them. The basis for determining such carrying capacity shall be that given by the National Electrical Code for "allowable carrying capacities of wires."

All wiring, particularly that outside of locked or explosion-proof enclosures, shall have adequate mechanical and electrical protection to minimize gas-ignition hazards as well as fire hazards. If for any reason rigid conduit is unsuitable or undesirable a good grade of rubber air hose or equivalent may be construed as meeting the requirement for mechanical protection if used where it will not be damaged by engine heat and oil. Flexible metal conduit is not acceptable. All conduit ends must be adequately clamped or otherwise secured to prevent their being pulled out. Inserts should be used to prevent collapse of hose conduit ends that are secured by external clamps.

Sharp edges and corners shall be removed at all points where there is possibility of damaging the insulation of wires, cables, or conduits by cutting or abrasion.

Wiring and conduits shall be wellcleated or otherwise held to prevent vibration and displacement.

The ends and terminal lugs of wires and cables shall be held or clamped in a manner that will minimize the possibility of the ends and lugs coming loose from their connections and swinging against sides of enclosing casings or against parts of opposite polarity.

(5) Electrical clearances and insulation. The clearance between live parts `and casings of electrical equipment shall be such as to minimize the possibility of arcs striking to the casings, or if space is limited the casings shall be lined with adequate insulation.

(6) Detailed requirements for class 1 electrical parts; enclosure casings-(i) Materials and construction. The casings forming the enclosure for class 1 parts shall be of suitable material and especially durable in order that, with proper care and maintenance, the explosionproof qualities will remain unimpaired not only when subjected to pressures developed during explosion tests, but also under the severe conditions imposed by mining service. Sheet metal used in the fabrication of explosion-proof casings shall be at least 4 inch thick for any wall or cover having an area of 216 square inches or more (12 by 18), unless adequate reinforcing ribs, or their equiv

alent, are used to prevent deformation. Less than 16-inch thickness is not recommended. If welding is employed to join the side and wall pieces, the joints shall be continuously welded gas tight both inside and out.

Casings may be either of the totally enclosed type, in which no provision is made for ventilation of the interior, or else the type having provision for ventilation or relief of pressure from internal explosions. Totally enclosed construction, however, is recommended by the Bureau. Complicated castings and fabricated housings should be pressuretested at the factory to reveal blowholes and other weaknesses.

If provision is not made for pressure relief through special devices, the casing will need to be strong enough to withstand explosion pressures approaching 100 pounds per square inch. However, if a casing communicates with another through a small passage or is itself divided by a partition, the effect of "pressure-piling" may be produced, and pressures considerably in excess of 100 pounds per square inch may be anticipated.

The use of phenolic and other insulating materials that give off highly explosive gases when decomposed by electric arcs should be avoided in mounting live parts within explosion-proof enclosures.

(ii) Joints and machining tolerances. Where an explosion-proof enclosure consists of two or more metal parts that are held together by bolts or other suitable means, the flanges comprising the joints between parts shall have surfaces making metal-to-metal contact. Glassto-metal joints are permitted in casings such as those for headlights and meters. Gaskets, if adequate, may be used to obtain a firm seat for the glass, but not elsewhere. Rubber, putty, and plaster of paris are not acceptable as gasket materials.

The surfaces comprising a flange joint need not be all in one plane. For enclosures having an unoccupied volume (air space) of more than 60 cubic inches, the total width of joint measured along the shortest path from inside to outside of the enclosure shall be not less than 1 inch, except as follows:

A rabbet joint having a total width of 34 inch may be accepted if neither the cylindrical nor the plane fit is less than 16 inch wide, with a maximum radial clearance of 0.002 inch for the cylindrical

fit. When the unoccupied volume (air space) is less than 60 cubic inches, a minimum width of 34 inch may be accepted for plane joints, but a 1-inch width of plane or rabbet joint is recommended.

The width of blow holes in joint surfaces will be deducted in measuring flange widths. Diameters of holes for bolts or screws required to maintain tight joints will also be deducted in such measurement: (a) If excessive clearance (over 64-inch radial) is allowed for the bolt in its hole and, (b) if the diameter of the bolt hole is more than half of the required metal-to-metal contact. It is recommended that such holes be located so that the shortest distance along the joint from the interior of the enclosure to the edge of the hole is not less than 16 inch. However, less than 1/4 inch will not be accepted for 1-inch joints, nor less than 16 inch for joints under 1 inch. (Exception may be made for narrow interpoles, in which case the distance from the edge of pole piece to the bolt hole in the motor frame shall not be less than 8 inch and the diametrical clearance around the bolt shall be as stated in the next paragraph of this subdivision (ii). Furthermore, the pole piece shall seat against the frame surface.)

Bolts and screws shall be close-fitting in holes that cut through joint surfaces. If the edge of a bolt or screw hole is less than 16 inch from the interior of the enclosure, the diametral clearance around the bolt or screw shall not exceed 164 inch and this clearance shall be maintained for a distance of at least 1⁄2 inch as measured from the joint.

When the flanges of a joint cannot be brought into actual contact with each other owing to warping or faulty machining of parts or necessity for sliding fits, the requirement for metal-to-metal contact will be construed as having been met for plane flanges under the following conditions:

1. If the separation does not exceed 0.004 inch at any point.

2. If the 0.004-inch separation does not extend over 6 inches along the joint.

3. Provided the joint does not permit discharge of flame during the explosion tests.

When it is necessary in manufacture to provide for a running fit between cylindrical surfaces other than motor shafts, a shoulder shall be included in the design to provide a change in direction through the flame path between the

parts. In joints of this type, the diametral clearance between cylindrical surfaces shall be kept as small as feasible, but in no case shall it exceed 0.01 inch. • Laminated motor frames having end rings assembled as an integral part under high pressure may be considered with less width of contact between the end rings and laminations than that specified in the preceding paragraphs of this subdivision (ii). It is recommended that the metal-to-metal contact be kept as near the 1-inch standard as practical, but less than 4 inch will not be accepted. If less than the 1-inch standard width is used for joints of this type, the construction must permanently preclude any separation between the end rings and lamination, and if a 0.0015inch thickness gage can be inserted % inch at any point, the construction will be considered unsatisfactory. The joint should not open under explosion pres

sures.

(iii) Bolts and similar fastenings. Bolts and similar means of clamping flange joints together shall be generously proportioned to minimize stripping of threads and to give adequate strength. Steel inserts shall be used when it is necessary to thread screws or bolts into aluminum castings. Clamping bolts and screws should be at least 4 inch in diameter and preferably not less than 1⁄2 inch.

Unless the design permits of especially rigid construction between bolts, spacings greater than 6 inches are not recommended for flange joints.

All bolts, nuts, and screws used in fastening flange joints, as well as those used in holding parts such as pole pieces, brush rigging, and bearing caps, shall be provided with lock washers or other suitable means to prevent loosening. The length of threads in bottomed holes and on bolts, screws, and studs shall be such that the joint can be made tight even though lock washers are omitted.

(iv) Through holes for bolts, screws, and rivets. Through holes into explosion-proof casings shall be kept to a minimum. Holes for bolts, screws, etc., shall be "blind" or bottomed if the omission of a bolt or screw would leave an unprotected opening into the casing. If unavoidable, holes may be made through casings for bolts, studs, or screws that are necessary to hold essential parts such as pole pieces and brush rigging, providing the bolts, etc. have an adequate

long close fit through the casing and providing at least two holes, studs, or screws are used for each part held. In addition one of the following optional conditions shall apply: (a) Each hole must be bottomed in the part held and adequate metal-to-metal contact provided between the part and the casing to insure an effective internal seal around the hole in the event that the bolt or screw is omitted or lost, or (b) if studs are used they must be permanently fastened in the part held, or (c) bolts passing entirely through pole pieces must be arranged so that they cannot be removed without removal of the armature, or (d) special nonremovable bolts must be adequate for the intended purpose.

Holes shall not be drilled through walls of explosion-proof casings for screws or rivets holding name plates or approval plates.

(v) Inspection openings and covers. The number of openings in explosionproof enclosures shall not exceed the minimum required for proper assembly and inspection of parts. Openings such as those necessary for inspection of motor commutator and brushes are permitted if suitable covers are provided. These covers must have the width of flange joint previously specified or a threaded joint with sufficient threads to give the required width of surface in contact.

Screw covers and those held by special clamps and screws must be secured against unauthorized opening by means of a lock or a nonrusting wire and seal. Where the seal wire alone is of insufficient mechanical strength, an additional fastening such as a set screw or a pin secured by a seal should be used. The distance between the two holes through which the seal wire is threaded shall not exceed 3 inches.

(vi) Bearing and shaft clearances. Armature, controller, switch, and other shafts or rods carried through walls of explosion-proof enclosures do not require stuffing boxes. For plain journaled bearings the diametral clearance between the shaft and bearing shall not exceed 0.01 inch to provide for a running fit, and this running fit shall not be less than 34 inch long for an enclosure having an internal air space under 60 cubic inches, nor less than 1 inch for enclosures having more than 60 cubic inches air space.

Roller and ball bearings are not accepted as suitable barriers for stoppage

of flames, and therefore the flame path provided between a shaft and the inner parts of bearing housings shall not be less than 1 inch long for enclosures of more than 60 cubic inches air space. If the air space is 60 cubic inches or less, this part of the flame path may be reduced to 3/4 inch. In either case the diametral clearance shall not be greater than 0.03 inch at any point in the 34- or the 1-inch path. This clearance is allowed providing it does not permit discharge of flame.

Reduced clearances may be required for the fit of bearing cartridges, collars and other parts forming cylindrical flame paths in bearing housings. Flame paths having radial clearances of more than 0.005 inch should have one change of direction in them.

Oil grooves in bearings and felt rings or oil grooves in bearing housings are not to be included in the measurement of the length of running fit along a shaft. Such grooves are not allowed if not of sufficient volume to reduce the effectiveness of the path. Openings made for purposes of filling and draining bearings shall be outside of the required length of path.

Labyrinths or other special arrangements may be accepted in place of straight paths if they provide equivalent lengths and clearances and are made up of rugged parts not likely to be readily omitted. A removable outer bearing cap is not considered as part of the required length of fit.

(vii) Cable entrances. All electrical conductors that pass through the walls of explosion-proof enclosures shall be provided with adequate insulation and guards at the point of entrance to the enclosure in accordance with one or more of the following:

(a) If stuffing-box cable entrances are used, the packing material shall be an untreated asbestos, such as woven valve-stem packing, and it shall be not less than 16 inch in diameter. The size, length, and kind shall be specified on the drawings or bills of material. The amount of packing material used in each stuffing box shall be such that when compressed, it will completely surround the wire or cable for a distance of not less than 1/2 inch measured axially.

The stuffing-box design and the amount of packing used shall be such

that, with the packing properly compressed, the gland still has a clearance distance of 1⁄2 inch or more to travel before it meets interference by parts other than packing. The glands shall be secured against loosening. The use of insulating bushings in stuffing boxes is recommended, especially for voltages that exceed 250. When an outer braid insulation covering is used on wires and cables passing through stuffing boxes it should be made of asbestos or slowburning material.

The width of space for packing material shall not exceed the diameter or width of the uncompressed material by more than 50 percent. At other points small clearances shall be maintained between the stuffing-box parts and the cables or wires passing through them. A clearance greater than 3/64 inch (i. e., 3/32 inch diametral clearance) will not be accepted.

Corners shall be well-rounded at all points where cables and wires emerge from bushings, glands, and stuffing boxes to prevent cutting of insulation. Stuffing boxes, if not made integral with enclosures, shall be securely held to enclosures on which they are used.

Stuffing boxes and the fittings connected to them shall be so placed or guarded that they are not likely to be damaged in derailments and other accidents.

The diameter of cables used in stuffing boxes and the dimensions of openings for cables in stuffing boxes shall be given in decimal rather than common fractions.

(b) If insulated studs are used they shall be designed and spaced to minimize the possibility of electrical creepage to parts of opposite polarity or to the casing. Terminal lugs shall be keyed to their studs or else shielded by insulating barriers so that they cannot come into contact with each other or with any metal not of the same potential or polarity. Adequate means shall be provided to prevent loosening of the studs and lugs by vibration or by expansion and contraction. Special attention shall be given to the shielding of external stud connections so that they cannot be shortcircuited or grounded by accidental or careless contact or by water when the machine is properly assembled.

(c) Insulating tubes or bushings shall not be used alone to take wires and cables through walls and partitions or explosion-proof enclosures unless both

ends of each tube are wholly within such enclosures. The length of each tube and the clearance around the wire or wires in it should be such as to prevent "pressure-piling" in the event of flame passing through it. (In general, a diametral clearance of 16 inch should not be exceeded for single cables in tubes.) Bushings and tubes shall be secured against loosening and shall be of incombustible material.

(d) If wires and cables are taken through openings which are closed with sealing compounds, the design of the opening and characteristics of the compounds shall be such as to hold the sealing material in place without tendency of the material to crack or flow out of its place.

(e) An explosion-proof connection box may be used to facilitate connection to external circuits, providing the wires are closely fitted or sealed in the opening between the casing and the connection box in a manner adequate to inhibit flame propagation from the box to the enclosure on which it is mounted. A short metal tube or length of rigid conduit permanently secured to both may be used between the casing and the connection box when necessary. The connection box shall comply with all requirements of this subparagraph (6) for class 1 compartments and the lead entrance for external connection shall comply with the preceding paragraphs of this subdivision (vii).

(f) Lead entrances that are not designed to prevent passage of flame from enclosures may be used in making electrical connections between separated explosion-proof enclosures, provided the conductors are carried in rigid metal conduit or equivalent. This conduit shall be fastened securely to the enclosure and shall be sealed or sufficiently filled with conductors to prevent the propagation of flame through it.

(7) Parts having special requirements (i) Motors and generators. The internal construction of bearings for motors and generators shall be capable of preventing the escape of flame during explosion tests with the outer bearing caps removed, unless the outer caps are essential to hold the bearing in place. If the outer caps are essential, they shall comply with the constructional requirements of subparagraph (6) of this paragraph (i) for class 1 parts as to width of joints, fastenings, and through holes.

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(ii) Rheostats and resistors. lar attention shall be given to the choice of conductors used both inside and outside of enclosures for rheostats and resistors and to the type of cable entrances in order to prevent grounds and short circuits that might result from failure of insulation due to heat. Rheostats and resistors shall be so designed and proportioned that the temperature of the external surfaces of the enclosure does not exceed 400° F. at any point.

(iii) Meters and instruments. Meters and instruments that are actuated electrically shall be insulated from the explosion-proof casing in which they are enclosed. The glass in meter and instrument casings shall be at least 2 inch thick for diameters of 5 inches or more. For diameters under 5 inches, safety glass shall be used and the thickness shall be at least 4 inch. Meters and instruments shall be shielded by position or have a guard to protect the glass against damage.

(iv) Headlights. Headlights shall be mounted in protected positions where they are not likely to be damaged by passing objects. The glass in headlights shall be at least 2 inch thick.

(v) Push buttons and push-button stations. Push rods passing through walls of explosion-proof casings shall not be less than 1/4 inch in diameter. They shall have a shoulder, head, or equivalent at the inside end to prevent accidental loss or removal from the outside. Cotter pins or parts held by cotter pins are not acceptable as means of preventing this loss or removal.

The diametral clearance between the push rod and its hole shall not exceed 0.01 inch to provide for sliding fit, and this sliding fit shall not be less than 34 inch long for an enclosure having an internal air space under 60 cubic inches, nor less than 1 inch for enclosures having more than 60 cubic inches air space. In either case, the required length of sliding fit shall not be decreased when the button is depressed.

When it is important that accidental operation of push buttons be prevented, the Bureau reserves the right to require suitable guards or shields for the protection of the external ends of push buttons.

(8) Class 2 electrical parts having special requirements—(i) Battery boxes and batteries. Battery boxes shall be made of material equivalent in strength to

sheet steel not less than 16 inch in thickness or of wood reinforced with steel, and shall have a substantial cover or covers lined with nonbrittle insulation of adequate strength, quality, and dimensions. The cover or covers shall be provided with suitable means for locking them in the closed position to prevent opening by unauthorized persons.

Battery boxes shall be provided with means for ample ventilation to prevent accumulation of explosive hydrogen-air mixtures above the battery. Ventilating openings shall be guarded to prevent access to the cell terminals from the outside.

Unless the battery cells are insulated from the trays in an acceptable manner, the trays shall be insulated from the battery box with rubber or equivalent insulators of adequate dimensions. For cells in metal containers mounted in "open"-type trays a lining of wood or equally suitable insulation shall be provided for the bottom of the battery box. All wood and other insulating linings shall be treated or painted with suitable material to resist destruction by battery electrolyte.

The number, type, rating, and manufacturer of the cells comprising the battery shall be specified.

A diagram showing the connections between cells and between trays shall be submitted. The connections shall be such that the maximum total battery potential will not be placed between any two adjacent cells.

$31.5 Methods of testing-(a) Tests of locomotive parts other than electrical (1) Detailed inspection. An inspection will be made by engineers of the Bureau of Mines of all parts of the locomotive covered by the requirements of this Part 31 or any other parts or features that are associated with safety in operation. This inspection will include the following items:

(i) A detailed inspection to determine the adequacy of materials, workmanship, and design.

(ii) A detailed check of parts or assemblies against drawings as to materials, dimensions, and position, making notations for necessary correction of any discrepancies that may exist between the drawings and the parts or assemblies.

(iii) Measurements of joints, flanges, and other possible flame paths in the intake and exhaust systems of the engine.