used to polish commutators or slip rings. Emery cloth should not be used on commutators since particles from the cloth may cause shorting and burning.

(i) Batteries and battery containers. The drain and venting provision for the battery or battery containers should be checked frequently.

(2) Miscellaneous check items. Frequent checks should be made for miscellaneous irregularities such as loose terminal connections, poorly soldered or loose swaged terminals, missing safety wire, loose quick-disconnects, broken wire bundle lacing, broken or inadequate wire clamps, and insufficient clearance between exposed current-carrying parts and ground. Replacement or repair should be accomplished as a part of routine maintenance.

(c) Major adjustments. Major adjustments of items of equipment such as regulators, generators, contactors, control devices, inverters, and relays should be accomplished outside the airplane on the test stand or test bench where all necessary instruments and test equipment are at hand. The adjustment procedures outlined by the equipment manufacturer should be followed.

(d) Equipment replacement. Damaged, wornout, and defective electric equipment should be replaced with identical items or with equipment equivalent to the original in operating characteristics, mechanical strength, and the ability to withstand the environmental conditions encountered in the operation of the airplane.

(e) Aircraft electric cable installation (1) Types of electric cable. Aircraft service imposes severe environmental conditions on electric cable. To assure satisfactory service, the cable should be of aircraft quality at least equivalent to that specified in Military Specification MIL-W-5086 (Copper) and MIL-W-7072 (Aluminum).

(2) Size of electric cable-(i) Criteria for selection. The criteria upon which the selection of electric cable size should be based, when considering an alteration, are current carrying capacity and voltage drop.

(a) The selected cable should not carry current continuously or intermittently in excess of the ampere values indicated by curves 1, 2, and 3 on figure 12-1.

(ii) Electric cable chart. This chart, figure 12-1, applies to cable carrying direct current and is based on copper conductor cable manufactured in accordance with Specification MIL-W-5086, whose current ratings are given in Specification MIL-W-5088. Curves 1, 2 and 3 thereon intersect the vertical cable size lines at the maximum ampere rating for the specified conditions indicated on the chart.

(a) Examples of how to use the electric cable chart-Figure 12-1-(1) Knowing the cable length and ampere load. Determine the required cable size so as not to exceed one volt drop as fol- | lows: Select the cable length from the scale at the left and follow it horizontally across the chart to the right until it intersects the required diagonal ampere line. Then read the cable size on the nearest or preferably the nearest vertical cable size line to the right.

Example. Measured cable length 50 feet, continuous current 25 amperes-determine cable size. From the left scale follow horizontal line 50 chart to the right until it intersects the diagonal 25-ampere line. The 25-ampere line is slightly more than midway between the 20- and 30-ampere lines since the scale is logarithmic. The vertical cable size to the right of this intersection is numbered 8, and therefore a No. 8 cable size will be needed. Note also that the point of intersection is above curve 1, indicating that No. 8 cable wire will carry 25 amperes in conduit or bundles without overheating.

(2) Knowing the cable size and ampere load. Determine the maximum

cable length so as not to exceed one volt drop as follows: Select the cable size from the scale at the bottom of the chart and follow the vertical cable size line until it intersects the required diagonal ampere line. Then read the maximum distance in feet that the cable can be run, by horizontally projecting the point of intersection to the scale at the left.

Example. Cable size No. 2, continuous current 150 amperes, determine maximum cable length in feet. From the bottom scale follow the No. 2 vertical cable size line until it intersects the diagonal 150-ampere line. Projecting this point horizontally to the scale at the left it is determined that 38 feet is the maximum distance that the No. 2 cable carrying 150 amperes can be run without exceeding one-volt drop. It should be noted, however, that the point of intersection falls below Curve 1 and if the cable is to be installed in a close fitting conduit or even a large bundle it would be preferable to use a No. 1 or No. 1/0 cable, depending on the known factors of the installation.

Naturally the maximum distance that these larger cables can be run without exceeding one-volt drop will also be greater than that previously determined for the No. 2 cable.

(3) For other than one-volt drop. Examples. Determine cable size for various voltage drops, measured cable length 100 feet, continuous current 20 amperes; also determine maximum cable lengths in feet for various voltage drops, using cable size No. 10, continuous current 20 amperes.

(iii) Resistance. The resistance of the current return path through the aircraft structure is always considered negligible. However, this is based on the assumption that adequate bonding of the structure or a special electric current return path has been provided which is capable of carrying the required electric current with a negligible voltage drop. The measured resistance from the ground point of a generator or the battery to the ground terminal of any electric device should not exceed 0.005 ohm.

(feet)

choice of commercial terminals may lead to overheated joints, vibration failures, and corrosion difficulties.

terminals.

(i) Solder vs. solderless The solderless (crimp-type) terminals have largely replaced the older solder lugs for most applications. Some of the disadvantages of the soldering process are listed as follows:

(a) A more skilled operator is required. (b) A corrosive flux may be used, and the terminal joint will rapidly deteriorate.

(c) Maintenance is extremely difficult.

(d) The cable strands are stiffened by the solder, and become more susceptible to breakage by vibration.

(e) The cable insulation may be charred during the soldering process.

The

(ii) Solderless terminal joints. terminal manufacturer will normally provide a special crimping or swaging tool for joining the solderless terminal to the electric cable. Aluminum cable presents special difficulty in that each individual cable strand is insulated by an oxide coating. The oxide coating must be broken down in the crimping process and some method employed to prevent its reforming. In all cases, the terminal manufacturer's instructions should be carefully followed.

(iii) Attachment of terminals to studs. Electrical equipment malfunction has frequently been traced to poor terminal connections at terminal boards. Loose, dirty, or corroded contact surfaces will produce localized heating which may ignite nearby combustible materials, or overheat adjacent cable insulation to the smoking point. Heavy currentcarrying connections should be available for periodic inspection to determine their condition.

(4) Terminal strips. Cable runs are usually joined at terminal strips. The terminal strip should be fitted with barriers to prevent terminals on adjacent studs from coming in contact with each other. The studs should be anchored against rotation and be long enough to accommodate a maximum of four terminals. When more than 4 terminals are to be connected together, 2 or more adjacent studs should be used, and a small strip-metal bus mounted across the studs. In all cases, the current should be carried by the terminal contact surfaces, and not by the stud itself.

(1) Terminal strip stud sizes. If the stud size is too small, it is easily sheared during servicing by applying too much torque on the nut. After a few failures of this sort, the electrician will become overcautious and not tighten the nut sufficiently, and a hazardous loose connection will result. Consequently, it is good practice to limit stud sizes to No. 10 or larger.

(ii) Terminal strip installation. Terminal strips should be designed or mounted in such a manner that 100se metallic objects cannot fall across the terminals or studs. It is good practice to provide at least one spare stud for future circuit expansion, or in case a stud is broken.

(5) Connector assemblies. Connectors (plugs and receptacles) are used to facilitate maintenance when frequent disconnection is required in service. Since the cable is soldered to the connector inserts, the joints should be individually insulated and the cable bundle firmly supported to avoid damage by vibration. Connectors have been particularly vulnerable to corrosion in the past, due to condensation of moisture within the shell. Special connectors with water-s proofing features have been developed t and a chemically inert water-proof jelly s is sometimes packed in the connector, to combat the corrosion difficulty.

(i) Connector assembly application. When two or more connectors are installed adjacent to each other, the design should be such that a plug cannot be inserted in the wrong receptacle. socket-type insert should be used on that half of the connector which is "hot" after the connector is disconnected.

The

(6) Through bolts. Through bolts are sometimes required to make feeder connections through bulkheads, fuselage skin or firewalls. Such bolts should be mounted in such manner that they are mechanically secure independent of the terminal mounting nuts. Sufficient cross-section should be provided to insure adequate conductivity against overheating, and the contact surface area should be large enough to minimize voltage drop. Particular care should be exercised to avoid dissimilar metals among the terminal mounting hardware.

(7) Splices in electric cable. Splicing of electric cable should be kept to a minimum, and avoided entirely in locations subjected to extreme vibrations. Sol

1

dered splices are particularly brittle and should not be used. When a mechanical (crimped or swaged) splice is used, it should be covered by insulating tubing which is supported at both ends to prevent any motion which will tend to disconnect the splice. Multiple splices in a cable bundle should be staggered along the cable run.

(8) Wiring installation practice-(1) General. Electric wiring may be installed in aircraft without special enclosing means (open wiring) or may be confined in conduit or ducts to provide additional mechanical protection. Open wiring offers the advantages of ease of installation, simple maintenance, and reduced weight. However, conduit or ducting (preferably made of an insulating fire-resistant material) should be considered for the following situations.

(a) To minimize the possibility of a cable fault which would result in the loss of the electrical system, or render essential electrical equipment inoperative.

(b) To protect the cable from detrimental substances such as hydraulic fluid or gasoline.

(c) To protect the cable from abrasion or damage by moving aircraft elements, such as aircraft control cables or shifting cargo.

installation—(a)

(ii) Open wiring Cable bundles. To simplify maintenance and to minimize the damage that may result from a single fault, cable bundles should be limited as to the number of wires in the run. Shielded cable, ignition cable, and cable which is not protected by a circuit breaker or fuse should be routed separately. The bending radius should not be less than 10 times the outer diameter of the bundle, to avoid excessive stresses on the cable insulation.

(b) Insulating tubing. Soft insulating tubing (spaghetti) cannot be considered as mechanical protection against external abrasion of cable, since at best it provides only a delaying action. Conduit or ducting should be employed in such cases.

(c) Clamping of cable bundles. Clamps, preferably of non-metallic material, should be used to support the cable bundle along the run. Lacing may be used between clamps, but should not be considered as a substitute for adequate clamping. Adhesive tapes are

subject to age deterioration and there-fore should not be used as a clamping means.

installation.

(d) Clamp Clamps should be installed in such manner that the cables do not come in contact with other parts of the aircraft when subjected to vibration. Sufficient slack should be left between the last clamp and the electrical equipment to prevent strain at the cable terminals, or to minimize adverse effects on shock-mounted equipment. Where cables pass through bulkheads or other structural members, a grommet or suitable clamping should be provided to prevent abrasion.

(e) Separation from flammable fluid lines. An arcing fault between an electric cable and a metallic flammable fluid line may puncture the line and result in a serious fire. Consequently, every effort should be made to avoid this hazard by physical separation of the cables from lines or equipment containing oil, fuel, hydraulic fluid, or alcohol. When separation is impractical, the electric cable should be placed above the flammable fluid line and securely clamped to the structure. In no case should the cable be clamped to the flammable fluid line.

(iii) Conduit installations. Conduit is available in metallic and non-metallic materials and in both rigid and flexible forms. Primarily its purpose is for mechanical protection of the cable within, although some radio interference shielding may be provided.

(a) Size of conduit. When selecting conduit size for a specific cable bundle application, it is common practice to allow for ease in maintenance and possible future circuit expansion by specifying the conduit inner diameter about 25 percent larger than the maximum diameter of the cable bundle. Large conduit sizes should be avoided, since simultaneous damage to many cables is possible and maintenance becomes difficult.

(b) Conduit fittings. From the abrasion standpoints, conduit is vulnerable at its ends. Suitable fittings should be applied to conduit ends, in such manner that a smooth surface comes in contact with the cable within. Conduit should be supported by clamps along the conduit

run.

(c) Conduit installation. Many of the past troubles with conduit can be

avoided by proper attention to the following design details:

(1) Conduit should not be located where operating or maintenance personnel would use it as a hand-hold or foot step.

(2) Drain-holes should be provided at the lowest point in a conduit run. Drilling burrs should be carefully removed.

(3) Conduit should be adequately supported to prevent chafing against structure, and to avoid stressing its end fittings.

(iv) Wiring identification. To facilitate installation and maintenance, all wiring should be indelibly marked for identification. Any consistent numbering system is considered adequate. The identification marking should be placed at each end of the cable, and also, preferably, at intervals along the length.

(f) Junction boxes and enclosures(1) Junction box construction. Junction boxes should be made from a fireresistant, nonabsorbent material, such as aluminum alloy or an acceptable plastic material. Where fireproofing is necessary, a stainless steel junction box is recommended. A rigid construction will prevent "oil-canning" of the box sides, which may result in internal short circuits. In all cases drain-holes should be provided.

(2) Internal arrangement. The junction box should be designed to permit easy access to all installed items of equipment, terminals and cable. Where marginal clearances are unavoidable, an insulating material should be interposed between current carrying parts and any grounded surface. It is not good practice to mount equipment on the covers or doors of junction boxes, since inspection of internal clearance is impossible when the door or cover is in the closed position.

(3) Junction box installation. It is desirable to mount junction boxes with their open side facing downward, so that loose metallic objects, such as washers or bolts, will tend to fall out of the junction box, rather than wedge between terminals.

(4) Junction box wiring. The original layout of the junction box should take into consideration the necessity for adequate wiring space, and possible future additions. Electric cable inside the box should be laced or clamped in such manner that terminals are not hidden,

relay armatures are not fouled, and motion relative to any equipment is prevented. Entrance openings for cable should be protected against chafing by grommets or other means.

(g) Bonding—(1) General. Bonding is defined as the process of electrically connecting the various metallic parts of the airplane, to achieve one or more of the following:

(1) A low resistance ground path for electrical equipment, thereby eliminating ground wires.

(ii) A reduction in radio interference. (iii) Less probability of lightning damage to such airplane elements as control hinges.

(iv) Prevention of the buildup of static charges between parts of the airplane, which may be a fire hazard.

(2) Bonding jumper installations. Bonding jumpers should be as short as practicable, and installed in such manner that the resistance of each connection does not exceed .003 ohm. Reasonable access for maintenance should be provided. The jumper should not interfere with the operation of movable aircraft elements, such as surface controls, nor should normal movement of these elements result in damage to the bonding jumper.

(i) Bonding connections. To assure low-resistance connections, nonconducting finishes, such as paint and anodizing films, should be carefully removed from the attachment surface under the bonding terminal.

(ii) Corrosion prevention. Electrolytic action may rapidly corrode a bonding connection, if suitable precautions are not taken. Aluminum alloy jumpers are recommended for most cases, except that copper jumpers are used to bond together parts made of stainless steel, cadmium plated steel, copper, brass or bronze. Where contact between dissimilar metals cannot be avoided, the choice of jumper and hardware should be such that corrosion is minimized, and the part likely to corrode would be the jumper or associated hardware. At locations where finishes were removed, a protective finish should be applied to the completed connection to prevent subsequent corrosion.

(iii) Bonding jumper attachment. The use of solder to attach bonding jumpers should be avoided for the same reasons outlined previously in reference