the rating of the fuse exceeds 25 amperes. Various types of enclosed fuse are shown in figs. 468 to 470.

There are numerous kinds of circuit breaker to meet the varied conditions of service of which may be mentioned the following:

1. Maximum circuit breaker;
2. Minimum circuit breaker;
3. Reverse current circuit breaker;
4. Maximum and reverse circuit breaker;
5. No voltage circuit breaker.

Figs. 487 and 488.—Reverse current circuit breaker; fig. 488, view looking at end of coils of cut out, showing direction of current. A to + bus bar; B, resistance lamp; C, brush of cut out; D, shunt coil; E, series coil; F, core that trips cut out; G, to—bus bar; H, to + pole of dynamo.

Of these the maximum, reverse, and maximum and reverse types are the more important.
A maximum circuit breaker is equivalent to a fuse, but has the advantage that it can be at once reset, whereas a fuse must be replaced.
A reverse breaker is used in connection with dynamos in parallel, to automatically cut out a machine if it takes more than say, 10 per cent. motor current.
Maximum and reverse circuit breakers are frequently used on dynamo panels.

Figs. 489 and 490.—Front and top views of I-T-E automatic overload circuit breaker. In fig. 489 the current in the circuit enters at A, passes through the solenoid coil B (which in its iron jacket becomes a powerful magnet), through the copper terminal C, to the contact blades D, across the bridge at E to the contact blades F, and out into the line at G. The path of the current as indicated above is more clearly indicated in the top view fig. 490. When the current in the solenoid coil produces sufficient magnetism to overcome the weight of the plunger, the latter is drawn up with constantly increasing velocity until it strikes a restraining latch or trigger which forces the arm out of the switch, thus automatically opening the circuit. The device is so constructed that in opening the circuit the arc is broken on the carbon contacts instead of the copper contacts.

Ques. Describe a reverse current circuit breaker or discriminating cut out.

Ans. This type of circuit breaker is arranged to open a circuit in the event of current flowing in the circuit in a direction reverse to the normal. This is sometimes effected by winding the electromagnet of the circuit breaker with two coils, one connected as a shunt across the main circuit and the other in series with the main circuit, the two coils being so arranged that when the main current flows in the normal direction their effects assist one another, whereas, when the main current reverses, the effects of the coils are neutralized and the breaker opens.

Fig. 491.—Roller-Smith "S.E." plain overload circuit breaker. In operation, current entering through the lower studs flows through the laminated strap windings C, from this into the arm D, through the contact plate E, into the stationary brush F, and finally out through the upper stud Q. In its passage through the laminated windings C, the square core A is of course magnetized to a degree dependent on the current strength. When this magnetization reaches a predetermined value, the attraction exerted on the ends K of the pivoted armature causes the same to rise with great and increasing velocity, finally bringing the finger D which forms part of the armature into violent contact with the face R of the corresponding projection on the housing which carries the handle and the roller H. This heavy blow causes H, in its rotation about the shaft J, to go over the center and consequently allows the strong outward pressure of the brush F and the resilient coil C to throw the arm outward with a high velocity and so break the circuit, first between the brush fingers and the contact plate and finally between the carbons S and F, the one of which is rigidly secured to the arm and the other of which is resiliently mounted on its supporting spring. To reset the breaker, the handle, which the act of opening has raised, is pulled down, thus bringing roller H into engagement with roller G once more and in that way forcing the arm back into its initial position.

Fig. 492.—Roller-Smith "S.E." combination overload and underload circuit breaker. Attached to the supporting frame B is the extension Z, which like B, is of non-magnetic material and carries a rectangular magnetic core around which there are wrapped laminated copper conductors. Hinged at U is a heavy cup-shaped mass of magnetic material, and hinged at V is a flat lever X which bears against the extension Y secured to the housing which carries the operating handle. The circuit through the breaker conveys the current around the windings of this underload coil carried by frame Z and passes from it to the regular overload winding C from which it pursues the same course and exercises the same function as in a plain overload breaker. The core of Z being thus magnetized, the cup-shaped member W is held in firm contact therewith and the lever X hangs free. Should, however, the current fall below the minimum value, W is no longer sustained by the magnetic attraction but drops away, swinging on its hinge U until the projection on the heel thereof strikes the lever X, which blow is transmitted through Y to the handle and thus trips the breaker. When closing to reset the breaker, the handle is manipulated just as in the case of a plain overload breaker, that is, it is pulled down, thus not only closing and locking the breaker as before but through the pressure exerted by Y on X and by X on W, putting the latter into contact with its rectangular core to which it will adhere if the necessary current be present.

Ques. State some disadvantages of a discriminating cut out.

Ans. If one current reverse very rapidly, and soon reach a large value in the opposite direction, it is possible the cut out may not open at the desired instant, and thereafter the effect of the heavy reverse current will be so great that the breaker will be held in more and more strongly; a second disadvantage is that should the supply fail, the breaker will open in any case, and have to be reset before the supply can be resumed, though in certain cases, as, for instance where there is a motor load, this feature is an advantage and not a disadvantage, since the breaker acts as a no-voltage cut out as well as a reverse current cut out.

Reverse breakers, however, can be made positive in their action; that is, they can be so arranged that a reverse current exerts a positive pull on the tripping gear, so that the greater the reverse current, the greater the tripping effect.

Ques. What are time limit attachments?

Ans. Devices which are fitted to circuit breakers and which act as dampers and prevent the too sudden operation of the breakers on what may be only a temporary overload or reverse current.

By having different time limits on feeder and dynamo breakers it can be ensured that the former operate before the latter, and suitably in other cases where it is desired that one breaker shall operate before another.

Ques. Describe a time limit attachment.

Ans. There are numerous types. It may consist of a clockwork device, a weight acting on a small drum or pulley, a modified dash pot arrangement, or a device operating by the expansion of a conductor due to the heat generated by a current passing through it.

Ques. How should a time limit device be arranged?

Ans. It should be so arranged that the heavier the overload the quicker the device acts, until with a short circuit the device is almost instantaneous in its action.

Fig. 493.—Diagram showing connections of a rheostat. The various resistance coils are connected to brass buttons or "contacts." The rheostat is connected in series in the circuit that it is to control. In operation when the lever is on contact 1, the current is opposed by all the resistance of the rheostat so that the flow is very small. As the lever is moved over contacts 1, 2, 3, etc., the coils are successively cut out, thus diminishing the resistance, and when contact 11 is reached all the resistance is short circuited allowing the full current to flow. In some types of rheostat the wire is wound around an iron framework which has been previously dipped into a fireproof insulating enamel. The advantage of this construction is that the heat from the wire is dissipated much more rapidly, so that a much smaller wire can be used to carry a given current. The size of such an enameled rheostat required for absorbing a given amount of energy is much smaller than one made of coils of wire stretched between an iron supporting framework.

Rheostats.—These devices consist of conductors inserted into a circuit for the purpose of diminishing, either constantly or in a variable degree, the amount of current flowing, or to develop heat by the passage of a current through them. Rheostats designed to be used in starting electric motors are frequently called "starting boxes."

Ques. Describe the construction of a rheostat.

Ans. In fig. 493, resistance coils, A, B, C, etc., are mounted in a frame or box, and are connected at intervals to the contacts 1, 2, 3, etc. The rheostat arm or lever L is pivoted at S, and when moved over the contacts, inserts more or less of the resistance in the circuit thus regulating the flow of the current. One terminal M of the rheostat is connected to the first contact and the other terminal O, to the lever at S.

Fig. 494.—Starter with no voltage release for a series motor. A helical spring coiled around the lever pivot P, and acting on the lever A, tends to keep it in the off position against the stop S. This lever carries a soft iron armature I, which is held by the poles of the electromagnet E, when, in starting the motor, the arm has been gradually forced over as far as it will go. Should anything happen to interrupt the current while the motor M is running, E will lose its magnetism and A will be released, and will fly over to the off position. E is usually shunted by a small resistance R, so that only a portion of the main current flows through it. This device constitutes the no voltage release, and ensures that all the resistance is in circuit every time the motor is started.

Ques. How is a starting box connected to a motor?

Ans. In series.

Ques. Why should a starting box be used with a motor?

Ans. If the line voltage should be applied directly to the terminals of the armature when not running, an excessive flow of current will result, on account of the low resistance. Accordingly, to prevent injury to the winding, a variable resistance or starting box is inserted between one supply terminal and the armature so that the pressure may be applied gradually while the motor is coming up to speed.

Fig. 495.—Starter with no voltage release for a shunt motor. The terminals of the motor are at M, M', m, and those of the starter at S, S', s. The lever SA is shown in the "on" position. The current enters the motor at the terminal M, and there divides, part going through the field coil F, and the main current through the motor armature A. The armature current enters the starter at the terminal S', and traversing the lever SA, leaves by the terminal S. The field current enters the starter at the terminal s, traverses the coil of the magnet E (which holds up the armature a linked to the lever) and thence completes its journey through the whole of the resistance R, and through the lever SA, to the terminal S. When the supply is cut off by opening Sw, or should the field circuit be accidentally broken, the magnet E will release a and the lever, which will thereupon fly to the "off" stop O. It should be noticed that when SA is off, A and F form a closed circuit with the resistance R and magnet E. The inductance of F has consequently no chance of causing destructive sparking when the current is shut off. In starting the motor, Sw is first closed, and then, as the lever is slowly moved, the resistance R, which at first is all in circuit with A, is gradually transferred from A to F. The resistance of R is too small to affect appreciably the current in F, which necessarily consists of a comparatively large number of turns of fine wire. The arrangement is adopted to render the breaking of the shunt circuit unnecessary and is rendered clearer by the diagram fig. 496. It should be noted that E may be provided with a short circuiting key or push if required.

Fig. 496.—Simplified diagram of the connections of fig. 495.

Fig. 497.—Starter with no voltage release and overload release connected to a compound motor. With a shunt motor, the only difference in the diagram would be that the series winding Se would be absent, and the armature A would then be connected straight across between the main terminals M and M'. When switch Sw is closed, the current will enter the starter at its terminal S, and pass through the magnet coil m' of the overload release to the switch lever L, which is shown in the off position. As soon as L is moved up to make contact with the first contact S the current divides; part going through the resistance R and the terminals S' and M' to the series coil Se (if a compound motor) and armature A; and part through the no voltage magnet E to the shunt winding Sh. As the lever L is moved up toward E, the effect is to take R out of the armature circuit and put it into the shunt circuit. When the iron armature a, fixed on the switch lever, comes against the poles of E, the laminated copper brush C bears against the blocks B, B, and so affords a better path for the current than through the spindle s. Should the supply voltage fail, either temporarily or permanently, E will release a, and L will fly off under the tension of a helical spring coiled round s. If there should be an overload on the motor, tending to pull it up and cause an excess of current to flow through the armature; this excess current, passing through m', will make it attract its armature, so bringing two contacts together at K which will short circuit E, and allow the switch to fly off. The connections between E and m' are not shown in the figure, but they are indicated at C in fig. 498, which is a simplification of fig. 497, and which should be carefully compared therewith. When only the normal current is flowing, the attraction between m' and its armature is not sufficient to pull the latter up. The actual forms and arrangement of parts on the starters are well shown in some of the figures.

Ques. What attachments should be provided on a starting box?

Ans. An overload release, and a no voltage release.

Ques. Describe these devices.

Ans. The overload release is an electromagnetic circuit breaker that opens the circuit if the motor become greatly overloaded. A no voltage release may consist of an electromagnet in series with the shunt field circuit; it holds the rheostat arm in the operating position as long as current flows through the shunt field from the line. If the line switch be opened or the shunt field circuit accidentally broken, the device becomes demagnetized and releases the arm, which returns to its starting position by the action of a spring.

Fig. 498.—Simplified diagram of the connections of starter connected to compound motor as shown in fig. 497.

The general arrangement of switches, cut outs and starting boxes should be in accordance with the requirements of the National Electrical Code as follows:

"Each motor and starting box must be protected by a cut out and controlled by a switch, said switch plainly indicating whether 'on' or 'off.' The switch and rheostat must be located within sight of the motor, except in cases where special permission to locate them elsewhere is given, in writing by the inspection department having jurisdiction.
"Where the circuit breaking device on the motor starting rheostat discs disconnects all wires of the circuit, this switch may be omitted.
"Overload release devices on motor starting rheostats will not be considered to take the place of the cut out required if they be inoperative during the starting of the motor.
"The switch is necessary for entirely disconnecting the motor when not in use, and the cut out to protect the motor from excessive currents, or careless handling when starting. An automatic circuit breaker disconnecting all wires of the circuit, may, however, serve as both switch and cut out."

Fig. 499.—View showing general arrangement of a switchboard. The wires are shown to illustrate the various connections, but in actual construction these wires are connected on the back of the switchboard.

Switchboards.—A switchboard consists of a panel or series of panels of slate, marble, soapstone or brick tile erected in an electric plant for the purpose of mounting in a convenient group the instruments for controlling and distributing the current and safeguarding the system. Switchboards may be divided according to operation into two classes:

1. Direct control;
2. Remote control.

A direct control switchboard has all its apparatus mounted directly on the board and controlled by hand, while in the remote control type, the main current carrying parts are at some distance from the operating board, the control being effected by mechanical devices or by electric motors or solenoids. When the control system of a plant is very extensive, it sometimes occupies a separate building known as the switch house.

Ques. What may be said with respect to the material for switchboards?

Ans. In order to avoid danger of fire from short circuits, the panel should be made of some non-combustible material, such as marble, slate, glass plates or earthenware tiles. If slate be used, care should be taken to have it free from conducting veins, or it should be marbleized, that is, subjected to a treatment that will fill up the pores of the veins and thus prevent the absorption of moisture.

Wood is seldom used, except in cases where the switches, fuse blocks, wire supports, etc., are all mounted on porcelain or other incombustible material.

Ques. How should the instruments and connections be arranged on a switchboard?

Ans. They should be arranged so as to provide the shortest possible path for the current, and preferably always in the same direction, that is, from right to left or from top to bottom, the connecting wires being brought in on one side and out on the other, and the crossing of wires avoided as far as possible.

All wires and current carrying parts should be kept far enough apart at all points to prevent accidental contact or the jumping across of the current where there is a great difference of voltage. Such wires should be also kept at a sufficient distance from screw heads, metal brackets, gas pipes, water pipes, and other conducting bodies, in order to prevent accidental grounds or short circuits.
All instruments and switches should be placed so as to be conveniently accessible for observation and operation, and sufficiently out of reach of accidental contact by persons; otherwise they should be protected by some form of insulating shield.

Fig. 500.—Small switchboard suitable for two dynamos; view showing ammeters and voltmeters, switches, circuit breakers, etc.

Ques. What type of switch is used on switchboards?

Ans. The "knife" switch.

Ques. Describe a small switchboard.

Ans. Fig. 500 shows one suitable for two dynamos. At the top is a voltmeter and two ammeters. Immediately below is a row of feeder switches serving to connect and disconnect the various feeders with and from the bus bars which are mounted behind the board. Below are two rheostat handwheels, and two large switches connecting the dynamos with the bus bars. V S is a voltmeter switch connecting the voltmeter with various parts of the system. Below the voltmeter switch is a double-throw switch to transfer the bus bars from connection with the dynamo switches to one with some other source of current such as a street circuit, in the event of a breakdown. At the bottom are two circuit breakers.

Fig. 501.—Diagram showing various connections of voltmeter switch of the small switchboard shown in fig. 500.

Ques. Describe the voltmeter switch.

Ans. Fig. 501 shows the connections, from which it can be seen that the voltmeter can be connected with the terminals of either dynamo or with the bus bars, or with either a central or remote part in the lamp circuits.

Under ordinary conditions it remains connected to the circuit at the central point of distribution. When one dynamo is already in circuit, however, and it becomes necessary to connect up the other one, the voltage of the latter must be the same as that at the bus bars. Accordingly, connections are provided to the voltmeter switch such that the attendant can compare the voltages at the dynamo terminals and bus bars before closing the dynamo switch. All the positive connections are on one side of the circle swept by the switch and all the negative connections on the other side.

Fig. 502.—Roller-Smith, single-pole, plain overload circuit breaker. As its name indicates, the function of the plain overload circuit breaker is to automatically interrupt the circuit in which it is placed when the flow of current through it exceeds the predetermined limit for which the apparatus is set. It is the most common of all of the types and is utilized for the protection of dynamos and motors and all other electrical apparatus which, by reason of the conditions of operation, may become subject to loads in excess of the normal. The single-pole type may be used separately for the protection of a single wire of a given circuit or grouped to protect the two or more wires of one circuit, becoming in the latter case the so called independent arm multi-pole apparatus. The action of this type of circuit breaker is fully explained in fig. 491.

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