Motion Picture Operation, Stage Electrics and Illusions / A Practical Hand-book and Guide for Theater Electricians, Motion Picture Operators and Managers of Theaters and Productions

As the wiring is always used in connection with gas piping, grounds are of frequent occurrence, and in order to indicate a ground as soon as it comes on, the bell and battery shown are provided. If a ground causes a continuous current, the spark coil will attract the armature, thus causing the bell to ring continuously.

National Electrical Code Rule for Gas Lighting.

Electric gas lighting, unless it is the frictional system, must not be used on the same fixtures with the electric light.

Grid Floor.

Grid Floor.—This is the term given to the framework which supports the pulleys over which the cables for handling curtains and scenery run. It is usually made up of parallel iron slats or bars; hence the name. A fairly good illumination should be provided here and all of the lights should be arranged on three-way switches. One light for every 400 square feet will be sufficient.

National Electrical Code Rule for Ground Clamps.

Connections to grounded pipes and to conduit must be exposed to view or be readily accessible, and must be made by means of approved ground clamps to which the ground wire must be soldered.

National Electrical Code Rule for Grounding.

The metal of conduit, armored cable, or metal molding must be permanently and effectually grounded to water piping, gas piping, or other suitable grounds, provided that, when connections are made to gas piping, they must be on the street side of the meter. If the conduit system consists of several separate sections, the sections must be bonded to each other, and the system grounded; or each section may be separately grounded, as required above.

Conduits and gas pipes must be securely fastened in outlet boxes, junction boxes, and cabinets, so as to secure good electrical connections.

If conduit, couplings, outlet boxes, junction boxes, cabinets, or fittings, having protective coating of non-conducting material such as enamel, are used, such coating must be thoroughly removed from threads of both couplings and conduit and such surfaces of boxes, cabinets and fittings where the conduit or ground clamp is secured in order to obtain the requisite good connection. Grounded pipes must be cleaned of rust, scale, etc., at place of attachment of ground clamp.

Connections to grounded pipes and to conduit must be exposed to view or readily accessible, and must be made by means of approved ground clamps to which the ground wires must be soldered.

Ground wires must be of copper, at least No. 10 B. & S. gauge (where largest wire contained in conduit is not greater than No. 0 B. & S. gauge), and need not be greater than No. 4 B. & S. gauge (where largest wire contained in conduit is greater than No. 0 B. & S. gauge). They shall be protected from mechanical injury.

The ground on the conduit system is not to be considered as a ground for a secondary system.

Guards.

Guards.—A guard rail should be provided around the stage switchboard to prevent actors from coming in contact with any live part of the switchboard. All incandescent lamps about the stage, dressing rooms, fly-floor, grid-floor, etc., should be provided with guards.

Hanger-Boards.

Hanger-Boards.—Hanger-boards are not compulsory but where not used, arc lamps must be hung from insulating supports other than their conductors.

National Electrical Code Rule for Hanger-Boards for Series Arc Lamps.

Hanger-boards must be so constructed that all wires and current-carrying devices thereon will be exposed to view and thoroughly insulated by being mounted on a non-combustible, non-absorptive, insulating substance. All switches attached to the same must be so constructed that they shall be automatic in their action, cutting off both poles to the lamps, not stopping between points when started, and preventing an arc between points under all circumstances.

Heaters.

Heaters.—Heaters are used sometimes in cold dressing rooms, in box offices and in other small spaces where other adequate heating arrangements have not been made. The fire hazard incident to the use of electric heaters is considerable and they should be carefully installed according to the rules given below.

From one-half to three watts per cubic foot will be required to heat a room. The quantity of heat necessary to be supplied depends largely upon the ventilation. It will be small in closed dressing rooms and large, for instance, in the box office. Always place a heater where the air enters a room; never where it leaves.

National Electrical Code Rules for Electric Heaters.

Each heater of more than 6 amperes or 660 watts capacity must be protected by a cut-out, and controlled by a switch or plug connector plainly indicating whether “on” or “off” and located within sight of the heater. Heaters of 6 amperes or 660 watts capacity, or less, may be grouped under the protection of a single set of fuses, provided the rated capacity of the fuses does not exceed 10 amperes; or may be connected individually to lighting circuits.

Flexible conductors for smoothing irons and sad irons, and for all devices requiring over 250 watts, must have an approved insulation and covering.

With portable heating devices, approved plug connectors must be used so arranged that the plug may be pulled out to open the circuit without leaving any live parts so exposed as to render likely accidental contact therewith. The connector may be located at either end of the flexible conductor or inserted in the conductor itself.

Smoothing irons, sad irons, and other heating devices that are intended to be applied to combustible articles, must be provided with approved stands.

Stationary heaters such as radiators, ranges, plate warmers, etc., must be so located as to furnish ample protection between the device and surrounding combustible material.

Must each be provided with a name-plate, giving the maker’s name and the normal capacity in volts and amperes.

High Potential.

High Potential.—The National Electrical Code classifies all voltages below 550 as low. Nevertheless voltages above 220 should not be considered in the auditorium, stage, or dressing rooms of any theater. And this voltage only in connection with a three-wire system where the high voltage exists only between the outside wires, and 110 volts are used for lamps and other devices. High potential systems should be used only on the outside.

Illumination.

Illumination.—Illumination is more an art than a science and the rules that can be given have only a very general application. The best practice, where really good illumination is desired, is to install a large number of circuits in proportion to the number of lights, so that lamps of large candle power may be used wherever desirable. The quantity of light needed in theaters varies greatly with the color of decorations and with their condition. Dust accumulates rapidly in theaters and may absorb half of the light. If there is plenty of capacity the candle power of lamps may be changed to suit conditions and obtain any result desired.

The number of sockets installed in the auditorium varies widely. In some of the cheaper theaters one light for each twenty seats is considered sufficient; while in elaborately lighted houses, there are cases where the number of sockets is equal to about half the number of seats. Good illumination requires that the light come from the back, but very often the purpose is to obtain a brilliant effect by placing lights in front of the audience. In such cases, however, the lamps are always of low candle power and should be frosted.

Incandescent Lamps.

Incandescent Lamps.—Practically all incandescent lamps are now rated in watts. They can be obtained in voltages ranging from 2 to 250 and may be operated in series or in multiple. All but the tantalum lamp operates equally well on alternating or direct current. The tantalum cannot be recommended for alternating-current circuits. No incandescent lamps will operate well on frequencies lower than forty cycles. The natural distribution of light is mostly in the horizontal plane and for good illumination reflectors should be provided.

The color value is in the following order: Tungsten, tantalum, graphitized filament, carbon filament. None are equal for color-matching purposes to the intensified arc or the Moore tube.

The life of all incandescent lamps varies inversely with the voltage. An increase in voltage will produce an increase in the efficiency of the lamp but shorten its life. The efficiency decreases with continued use, and it is generally considered that the useful life of a lamp is over when its efficiency has fallen to 80 per cent of its original value. Frosting or coloring shortens the life of lamps from 30 to 50 per cent and reduces the candle power from 3 to 10 per cent, but the lamp yields a more pleasing light. Frosting is ordinarily used only where lamps are placed so as to come in the line of vision. Bowl frosting does not materially reduce the life of a lamp.

Efficiency of Lamps.

With incandescent lamps the term “efficiency” has a meaning quite different from that usually given, it being the number of watts per candle power. The lower the efficiency of an incandescent lamp, the better it is and the more light it yields per watt consumed.

Mazda Lamp.

The Mazda lamp has an efficiency of about 1.25 and is the most efficient of all. It may be recommended in all places where lamps may be suspended vertically; where there is not too much jarring; where there is ordinary care in handling; and where the lamps are burning a large part of the time. The operating expense of this lamp is low, but the initial cost is high and the breakage is likely to be considerable. If lamps are much handled and not burned much, the cost of broken lamps may exceed the saving in energy. It is best not to clean Mazda lamps when cold. Shock absorbers should be used where there is much jarring. The illumination should be laid out for the use of lamps not smaller than 60 watts. The lamps should not be used for temporary work or for coloring. Arrange lamps as far as possible to be controlled by switches. Broken filaments can often be united again by shaking the lamps gently until the broken ends come together. The current will then weld them. When a bank of Mazda lamps is turned on there is an excessive current for an instant.

Tantalum Lamp.

The efficiency of this lamp is from 1.8 to 2 watts per candle power. This lamp is much used for street-car and similar illumination because it stands jarring very well. It should not be used on alternating-current circuits. The filament can often be united in the same manner as the Mazda.

Graphitized Filament Lamp.

This lamp has an efficiency of 2.5 watts per candle power.

Carbon Filament Lamp.

This lamp used on 110-volt circuits has an efficiency of from 3 to 3.2 watts per candle power. The efficiency of the smaller sizes is from 4 to 5 watts per candle power. The carbon filament lamp is the most expensive of all to operate, but on account of the strength and cheapness of the lamp it can be recommended in places where the breakage is liable to be great or where the light is used for comparatively brief periods.

National Electrical Code Rules for Incandescent Lamps.

Must be provided with guards when liable to come in contact with inflammable material or subject to rough usage. Must be protected by vapor-proof globes where inflammable gases exist.

National Electrical Code Rules for Insulating Joints.

When supported at outlets in metal conduit, armored cable, or metal molding systems, or from gas piping or any grounded metal work; or when installed on metal walls or ceilings, or on plaster walls or ceilings containing metal lath, or on walls or ceilings in fireproof buildings, fixtures must be insulated from such supports by approved insulating joints placed as close as possible to the ceilings or walls. The insulating joint may be omitted in conduit, armored cable, or metal molding systems with straight electric fixtures in which the insulation of conductors is the equivalent to insulation in other parts of the system, and provided that approved sockets, receptacles, or wireless clusters are used of a type having porcelain or equivalent insulation between live metal parts and outer metal shells, if any.

Where insulating joints are required, fixture canopies of metal must be thoroughly and permanently insulated from metal walls or ceilings, or from plaster walls or ceilings on metal lathing, and from outlet boxes.

Canopy insulators must be securely fastened in place, so as to separate the canopies thoroughly and permanently from the surfaces and outlet boxes from which they are designed to be insulated.

Inverted Lighting.

Inverted Lighting.—In this method of lighting the light is first thrown upward against the ceiling and then reflected back. This method can be used to advantage with light colored ceilings only. It is especially suited for low ceilings and with high ceilings the advantage disappears. The light obtained in this manner is very even and almost shadowless. Much light is lost through absorption but, owing to the fact that the light is of such an even quality, the eye readily accommodates itself to a lower quantity and the net increase in energy required to illuminate suitable spaces is not so very great. Those who wish to go into the subject of illumination thoroughly will find it treated fully in “Modern Illumination Theory and Practice”.

Joints.

Joints.—Methods of making joints are illustrated in Figure 164. Be careful not to overheat, especially at points where there is a strain on the wire.

National Electrical Code Rule for Joints.

Wires must be so spliced or joined as to be both mechanically and electrically secure without solder. The joints must then be soldered unless made with some form of approved splicing device, and covered with an insulation equal to that on the conductors.

Stranded wires (except in flexible cords) must be soldered before being fastened under clamps or binding screws; and, whether stranded or solid, when they have a conductivity greater than that of No. 8 B. & S. gauge they must be soldered into lugs for all terminal connections, except where an approved solderless terminal connector is used.

Junction Boxes.

Junction Boxes.—Junction boxes are installed in conduit systems for the purpose of facilitating the drawing in of the wire or of branching off from a main run. See “Conduit Work.”

Lamps.

Lamps.—See “Incandescent Lamps.”

Lobby.

Lobby.—The lobby generally requires a number of lights and the aim often is to create a lavish display. Very often a cut-out center is arranged at some convenient place. The exit and emergency lights must be controlled from the lobby. Sometimes outlets are provided for electric bulletin boards or small signs.

Lugs.

Lugs.—For solderless lugs, such as are used on rheostats or for arc lamps, see “Portable Stage Equipment.”

National Electrical Code Rule for Lugs.

For fuses rated over 30 amperes lugs, firmly screwed or bolted to the terminals and into which the conducting wires are soldered, must be used. On rheostats, resistances, etc., lugs will not be required when leads are provided as a part of the device.

Switches for current of over 30 amperes must be equipped with lugs, firmly screwed or bolted to the switch, and into which the conducting wires shall be soldered. For the smaller sized switches simple clamps can be employed, provided they are heavy enough to stand considerable hard usage.

Where lugs are not provided, a rugged double-V groove clamp is advised. A set screw gives a contact at only one point, is more likely to become loosened, and is almost sure to cut into the wire. For the smaller sizes, a screw and washer connection with up-turned lugs on the switch terminal gives a satisfactory contact.

National Electrical Code Rules for Metal Moldings.

Must not be used on stage side of proscenium wall.

Must not be used for circuits carrying more than 1,320 watts.

Wire used must be standard rubber covered, but may be single braid.

Must never be concealed or run in damp places.

Must not be used where the difference of potential exceeds 300 volts.

Must be continuous from outlet to outlet, to junction boxes, or approved fittings designed especially for use with metal moldings, and must at all outlets be provided with approved terminal fittings which will protect the insulation of conductors from abrasion, unless such protection is afforded by the construction of the boxes or fittings.

Such molding where passing through a floor must be carried through an iron pipe extending from the ceiling below to a point five feet above the floor, which will serve as an additional mechanical protection and exclude the presence of moisture often prevalent in such locations.

Where the mechanical strength of the molding itself is adequate, this ruling may be modified to require the protecting piping from the ceiling below to a point at least three inches above the flooring.

Where such moldings pass through a partition the iron pipe, required for passing through floors, may be omitted and the molding passed directly through, providing the partition is dry and the molding is in a continuous length with no joint or coupling within the partition.

Backing must be secured in position by screws or bolts, the heads of which must be flush with the metal.

Must have the metal of molding permanently and effectually grounded to water piping, gas piping, or other suitable grounds, provided that when connections are made to gas piping, they must be on the street side of the meter. If the metal molding system consists of several separate sections, the sections must be bonded to each other and the system grounded, or each section may be separately grounded, as required above.

Metal moldings and gas pipes must be securely fastened to outlet boxes, junction boxes, and cabinets, so as to secure a good electrical connection. Molding must be so installed that adjacent lengths of molding will be mechanically and electrically secured at all points.

If metal molding, couplings, outlet boxes, junction boxes, cabinets, or fittings having protective coating of non-conducting material such as enamel are used, such coating must be thoroughly removed from threads of couplings and metal moldings, and from the surfaces of boxes, cabinets, and fittings, where the metal molding or ground clamp is secured in order to obtain the requisite good connection. Grounded pipes must be cleaned of rust, scale, etc., at the place of attachment of the ground clamp.

Connection to grounded pipes and to metal moldings must be exposed to view, or readily accessible, and must be made by means of approved ground clamps, to which the wires must be soldered.

Ground wires must be of copper, at least No. 10 B. & S. gauge. They shall be protected from mechanical injury.

Must be installed so that for alternating-current systems the two or more wires of a circuit will be in the same metal molding. It is suggested that this be done for direct-current systems also so that they may be changed to the alternating-current system at any time, induction troubles preventing such change if the wires are in separate moldings.

Meters.

Meters.—A good job of meter setting requires that the meter fittings which are now on the market be used. Two separate meters will be required in each theater; one for the general lighting and one for the emergency system.

Meter Reading.

Meter Reading.—Meter readings are indicated by pointers, arranged to move over dials as shown in Figure 165. The various pointers are connected together by gearing in such a manner, that alternate pointers move in opposite directions, as indicated by the figures on the dials. The gearing moving the pointers in Figure 165 is of such proportions that a total revolution of any pointer represents one-tenth of a revolution of the pointer to the left of it. Thus ten revolutions of one pointer causes one revolution of the one at the left.

At the top of each dial the value of the reading of that dial is shown. Where the figures given are followed by the letter “s”, as in Figures 167 and 168, it signifies that each division of the dial represents the amount of energy indicated by the figures at the top. For instance, in Figure 168 each division of the dial at the right represents one-tenth of one kilowatt hour and a total revolution of the pointer ten-tenths, or one kilowatt hour.

If the figures given at the top of the dial are not followed by the letter “s”, or as shown in Figure 166, each division of the dial represents one-tenth of the amount shown at the top of the dial, the dial at the right of Figure 166 indicating nine-tenths of ten kilowatts or nine kilowatts.

The meter must always be read from right to left, the lowest indicating dial being the one at the extreme right, and the lower reading ones always being used to check the higher ones just at the left. The following example will illustrate the manner of reading meters:

In Figure 165 the right-hand pointer registers nine-tenths of 1000, or 900 watt hours; the pointer next to it registers eight, since it cannot be considered as fully up to any number unless the pointer at the right of it has just arrived at or passed 0. By the same token the middle pointer also registers 8 and as the middle pointer has not reached 0 the one at the left of it must be read as one, the last dial also indicates one and the total reading is 1,188,900. On some types of meters a multiplier is used. This is generally given on the meter dial and the readings given by the pointers must be multiplied by this number to obtain the correct reading of the meter.

Motors.

Motors.—Motors are used in theaters for ventilation, for raising and lowering the steel curtain in cities where such are required, and also in some cases for operating drop curtains, but for this last purpose they have not found much favor. In some localities motors are required to keep up a certain water pressure to be used in case of fire. Voltages higher than 550 are not considered in theater work.

National Electrical Code Rules for 550-volt Motors and Less.

Motors operating at a potential of 550 volts or less must be thoroughly insulated from the ground wherever feasible. Wooden base frames used for this purpose, and wooden floors, which are depended upon for insulation where, for any reason, it is necessary to omit the base frames, must be kept filled to prevent absorption of moisture, and must be kept clean and dry. Where frame insulation is impracticable special permission in writing may be given for its omission, in which case the frame must be permanently and effectively grounded.

The motor leads or branch circuits must be designed to carry a current at least 25 per cent greater than that for which the motor is rated. Where the wires under this rule would be over-fused in order to provide for the starting current, as in the case of many of the alternating-current motors, the wires must be of such size as to be properly protected by these larger fuses.

Each motor and resistance box must be protected by a cut-out and controlled by a switch; said switch plainly indicating whether “on” or “off”.

Small motors may be grouped under the protection of a single set of fuses, provided the rated capacity of the fuses does not exceed 6 amperes. With motors of one-fourth horse power or less, on circuits where the voltage does not exceed 300, single-pole switches may be used. 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.

Where the circuit-breaking device on the motor-starting rheostat disconnects all wires of the circuit, the switch called for in this section may be omitted. Overload-release devices on motor-starting rheostats will not be considered to take the place of the cut-out required by this section. An automatic circuit-breaker disconnecting all wires of the circuit may serve as both switch and cut-out.

Auto starters, unless equipped with tight casings enclosing all current-carrying parts, in all wet, dusty, or linty places, must be enclosed in dust-tight, fireproof cabinets. Where there is any liability of short circuits across their exposed live parts being caused by accidental contacts, a railing must be erected around them.

Must not be run in series-multiple or multiple-series, except on constant-potential systems, and then only by special permission.

Must, when combined with ceiling fans, be hung from insulated hooks, or else there must be an insulator interposed between the motor and its support.

Must each be provided with a name-plate, giving the maker’s name, the capacity in volts and amperes, and the normal speed in revolutions per minute.

All varying (or variable) speed alternating-current motors except those used for railway service must be marked with the maximum current which they can safely carry for 30 minutes, starting cold.

Terminal blocks when used on motors must be made of approved non-combustible, non-absorptive, insulating material such as slate, marble, or porcelain.

Adjustable-speed motors, unless of special and appropriate design, if controlled by means of field regulation, must be so arranged and connected that they cannot be started under weakened field.

The use of soft rubber bushings to protect the lead wires coming through the frames of motors is permitted, except when installed where oils, grease, oily vapors, or other substances known to have rapid deleterious effect on rubber are present in such quantities and in such proximity to motors as may cause such bushings to be liable to rapid destruction. In such cases hardwood properly filled, or preferably porcelain or micanite bushings must be used.

The following table shows the sizes of wire recommended to be used with motors of the horsepower given. This table is an extract from the rules of the Department of Gas and Electricity of the City of Chicago. The column headed “Mains” may be used when there are a number of motors fed by a single line. For all lines which supply a single motor only, the column headed “Branches” must be used.

The difference between the two is due to the fact that it is not believed that several motors fed by a single line will all be started at the same time; hence it is not necessary to provide the overload capacity for all of the motors as it is where but a single motor is installed.

Table VIII.
SIZE OF WIRES FOR MOTORS OF DIFFERENT HORSE POWER.

Music Stands.

Music Stands.—Music stands are used by the musicians and are generally placed between the first row of seats and the stage. Each musician should be provided with an individual stand, although if necessary, two or three can get along with one stand.

No first-class theater should be fitted up with less than twenty outlets for musicians’ lights. In the Metropolitan Opera House of New York there are one hundred. Houses used exclusively for vaudeville do not, however, need that many. In houses devoted to grand opera, often as many as one hundred or more musicians are employed at the same time. To take care of such a number, the music stands should be wired with pin-plug connectors so that connections may be made from one to the other. Long flexible connections are necessary for most music stands.

Sometimes it is necessary to crowd the orchestra under the stage and at other times, with musical comedies, for instance, they must be brought out where they can see the movements of the actors.

The more the circuits are subdivided, and the different lights made independent of each other, the better it will be and the less the annoyance in case a fuse blows. The fuses should always be arranged at the switchboard so that it will not be necessary for the electrician to work in front of the audience in cases of trouble. For this reason extra stands should also be kept on hand.

A main switch controlling the lights should be placed where one of the musicians can handle it. In dark scenes these lights must often be turned out. If these lights are left under the control of the stage electrician they will be more likely to be forgotten at the critical moment than if under the control of the men who need them.

An eight candle-power lamp for each stand will be sufficient and this is usually placed inside a special reflector which allows the light to fall upon the music sheet only. Stage cable of good quality should be used for the connections; there is too much rough handling for reinforced cord. The use of the ordinary attachment plug should be avoided; use approved pin-plug connectors.

Open Work.

Open Work.—Open work is not allowed in theaters.

Operating Room.

Operating Room.—See special chapter on “Operating Room.”

Panel Boards.

Panel Boards.—Panel boards are really small switchboards, the switches and cut-outs being mounted usually upon slate. The slate must be free of metal seams; these, if present, often manifest themselves by heating. Panel boards, unless located in the immediate vicinity of the main switchboard, and where they are enclosed in a compartment, must always be placed in standard metal cabinets.

National Electrical Code Rules for Panel Boards.

The following specifications are intended to apply to all panel and distributing boards used for the control of light and power circuits, but not to such switchboards in central stations, sub-stations, or isolated plants as directly control energy derived from generators or transforming devices.

Design.

The specifications for construction of switches and cut-outs given in the following pages must be followed as far as they apply.

In the relative arrangement of fuses and switches, the fuses may be placed between the bus-bars and the switches, or between the switches and the circuits, except in the case of service switches. When the branch switches are between the fuses and the bus-bars, the connections must be so arranged that the blades will be dead when the switches are open.

When there are exposed live-metal parts on the back of a board, a space of at least one-half inch must be provided between such live metal parts and the cabinet in which the board is mounted.

Spacings.

Table IX.
THE MINIMUM DISTANCE THAT MUST BE MAINTAINED
BETWEEN BARE LIVE METAL PARTS (BUS-BARS, ETC.)

At switches or enclosed fuses, parts of the same polarity may be placed as close together as convenience in handling will allow. It should be noted that the above distances are the minimum allowable, and it is urged that greater distances be adopted wherever the conditions will permit.

The spacings given in the first column apply to the branch conductors where enclosed fuses are used. Where link fuses or knife switches are used, the spacings must be at least as great as those given in the following section on fuse spacing. The spacings given in the second column above apply to the distance between the raised main bars and between these bars and the branch bars over which they pass. The spacings given in the third column are intended to prevent the melting of a link fuse by the blowing of an adjacent fuse of the same polarity.

Panel boards of special design, in which the insulation and separation between the bus-bars, and between the other current-carrying parts is secured by means of barriers or insulating materials instead of by the spacings given above, must be submitted for special examination and approved before being used.

Fuse Spacings.

Spacings must be at least as great as those given in Table X, which applies only to plain, open link fuses. The spaces given are correct for fuse blocks to be used on direct-current systems, and can therefore be safely followed in devices designed for alternating currents. If the copper fuse tips overhang the edges of the fuse block terminals, the spacings should be measured between the nearest edges of the tips.

A space must be maintained between fuse terminals of the same polarity of at least one-half of an inch for voltages up to 125 and of at least three-quarters of an inch for voltages from 125 to 250. This is the minimum distance allowable, and greater separation should be provided when practicable.

For 250 volts, boards, or blocks with the ordinary front-connected terminals, except where these have a mass of compact form equivalent to the back-connected terminals usually found in switchboard work, a substantial barrier of insulating material, not less than one-eighth of an inch in thickness, must be placed in the “break” gap—this barrier to extend out from the base at least one-eighth of an inch farther than any bare live part of the fuse-block terminal, including binding screws, nuts and the like.

For three-wire systems cut-outs must have the break-distance required for circuits of the potential of the outside wires.

Table X.
FUSE SPACINGS.

Switch Spacings and Dimensions.

When designed with 250-volt spacings between adjacent blades, triple-pole switches must be marked 250 volts and may be used on 3-wire D. C. or single-phase systems having not more than 250 volts between adjacent wires and not more than 500 volts between the two outside wires.

Spacings and dimensions must be at least as great as those given below:

Table XI A.
SWITCH SPACINGS AND DIMENSIONS
FOR SWITCH-BOARDS AND PANEL BOARDS.

Table XI B.
SWITCH SPACINGS AND DIMENSIONS
FOR INDIVIDUAL SWITCHES.

A 300-ampere switch with the spacings of the 200-ampere switch above may be used on switchboards.

Table XI C.
SWITCH SPACINGS AND DIMENSIONS FOR ALL SWITCHES.

Table XI D.
SWITCH SPACINGS AND DIMENSIONS FOR ALL SWITCHES.

A 300-ampere switch with the spacings of the 200-ampere switch above may be used on switchboards.

Cut-out terminals on switches for over 250 volts must be designed and spaced for 600-volt fuses.

Table XI E.
SWITCH SPACINGS AND DIMENSIONS FOR ALL SWITCHES.

Paint Bridge.

Paint Bridge.—The paint bridge is usually a scaffold which can be raised and lowered, and which serves the purpose of carrying scene painters at work on the curtains. A long strip is the best means of illumination and it must be connected with long stage cable so as to be portable.

Paint Room.

Paint Room.—Rooms in which paints are stored should not contain switches or cut-outs. Lamps should be incased in vapor-tight globes.

Program Board.

Program Board.—A simple form of program board is shown in Figure 169. Except at the top and the bottom, where three lamps are shown together in one compartment, each lamp is incased by itself. In front of the lamps is usually colored glass, bearing numbers or letters. The lamp behind any number being turned on, that number becomes visible. The top and bottom are usually labeled, “Special”, or “Extra”.