The National Electrical Code. AN ANALYSIS AND EXPLANATION OF THE UNDERWRITERS' ELECTRICAL CODE, INTELLIGIBLE TO NON-EXPERTS. PIERCE and RICHARDSON, | V ELECTRICAL ENGINEERS. CHICAGO. PUBLISHED BY CHARLES A, HEWITT, 510 ROYAL BUILDING, CHICAGO, ILL. COPYRIGHTED 1896 BY CHARLES A. HEWITT. Publisher's Announcement. In response to many requests, the notable articles on "The National Electrical Code," by Pierce and Richardson, in "The Insurance Post " of Chicago, have been revised and extended and are published herewith in convenient book form. As stated in the sub-title, this little book- is "An Analysis and Explanation Of The Underwriters' Electrical Code, Intelligible to Non- Experts." Most writers on electrical topics confuse the non-expert by their continued use of technical terms. Messrs. Pierce and Richardson, whose com- petency is unquestioned, have undertaken "to explain the matter in ordinary language, "for the special benefit of insurance inspectors and electrical students. Hence, also, the common analogies and simple definitions. On the authority of eminent electricians, the Un- derwriters' Code is to-day the best guide to safe con- struction and certain provision against loss by fire. The joint work of advanced underwriters and expe- rienced electricians, it represents years of patient re- iv. PUBLISHER'S ANNOUNCEMENT. search and the accumulated knowledge of recognized experts. A study of its rules and requirements has been often urged upon central station men and engi- neers. Insurance companies and their representatives, meanwhile, have come to look to the Code for instruc- tion and guidance, but without knowing why or where- fore. The whys and wherefores are clearly set forth in the following pages; and now that Electricity is coming into general use, the necessity of a better knowledge of electrical hazards is freely admitted by all those engaged in fire insurance. Supplemental to the general exposition, properly classified and indexed, is the "Appendix,' ' in which will be found tables and curves for measuring wires, and the full text of the Underwriters' National Elec- trical Code for 1896. The writings of Messrs. Pierce and Richardson, it is proper to add, have attracted the ' favorable notice of prominent underwriters, architects, builders and engineers. It is believed that this little book will prove indispensable to most insurance in- spectors and special agents, and of practical interest and value to many electricians. C. A. H. CHICAGO, NOVEMBER, 1896. TABLE OF CONTENTS. CHAPTER I. PAGES. INTRODUCTORY AND DEFINITIONS OF ELECTRICAL TERMS The National Code Electricity Electro-Motive Force Potential or Pressure Dynamo Conductor Re sistance Insulator Polarity, . . . g to 18 CHAPTER II CENTRAL STATIONS FOR LIGHT AND POWER PART I. Central Station Generators Care and Attendance Conductors Switchboards, . 19 to 27 CHAPTER III CENTRAL STATIONS FOR LIGHT AND POWER. PART II Switchboards Their Location and Construction Resist- ance Boxes and Equalizers Their Uses and How Installed Lightning Arresters, Why Needed and Where to Place Them, . 28 to 35 CHAPTER IV CENTRAL STATIONS FOR LIGHT AND POWER. PART III. Testing Series Circuits Multiple-arc Circuits Al- ternating Circuits Motors . 36 to 46 CHAPTER V. CLASS B, HIGH POTENTIAL SYSTEMS PART I High Potential Circuits Outside Conductors Class of Wire Allowed Crosses Wires Entering Buildings Switches Interior Conduits Insulation of Wires List of Approved Wires Joints in Wires, . 47 to 55 VI. TABLE OF CONTENTS. CHAPTER VI. PAGES. HIGH POTENTIAL SYSTEMS. PART II. Arc Lamps Spark Arresters Hanger Boards Auto- matic Switches Suspension of Arc Lamps Incandes- cent Lamps on Series Arc Circuits Definition of " Mul- tiple Series" and "Series Multiple," . 56 to 65 CHAPTER VII CLASS C. Low POTENTIAL SYSTEMS. PART I Two and Three Wire Systems Outside Conductors "Constant Current" and "Constant Pressure Sys terns Fusible Cut-outs, . . 66 to 72 CHAPTER VIII. CLASS C Low POTENTIAL SYSTEMS. PART II. Underground Conductors Inside Wiring, General Rules Insulation of Wires Minimum Size of Wires Tubes or Bushings Protection of Low Potential from High Potential Wires Joints in Wires and in Insulation, 73 to 83 CHAPTER IX. CLASS C Low POTENTIAL SYSTEMS. PART III. Special Rules Wiring Not Encased in Moulding or Approved Conduit Various Methods of Running Wires Methods Permitted by the Code, . 84 to 90 CHAPTER X CLASS C, Low POTENTIAL SYSTEMS. PART IV. Mouldings Special Wiring in Breweries, Packing Houses, Stables Dye Houses, Pulp Mills, etc., 91 to 96 CHAPTER XI CLASS C, Low POTENTIAL SYSTEMS PART V. Interior Conduits. Their Object and Their Evolution How They Should be Installed, . . 97 to 105 TABLE OF CONTENTS. VII. CHAPTER XII PAGES. CLASS C, Low POTENTIAL SYSTEMS PART VI Safety Cut-outs A System of Conductors How Pro- tected by Cut-outs Double -pole Cut-outs Fuse Blocks, . . . 106 to 113 CHAPTER XIII CLASS C, Low POTENTIAL SYSTEMS PART VII. Cut-outs and Cut-out Boxes Maximum Safe Carrying Capacity of Wires, (Table) The Heating of Wires Car- rying Electric Currents, . 114 to 123 CHAPTER XIV. CLASS C, Low POTENTIAL SYSTEMS. PART VIII. Switches Snap Switches Fixtures Combination Fixtures, . . . 124 to 131 CHAPTER XV. CLASS C, Low POTENTIAL SYSTEMS. PART IX. Arc Lamps on Low Potential Systems Electric Gas Lighting Lamp Sockets Flexible Cord, . 132 to 139 CHAPTER XVI. CLASS D, ALTERNATING SYSTEMS, CONVERTERS OR TRANSFORMERS. Direct and Alternating Currents Transmission of Energy by Electricity Transformers Primary and Secondary Circuits Transformers in Buildings, 140 to 149 CHAPTER XVII. CLASS E, ELECTRIC RAILWAYS. Ground Return Circuit Breakers Bonds Lighting and Power from Railway Circuits Electrolysis, 150 to 156 CHAPTER XVIII. CLASS F, STORAGE OR PRIMARY BATTERIES. Description of Primary and Secondary Batteries Points to be Observed in Installation of Storage Bat- teries, .... 157 to 164 VIII. TABLE OF CONTENTS. CHAPTER XIX. PAGES. MISCELLANEOUS. Insulation Insulation Resistance Dead Ground Short Circuit Insulation Resistance Required by Code, (Table) Ground Wires Protection of Telephone, Clock and Similar Circuits How to Secure High Insula- tion, . 165 to 174 CHAPTER XX. EDITION OF 1896. General Suggestions Changes from Code of 1895 Grounding of Frames of Direct Coupled Generators Lightning Arresters Motors Weather-proof Wire Interior Conduits Closed Arc Lamps, . 175 to 182 CHAPTER XXI. EDITION OF 1896. Changes from Code of 1895 Mechanical Protection of Wires Mouldings Iron-armored Conduit Fixture Work Flexible Cord Decorative Series Lamps Transformers in Buildings Car Houses Electric Heaters New List of Approved Wires The Object and Limitations of the Code, . . 183 to 196 APPENDIX TABLES AND CURVES, . .- 197 to 200 APPENDIX (CONTINUED) FULL TEXT OF THE UNDERWRITERS NATIONAL ELECTRICAL CODE, . . 201 to 222 The National Electrical Code, CHAPTER I. INTRODUCTORY AND DEFINITIONS OF COMMON TERMS. With the exception of those who are directly engaged in electrical employments, or are investing their money in electrical plants or enterprises, no one has a more direct interest in the proper construction of electrical work than the representatives of insurance companies. To-day the insurance agent duly appreciates if he does not even overestimate, the hazard that may be caused by faulty electrical construction. How to recognize this hazard when he sees it, and how to avoid it, is, however, a difficult and often an unsolved problem. When the question of safety is one of ordinary building construction, the hazard is easily seen, when once it has been pointed out, and the remedy is usually as easily described and understood. When we come to electrical matters, however, we often find that the mind of nearly every one, except the so-called electrical expert, is more or less befogged. An explanation does 10 THE NATIONAL ELECTRICAL CODE. not always explain, and even when one concludes that he has obtained a clear and satisfactory understanding of a hazard and its remedy, he is straightway confronted with some astounding statement or some apparently mysterious phenomena. It has occurred to us that possibly the principal reason why most persons have such an indefinite knowledge of even the simple things in the application of electricity is, that electricity has almost a language of its own. Every description of anything electrical by an electri- cian, is filled with such terms as "volts," "amperes," "ohms," "rheostats," "induction," "electro-motive force," etc., etc. Every day adds new words to this peculiar vocabulary, so that even the electrical engineer has to be well-read, to keep track of all these electrical terms. A good sized electrical dictionary has already been published, but this would have to be revised as often as the Chicago city directory to keep up with the inventive genius of the men who coin electrical terms. While these electrical terms are useful and necessary, still it seems to us that their great number has discour- aged many from the study of practical things which they wish to know, and which are easily understood if expressed or explained in common, every-day language. It is our intention in this and succeeding chapters, to discuss that part of electrical construction which has a direct bearing on the question of safety, and it will be our aim to do this, as far as is possible, in ordinary language, avoiding technical expressions when we can, and when they cannot be avoided, as must often be the case, we shall try to give a simple explanation or DEFINITIONS OF COMMON TERMS. II definition of the expressions, not going into a scientific discussion, or trying to give definitions which shall be logically and mathematically precise, but simply giving such explanation as will, we hope, enable the unscien- tific reader to form a conception of what is going on in a system of electrical wires, and to see clearly why one thing is safe and another is not. We do not propose to assume that the reader knows anything at all about electricity, and we offer our apol- ogies right here to any who may feel that their intelli- gence is insulted. We also apologize to those who have struggled to explain the same thing in technical language and mathematical formulae, for our audacity of under- taking a task at which so many of them have been unsuccessful. We are not prompted by conceit in our undertaking. Like all who have long been engaged in electrical work, we have been met daily, for years, with questions of all kinds from men who had no time to study technical works, but who wanted a simple expla- nation of something electrical. Usually we have been able to explain the matter in question in ordinary language to the satisfaction of the inquirer, and often we have had a man go away with the knowledge of the subject which answered his purpose perfectly, and was doubtless infinitely more satisfactory to him than our understanding was to us. We feel, therefore, that our undertaking is a laudable one, and if our success is not equal to our expectation, we hope that we shall, at least, help to open a path for others. It is, of course, impossible in this book, to cover the entire field of electrical construction or even the applications of 12 THE NATIONAL ELECTRICAL CODE. electricity that might interest our readers. We have, therefore, decided to take up the subject purely from the insurance man's point of view. We shall consider only electricity as it may or may not create a hazard. This subject has been carefully and very completely covered in the rules known as the "National Code." This code in its original form, or as revised and incor- porated into the regulations of the various insurance associations, is familiar to every one in insurance lines. It is, however, condensed in form, and full of technical terms. We have, therefore, thought it proper to take this code as our text. We shall endeavor to explain the code in every-day language, giving such definitions and explanations as will, we hope, enable any one to understand its meaning without the assistance of an electrical dictionary or electrical text books. We shall take up the various points in the same order that they are taken up in the code. The National Code is the outcome of experience. It is the result of a great amount of study and discussion. It stands to-day as the best expression of what is definitely believed by the ablest men of both insurance and electrical lines, and if we shall succeed in assisting any one to a better understanding and appreciation of its principles, \ve shall feel repaid for our efforts. The following defini- tions are absolutely necessary to describe the most common terms which constantly appear in the code. Electricity. We do not know what electricity is, and it is useless for us to try to define it. We perceive it as a manifestation of energy. All we need to know of its nature for our present use, is that we can transform the DEFINITIONS OF COMMON TERMS. 13 work of an engine or water-wheel into electricity, and that we can direct and regulate its distribution by wires or conductors, and transform it again into energy in the form of light, heat, or work, in moving a machine. There is but one kind of electricity, as far as we know, but it manifests itself to us in various ways. In the production of light, heat and power, we deal only with dynamical electricity, or electricity in motion. Electric Current. While we have to deal with elec- tricity in motion, still, as its nature is not known, its motion is but imperfectly understood. It is necessary for us, however, to have some theory to account for its actions. For all ordinary purposes, we may consider electricity to be a fluid, and to have a motion in a wire like water in a pipe. Following this analogy, we speak of a "current" of electricity. As with water in a pipe or a river, so with electricity, the amount of flow is proportional to the strength of the current, and to the time during which it flows. The rate of flow of elec- tricity is called "the current strength," or, more com- monly, the "current" It is measured by an arbitrary standard called an "ampere." The ampere is the unit of current strength. The total amount of electricity flowing will, of course, be measured in "ampere hours." For example, an ordinary 2,000 c. p. arc lamp, such as is commonly used in street illumination, requires a cur- rent of 10 amperes. If the lamp burns for 10 hours, the total amount of current consumed will be 100 ampere hours. When we speak of a wire carrying a current of 10 amperes, we mean that 10 units of current are flowing in the wire, and we use the expression in 14 THE NATIONAL ELECTRICAL CODE. the same way as if we were to say of water, that a cur- rent of 10 gallons per minute was flowing through a pipe. Electro-Motive Force Pressure or Potential. These terms are all used in the code to express the same thing. We will use the more common term of "elec- tro-motive force," which is commonly abbreviated to E. M. F. E. M. F. may be defined as a force which causes, or tends to cause, a current of electricity to flow. To use our same analogy, suppose we have a tank full of water; we shall have upon the bottom of a tank a pressure due to the head. If we bore a hole into the bottom of the tank, we shall immediately have a flow of water. If we place our hand over the hole, the pressure will still tend to cause a flow and the flow will be instantaneous as soon as the obstacle is removed. In electricity we measure the pressure by an arbitrary unit called a " volt." This corresponds with the pounds of pressure to the square inch with water. With water, the greater the pressure the greater will be the flow, the outlet remaining the same; so with electricity. The greater the E. M. F., the greater will be the current, provided other conditions remain unchanged. In fact, with electricity, the relation of flow to pressure is more simple than with water, for, other things remaining the same, if we double our pressure, we double our current, /. e., the current will be exactly proportional to the pressure; or the amperes will be proportional to the volts. Dynamo. A dynamo is a machine which, when driven by an engine or other source of power, trans- DEFINITIONS OF COM forms the work of the engine or prime mover into elec- tricity. It may be compared to a pump driven by a belt. When motion is transmitted to the pump, it sets up a pressure which will cause, or tend to cause, a flow of water. So, when a dynamo is set in motion by any mechanical means, an electrical pressure will be set up, and this is called the "pressure "or " E. M. F." of the machine. This pressure will tend to cause a flow of electricity, or an electrical current. A dynamo is a reversible machine, /. e., it can be used to generate a current of electricity, or, if the cur- rent of electricity is produced by another source, and sent through it, its armature or moving part will be set in rotation and it can be used as a source of power to drive other machines. When thus used it is called a "motor." The distinction between a dynamo and a motor is one of application, and not necessarily of con- struction. To make the distinction clear, it is now common to speak of a machine which generates elec- tricity as a "generator," and of one which transforms electricity into mechanical work as a "motor." The words "dynamo" and "generator" are used in the code to mean the same thing. Conductor. Unlike water, electricity does not flow most readily when not impeded by a solid substance. In fact, while no ordinary pressure will cause electricity to pass through the air, a small pressure may cause an immense current to flow through a mass of metal. We therefore say that metal is a conductor of electricity, and that air is a non-conductor. When we wish to direct a current of water from one 1 6 THE NATIONAL ELECTRICAL CODE. point to another, we provide a path free from solid obstruction and we confine the water through this path by some solid substance, such as the bank of a canal or a metal pipe. With electricity, however, we provide a metallic path, such as a copper wire, and the elec- tricity is kept from leaving the wire by the surrounding air, or some other non-conducting material which sep- arates the wire or metal path from other paths into which it might flow if there was no such barrier. Resistance. Although electricity will readily flow in a metal wire, a given pressure will not produce the same flow in wires of different metals, or in different sized wires of the same metal. Just as with water, a given pressure will not send the same amount through a small hole as through a large one, or through a long pipe of small diameter, as through a short one of large diame- ter, so a given E. M. F. will send a small current through a long, thin wire, and a strong current through a short and thick one. We explain the different results by saying that the long, thin wire offers a "resistance" to the flow of the current. This we call " electrical resist- ance," or simply "resistance." Resistance is measured in "ohms," an ohm being an arbitrary unit. Electrical resistance may be compared to friction in a pipe carrying a current of water ; the greater the pressure, and the less the friction, the greater will be the flow. So, in electricity, the greater the E. M. F., and the less the resistance in the path or conductor, the greater will be the current. Or, to express the same thing in electrical terms, the greater the voltage of our dynamo, and the less the resistance of our conductor DEFINITIONS OF COMMON TERMS. l^ in ohms, the greater will be the current in amperes which will flow through the conductor. Insulator. If we are to confine electricity to cur "conductor," we must separate it from other conduct- ors ; or, as we say, we must "insulate" it. This is accomplished by surrounding our conductor with a non-conducting substance, or by supporting it by a non-conducting substance in the air, which is itself a non-conductor. Any such non-conducting material used to support or surround a wire is called an insu- lator. In practice it is customary to speak of a non- conducting support as an " insulator," and of a non- conducting material surrounding a conductor or wire as "insulation." It is evident that the distinction between "insulators" and "conductors" is simply relative. A non-conducting or insulating substance is simply a sub- stance of very high resistance, but the resistance of materials used for insulation are so enormous, that we are justified in calling them "non-conductors." Of all insulators, dry air is the best, and dry glass the next best; rubber, porcelain, oil, shellac, mica, paper, cot- ton, silk, etc., are the substances most commonly used. As safety in electrical work depends solely upon the confining of the current to its proper circuit, the prob- lem of safety is very largely one of insulation, and it will be seen that the greater part of the "code" is devoted to specifying material and methods which will secure good insulation. Polarity. The flow of current from a dynamo is not exactly analogous to the flow of water from a tank, since to have a continuous current of electricity, we must l8 THE NATIONAL ELECTRICAL CODE. have a continuous conducting circuit; i. e., the current must tovt from the dynamo through the circuit and back again to the dynamo. In this respect the dynamo is more like our pump. We must continually supply water to the pump in order to have a continuous flow. Any electrical circuit must be continuous in order to have a current, and we may, if we like, imagine the condition similar to that of water flowing round and round in an endless pipe. We assume that any ordinary current flows always in the same direction, and that the current from a dynamo goes out from the machine on one conductor, and back to the machine on another. We express this by saying that these two conductors are of "opposite polarity " and the points where they join the dynamo we designate as the two "poles" of the machine. The pole where the current emerges is called the "positive" pole, and the one to which it returns is called the " negative " pole. " Polarity " is naturally relative, and we speak of any part of a circuit as being positive with reference to another, when the current flows from the first point to the second, or when the pressure tends to set up such a current. Other electrical terms which appear in the "code "will be defined as we come to them. CHAPTER II. CENTRAL STATIONS FOR LIGHT AND POWLR. PART I. TEXT OF CODE ON CENTRAL STATIONS. CLASS A. (These Rules also apply to dynamo rooms in isolated plants, connected with or detached from buildings used for other purposes; also to all varieties of apparatus therein of both high and low potential.) 1. GENERATORS: a. Must be located in a dry place, b. Must be insulated on floors or base frames, which must be kept filled, to prevent absorption of moisture, and also kept clean and dry. c. Must never be placed in a room where any hazardous process is carried on, nor in places where they would be exposed to inflam- mable gases, or flyings, or combustible material, d. Must each be provided with a waterproof covering. 2. CARE AND ATTENDANCE: A competent man must be kept on duty in the room where generators are operating. Oily waste must be kept in approved metal cans, and removed daily. 3. CONDUCTORS: From generators, switch boards, rheostats or other instruments, and thence to outside lines, conductors a. Must be in plain sight and readily accessible, b. Must be wholly on non-combustible insulators, such as glass or porcelain, c. Must be separated from contact with floors, partitions or walls through which they may pass, by non-combustible insulating tubes, such as glass or porcelain, d. Must be kept rigidly so far apart that they cannot come in contact, e. Must be covered with non- inflammable insulating material sufficient to prevent accidental contact, except that "bus bars" may be made of bare metal. f. Must have ample carrying capacity, to prevent heating. The name "Central Station" is applied to any elec- trical plant from which electricity is furnished for (19) 20 THE NATIONAL ELECTRICAL CODE. operating street lights or for supplying electricity for lamps or motors, in a similar manner to that in which a gas company furnishes gas for heat and light. By an " Isolated Plant " is meant an electrical plant for light- ing a single building or a number of buildings owned by one person or company. The distinction is not arbitrary, and it will be noted that the code lays down the same rules for the installation of machinery and apparatus in the dynamo room of an isolated plant as in a building devoted exclusively to an electric plant. So we will not try to give any more precise definition than the above. A Central Station has extra hazards due to its outside lines on poles or underground, which may accidentally come into contact with the ground, with one another or with the wires of other systems; or may, if improperly installed, conduct lightning into the station. In installing an isolated plant, on the other hand, equal care is necessary, as any hazard to the plant endangers the building in which it is located, and also the contents of the building. Before discussing electrical construction, let us con- sider how it is that an electrical current can cause a hazard. If a current flows in a wire, we have found that its flow is opposed by something analogous to fric- tion, which we have called resistance. The energy required to overcome this resistance is transformed into heat just as mechanical work expended to overcome friction is transformed into heat. The greater the cur- rent and the greater the resistance, the more energy will be absorbed in heating our wire. With a current of sufficient strength, we can heat any wire red hot, or, CENTRAL STATIONS. 21 for that matter, we can melt it. If, therefore, our con- ductors are so installed that an accident may load them with an excessive current, we may have enough heat generated to char their insulation, or even to set fire to the insulation or any adjacent combustible material. We can prevent undue heating by using wires of proper size for the required current, and protecting them from a greater current by safety devices (described later); and we can secure additional safety by installing the wires in such a manner that even if they do become excessively hot, the heat will not be conducted to com- bustible material. Again, if we have a wire carrying a small current from a dynamo, and if we open the cir- cuit, for example by cutting the wire, we will, upon separating the two ends, have a flash or " spark," as it is called. This spark gives out but little heat, but it will ignite an explosive mixture; we are all familiar with its use in electric gas lighting. If, however, the current be one of several amperes, when we separate the ends of our wires for a short distance, the current will flow across the gap. This is called an "electric arc." It is a flame, and its heat is so intense that it will melt steel like wax in the flame of a candle. It is this arc that gives the light in an " arc " lamp, and this is the thing we may get in a system of conductors when wires of opposite polarity come in contact with one another, or when any accident breaks a wire carrying a large current. The current required for six or eight ordinary incandescent lamps will maintain an arc equal to that of a small arc lamp. The accidental forming of an arc may be prevented 22 THE NATIONAL ELECTRICAL CODE. by proper insulation and workmanship. Fortunately, we can, by proper workmanship and material and the use of simple devices, install our conductors so that we can obtain light and power with less hazard from elec- tricity than from any other source. Poor insulation of conductors causes trouble by allowing the current to leave the conductor and flow through some other path. This improper path, like any other, must be continuous, i. e., the current will not leak from the wire to a con- ducting substance in contact with it unless there is another contact where it can return to the circuit. Two contacts to a conducting substance are necessary to set up a leak. The substance may be some metal or any substance which can absorb moisture. Although pure water is a poor conductor, water which is dirty or which contains any salt or acid, in solution, is a good conductor. Most of the non-metallic material used in building construction is non-conducting when dry, but will conduct the current readily when wet. The enemy of insulation is moisture. Generators. A dry location is especially necessary for a generator. If moisture condenses upon the dynamo while it is not running, it may destroy the insulation of the wire with which it is wound. If the wire is simply covered with a thin covering of cotton, as is the usual practice, leaks may be set up between the different wires, resulting finally in the burning of the insulation; or the insulation between the earth and the conductors on the machine may be destroyed, thus endangering both dynamo and circuits. Further, where machines of very high E. M. F. are used, a damp floor CENTRAL STATIONS. 23 or poorly insulated machine may endanger the life of the attendant, or at least discourage him from properly attending to the machine. As the current always flows in a closed circuit, we must have two "faults" in our insulation in order to have a leak. When we have one fault, the second may come at any instant, and the first fault may set up a strain upon a weak point in the insulation and help to develop a second one. The most common cause of leakage in a Central Station system is formed by the conductor of a circuit coming in contact with the earth. This contact is called a "ground." It is common usage to speak of the contact of a wire with any con- ducting substance as a " ground," and when we have such a contact, we say that the Wire is grounded upon the substance. For example, if a wire touched a gas pipe it would be grounded, and we would speak of it as being grounded on the pipe, even if the pipe were not connected to the earth. The mounting of a dynamo upon an insulating base or frame as required by the code is desirable in a light- ing plant, as an extra precaution. The wires upon the dynamo are insulated from the iron frame of the machine; but, as this insulation may be injured by an over-load, by lightning or by mechanical injury, the further precaution is taken of insulating the frame from the earth, as a leak from dynamo to earth may be more serious than one upon a circuit, as its extent is not lim- ited by safety devices. The observance of rule "c" concerning generators is important. An arc may be formed by the breaking of a wire, or by the intentional 24 THE NATIONAL ELECTRICAL CODE. opening of a circuit carrying current. It is impossible to guard against a spark in a dynamo room. The cur- rent in a dynamo is generated in the moving part or armature, and it is led to the circuit through an attach- ment to the armature called a " commutator " or "col- lector" (according to its design), the conductors of the circuit being electrically connected to the commutator or collector by stationary strips of copper or blocks of carbon called ' ' brushe s. " Any defect in design or workmanship, improper adjustment, overloading of the machine, or even the presence of dust, may at any time cause "sparking" at the brushes. The cover specified in rule " d " is to protect the machine while not in ser- vice from dirt and moisture, as from leaky pipes, defective roofs, etc. It may also prove useful to pre- vent damage by water in the extinguishing of an incip- ient fire. Care and Attendance. As a variety of accidents may cause a dynamo to spark so badly as to throw off sparks to a considerable distance, or may even cause the " burning out " of an armature, it needs no argument to show that an attendant ought always to be near. Any serious trouble on the circuit will usually be indi- cated in some manner in the dynamo room, and prompt inspection may discover a hazard in time to remove it without loss. The rule about oily waste should, of course, apply wherever waste is used. Its habit of indulging in spontaneous combustion, especially when it has a good chance to set fire to something else, is too well known to insurance men to call for comment. Conductors. Originally it was customary to place CENTRAL STATIONS. 25 electrical instruments and regulating and controlling devices either directly upon the wall of the dynamo room or, at best, to mount them upon a wooden board fastened to the wall. The board was called a " switch board." This name is now applied to any structure carrying instruments and regulating and controlling devices. In a dynamo room or station, all circuits from the dynamos are led to a main switch board, and thence the cunent is distributed by the necessary cir- cuits to the lamps or motors. The switch board con- trols the entire output of the plant, and often carries many regulating and controlling devices and a large amount of complicated wiring. Many of the fires in the poorly constructed stations of the past have origi- nated in the switch boards or their vicinity. It would seem evident that the wires to and from the switch board should be in sight and accessible; yet, in many plants, wires have been run from dynamo to switch board under wooden floors, and all the wires to and from the switch board have been crowded between the board and the wall in an inaccessible space. This has been done for appearance sake, it being easier for the constructor to conceal poor work than to do work in a mechanical and workmanlike manner. The unfortunate experience of insurance companies with central stations has been largely due to this kind of engineering. Rules " b " and " c " describe the safest possible kind of con- struction; i. the flow would be proportionate to the number of holes. If our tank is fed by a pump as before, the water being carried back into the tank through a pipe, we shall have a fair illustration of a "multiple arc" system. Our pump (dynamo) maintains a large rate of flow (amperes) at a constant pressure upon bottom of the tank of so many pounds to the square inch (volts). Alternating Circuit. Up to the present we have talked of electricity in the form of a current flowing, like water in a pipe, in one direction. We have, how- ever, to consider in electricity what is called an "Alter- nating Current," i. e., a current which flows in a wire first in one direction and then in the other direction. We can only use our water analogy by comparing this kind of current to what we would get provided we had a cylinder containing a piston and should connect the two ends of the cylinder to one another by a pipe. If now the cylinder and pipe are both full of water, we will get a flow in the pipe with each movement of the piston. If our piston moves back and forth, our cur- rent will first flow in one direction, then cease and then flow in the opposite direction. We do not, at this time, need to go further into a discussion of the nature of an "alternating current" than to give this simple analogy. The application of alternating current will be consid- ered more fully when we come to that part of the code which considers "alternating systems." For the pres- ent we need only say that the alternating current is used extensively for incandescent lighting, and to a limited extent for operating arc lamps and motors. The CENTRAL STATIONS. 43 object of using an alternating current for incandescent lamps is that it enables us to operate the lamps in mul- tiple arc, at a great distance from our dynamo, with the use of much smaller conducting wires than would be required for the same number of lamps if a direct current were used. The higher the pressure or voltage at which we transmit electricity, the smaller will be our wire for a given distance of transmission, and a given loss of energy. For example, suppose we wish to trans- mit electricity for 1,000 sixteen-candle electric lamps a distance of one mile with a loss of only 10 per cent, of our power in overcoming the resistance of our conduc- tors. If we undertake to run our dynamo at 120 volts, and supply direct current to our lamps at a pressure of no volts, our wire will be of enormous size, /.