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.
[Illustration: 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.]
[Illustration: 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.
[Illustration: 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.
[Illustration: 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.
[Illustration: 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.]
[Illustration: 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.
[Illustration: 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.
[Illustration: 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.
[Illustration: 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
sub-dividing 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.
[Illustration: 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.
[Illustration: 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.
[Illustration: 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 take more
than say, 10 per cent. motor current.
Maximum and reverse circuit breakers are frequently used on
dynamo panels.
[Illustration: 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.
[Illustration: 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 pre-determined 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.]
[Illustration: 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.
[Illustration: 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 frame-work 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.
[Illustration: 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.
[Illustration: 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.]
[Illustration: FIG. 496.--Simplified diagram of the
connections of fig. 495.]
[Illustration: 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.
[Illustration: 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."
[Illustration: 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.
[Illustration: 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. VS 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.
[Illustration: 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.
[Illustration: 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 multipole apparatus. The action of
this type of circuit breaker is fully explained in fig. 491.]
HAWKINS PRACTICAL LIBRARY OF
ELECTRICITY
IN HANDY POCKET FORM PRICE $1 EACH
_They are not only the best, but the cheapest work published on
Electricity. Each number being complete in itself. Separate numbers
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will be mailed free on request._
=ELECTRICAL GUIDE, NO. 1=
Containing the principles of Elementary Electricity, Magnetism,
Induction, Experiments, Dynamos, Electric Machinery.
=ELECTRICAL GUIDE, NO. 2=
The construction of Dynamos, Motors, Armatures, Armature Windings,
Installing of Dynamos.
=ELECTRICAL GUIDE, NO. 3=
Electrical Instruments, Testing, Practical Management of Dynamos and
Motors.
=ELECTRICAL GUIDE, NO. 4=
Distribution Systems, Wiring, Wiring Diagrams, Sign Flashers, Storage
Batteries.
=ELECTRICAL GUIDE, NO. 5=
Principles of Alternating Currents and Alternators.
=ELECTRICAL GUIDE, NO. 6=
Alternating Current Motors, Transformers, Converters, Rectifiers.
=ELECTRICAL GUIDE, NO. 7=
Alternating Current Systems, Circuit Breakers, Measuring Instruments.
=ELECTRICAL GUIDE, NO. 8=
Alternating Current Switch Boards, Wiring, Power Stations, Installation
and Operation.
=ELECTRICAL GUIDE, NO. 9=
Telephone, Telegraph, Wireless, Bells, Lighting, Railways.
=ELECTRICAL GUIDE, NO. 10=
Modern Practical Applications of Electricity and Ready Reference Index
of the 10 Numbers.
=Theo. Audel & Co., Publishers. 72 FIFTH AVENUE, NEW YORK=
[Transcriber's Note:
Inconsistent spelling and hyphenation are as in the original.]
End of Project Gutenberg's Hawkins Electrical Guide Number 2, by Hawkins