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.

=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.

=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.

=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.

=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.

=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.

=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.

=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.

=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.

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.

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.

=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.

=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.

=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.

=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.

=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.

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."

=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.

=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.

=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.

HAWKINS PRACTICAL LIBRARY OF

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=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.

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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