be well done, and that unless good insulation be provided the cases be definitely grounded. The object of an air-tight enclosure is to prevent smoke from escaping or fire from spreading, in case the transformer coils should become overheated from an overload or should be ignited by a break-down in the insulation between the primary and secondary coils. This is especially important with oilcooled transformers in which the danger from an oil-fire is added to the usual electrical hazards involved.

Wherever high-potential circuits are brought into a building only rubber-covered wire should be used and special care should be taken with the wire supports to protect the wires from mechanical injury since a failure resulting in an arc is very dangerous at this high voltage. Substantial boxing about wires on side walls and running boards where circuits cross floor timbers are very necessary, as, in fact, are all the precautions and rules for good wiring and good workmanship throughout. A very large number of motors are now used taking 2,200 to 2,500 volts at the motor terminals. Such power installations can be made reasonably safe by careful planning of the circuits and by excellence of installation and upkeep of all wiring and connected apparatus. No multiple-series or series-multiple system of lighting is allowed on high potential circuits.

Extra high-potential circuits are not allowed either in or over buildings except power stations and substations. Where extra high-potential primary circuits (over 3,500 volts) supply transformers the secondary wiring must be installed as high-potential circuits (550 to 3,500 volts) unless the primaries are installed in complete compliance with the rules governing outside work on constantpotential pole lines over 5,000 volts or are wholly underground within city, town, and village limits.

In concluding the consideration of these higher voltage circuits, it is proper to refer again to the fact that the higher the voltage employed the greater the need of care in installation, upkeep, and operation since the results of accidental arcs are more serious.

SIGNALING SYSTEMS

Wiring Requirements. The wiring and other devices in the great variety of signal systems now employed in buildings, present as a general rule no hazard except their liability to become crossed

either outside or inside buildings with electric light, heat, or power circuits. Such signal systems include telephone, telegraph, district messenger and call bell, fire and burglar alarm, and all similar appliances and circuits. It is seldom that the wires of any of these systems are installed in buildings with the same care as are those for lighting or power, and the insulations employed on signal wires are very generally far inferior to those specified for light and power circuits. Furthermore, since signal systems are usually operated from either primary or secondary (storage) batteries of low voltage and limited current capacity, they may be and commonly are installed with little attention to separation of the wires, either from each other, or from the surfaces, walls, and floors to which they are attached. It follows, therefore, that all care should be taken to prevent light and power wires carrying currents of large capacity and relatively high voltages from coming in contact with signal wires since in such event dangerous fires might very readily be caused. The same advantages in having wires underground instead of being placed on poles apply to signal as well as to light and power circuits, but the two classes should never occupy the same underground duct, manhole, or handhole even when cables are used, since in the mechanical work or repairs on the lines an injury resulting in a cross between the systems might cause a dangerous current from the higher voltage lines, to outer buildings, over the weakly insulated and poorly protected signal wires. The liability of accidental crossing of overhead signaling circuits with electric light and power circuits may be guarded against to a considerable extent by endeavoring to keep the two classes of circuits on different sides of the same street. The Code prescribes that signal wires on pole lines also carrying electric light or power wires shall generally be placed on the lower cross-arms. This arrangement is not, however, favored by many engineers or by all municipal authorities, who prefer that signal wires be put on the top arms or on extensions above the tops of the poles. The arguments in favor of putting the signal wires lowest on the poles are that they are by many judged to be more liable to break and fall, especially when loaded with sleet, because of their lesser size and strength; also in case of city fire alarm circuits which are, of course, very important, the lower position enables linemen making repairs to get at the fire-alarm wires without

passing up through the light and power wires which may be charged with dangerous voltages.

The arguments in favor of putting signal wires at the top are— they are small and, therefore, collect less ice or sleet and so are less liable to break; the light and power wires are often better insulated and a signal wire breaking is less liable to make a real live contact with them; the fall of a heavy power cable may wreck all of the signal wires if the latter are below; in the case of city fire-alarm circuits the upper location removes them from misuse and injury when wiremen are working on the other lines. However, it may be said that there is no general agreement either in theory or practice on this subject.

Single wires of signal circuits on the outside of buildings should have rubber insulation and where attached to frame buildings should be secured to glass or porcelain insulators or knobs. Only copper wire should be used for the span from the last pole to the building and the wires should pass through outside walls through insulating bushings and have drip loops the same as electric light wires. Inside of buildings neat arrangement and secure fastening of all wires is essential to keep them properly placed and no signal wire should come nearer than three inches to any light or power wire, unless separated therefrom by some continuous and firmly fixed non-conductor creating a permanent separation, this non-conductor to be in addition to the regular insulation on the wire as the wires would ordinarily be insulated, but the kind of insulation is not specified, as the protector described below is relied upon to stop all dangerous currents. Porcelain tubing, approved flexible tubing, or rigid conduit may be used for encasing wires where required as above. Wires where bunched together in a vertical run within any building should have a fire-resisting covering sufficient to prevent them from carrying fire from floor to floor unless they are run either in noncombustible tubing or in a fireproof shaft, which shaft should be provided with fire stops at each floor.

Signaling wires and electric light or power wires may be run in the same shaft, provided that one of these classes of wires is run in non-combustible tubing, otherwise the two classes of wires should be separated from each other by at least two inches. In no case should signaling wires be run in the same tube with light or power wires.

Protecting Devices. In signal installations where the currentcarrying parts of the apparatus installed are capable of carrying indefinitely without overheating a current of ten amperes (as in some telegraph or special systems) the inside wires should be of copper at least as large as No. 16 B. & S. gauge and must have the same insulation and be supported the same as electric light or power wires for 600 volts. At the entrance to the building each wire should be protected by a 10-ampere 600-volt fuse. Such signal circuits as the above are much less common than those not suited for a 10ampere current. Telephone, district messenger, private watchmen's time recorders, burglar alarms, and fire-alarm circuits, are never capable of carrying 10 amperes continuously and for these a special "protector" is required located as close as possible to the entrance of the building. The purpose of this protector is to prevent any foreign current or any lightning discharge from entering the building over

GROUND

Fig. 135. Circuit Showing Proper Placing of Protecting Devices

the signal wires. For telegraph circuits this protector takes the form of a 2,000-volt fuse in each wire. The commoner "protector" such as is used on telephone lines should have the following parts mounted on a porcelain or slate base on which all parts are well insulated: a lightning arrester which will operate at 500 volts or more from a ground wire not less than No. 18 B. & S. gauge; a fuse in each side of the circuit which will blow with small currents (to 8 amperes) and will operate well on the voltages likely to reach the protector in case of accident. Where very sensitive instruments are in the circuit, such as contain magnet windings which are easily overheated, there must be a heat coil on each side of the line. The heat coil is designed to warm up and melt out with a current large enough to endanger the instruments if continued for a long time, but so small that it would not blow the fuses ordinarily found necessary for such instruments. The smaller currents are often called sneak currents. On

those telephone circuits which are supplied with current entirely from the central telephone headquarters sneak or heat coils are not

Fig. 136. Common Form of Telephone Protector

necessary. The fuses must be so placed as to protect the arrester and heat coils, and the protector terminals must be plainly marked line, instrument, or ground.

The relative arrangement of parts is shown in diagram in Fig. 135 and a common form of protector in Fig. 136.

TESTING

Where possible, two tests of the electric wiring equipment should be made, one after the wiring itself is entirely completed, and switches, cut-out panels, etc., are connected; and another one after the fixtures have all been installed. The reason for this is that if a ground or short-circuit is discovered before the fixtures are installed, it is more easily remedied; and also, because there is no division of the responsibility, as there might be if the first test were made only after the fixtures were installed. If the test shows no grounds or short-circuits before the fixtures are installed, and one does develop after they are installed, the trouble, of course, is that the short-circuit or ground is one or more of the fixtures. As a matter of fact, it is a wise plan always to make a separate test of each fixture after it is delivered at the building and before it is installed.

While a magneto is largely used for the purpose of testing, it is at best a crude and unreliable method. In the first place, it does not give an indication, even approximately, of the total insulation resistance, but merely indicates whether or not there is a ground or short-circuit. In some instances, moreover, a magneto test has