Thermoplastic Slow-burning Size Rubber or asbestos, var- Impregnated or weatherthermo- cam, or asbestos asbestos Asbestos proof plastic var-cam 0000 300 385 475 510 370 000 260 330 410 430 320 00 225 285 355 370 275 195 245 305 325 235 1 165 210 265 280 205 2 140 180 225 240 175 3 120 155 195 210 150 105 135 170 180 130 6 80 100 125 135 100 8 55 90 100 70 10 40 55 70 75 55 12 25 40 50 55 40 14 20 30 40 45 30 FIGURE 11-5. Current-carrying capacity of wire.
Voltage Drop in Aircraft Wire and Cable It is recommended that the voltage drop in the main power cables from the aircraft generation source or the battery to the bus should not exceed 2% of the regulated voltage when the generator is carrying rated current or the battery is being discharged at a 5-min. rate. The tabulation in figure 11-6 shows the recommended maximum voltage drop in the load circuits between the bus and the utilization equipment.
FIGURE 11-6. Recommended maximum voltage drop in load circuits.
capable of carrying the required electric current with a negligible voltage drop. A resistance meas urement of 0.005 ohm from the ground point of the generator or battery to the ground terminal of any electrical device is considered satisfactory. Another satisfactory method of determining circuit resistance is to check the voltage drop across the circuit.
If the voltage drop does not exceed the limit estab lished by the aircraft or product manufacturer, the resistance value for the circuit is considered satisfactory. When using the voltage drop method of checking a circuit, the input voltage must be maintained at a constant value.
Instructions For Use of Electric Wire Chart Allowable voltage drop The charts in figures 11-7 and 11-8 apply to Nominal copper conductors carrying direct current. Curves system Continuous Intermittent 1, 2, and 3 are plotted to show the maximum amvoltage operation operation pere rating for the specified conductor under the specified conditions shown. To select the correct 115 14 28 0.5 1 4 2 8 size of conductor, two major requirements must be met. First, the size must be sufficient to prevent an 200 7 14 excessive voltage drop while carrying the required current over the required distance. Secondly, the The resistance of the current return path through size must be sufficient to prevent overheating of the cable while carrying the required current. The the aircraft structure is always considered negligi- charts in figures 11-7 and 11-8 can simplify these ble. However, this is based on the assumption that determinations. To use these charts to select the adequate bonding of the structure or a special elec- proper size of conductor, the following must be tric current return path has been provided and is known:
CONTINUOUS
CIRCUIT
VOLTAGE 115 200 14 28 ELECTRIC WIRE CHART 800 100 200 20 18 16 10 NTI
AMPERES 600 75 150 400 700 50 100 360 630 45 90 320 360 40 80 280 490 35 70 240 420 30 60 200: 350 251 50
LET WIRE LENGTH IN FEET CURVE →0005 CURVE 2 CURVES:
CONTINUOUS RATING - AMPERES
WIRE IN CONDUIT AND BUNDLES
2. CONTINUOUS RATING - AMPERES
SINGLE WIRE IN FREE - AIR 3 5 20 18 16 10 1/02/03/0 4/0
WIRE SIZE FIcURE 11-7. Conductor chart, continuous flow. (Applicable to copper conductors.)
INTERMITTENT
CIRCUIT
VOLTAGE 115 200 14 28 ELECTRIC WIRE CHART 1600 200 400 20 10 771
AMPERES 1200 150 300 180 10 20 00 800 1400 100 200 720 1260 90 180 400- WIRE LENGTH IN FEET CURVE 3
INTERMITTENT RATING - AMPERES MAXIMUM OF 2 MINUTES 14 12 10 8 2 1 1/0 2/0 /0 4/0
WIRE SIZE FICURF 11-8. Conductor chart, intermittent flow.
(1) The conductor length in feet.
(2) The number of amperes of current to be carried.
(3) The amount of voltage drop permitted.
(4) Whether the current to be carried will be intermittent or continuous, and if continuous, whether it is a single conductor in free air, in a conduit, or in a bundle.
Assume that it is desired to install a 50-ft. conductor from the aircraft bus to the equipment in a 28-volt system. For this length, a 1-volt drop is permissible for continuous operation. By referring to the chart in figure 11-7, the maximum number of feet a conductor may be run carrying a specified current with a 1-volt drop can be determined. In this example the number 50 is selected.
Assuming the current required by the equipment is 20 amperes, the line indicating the value of 20 amperes should be selected from the diagonal lines.
Follow this diagonal line downward until it intersects the horizonta) line number 50. From this point, drop straight downward to the bottom of the chart to find that a conductor between size No. 8 and No. 10 is required to prevent a greater drop than 1 volt. Since the indicated value is between two numbers, the larger size, No. 8, should be selected. This is the smallest size conductor which should be used to avoid an excessive voltage drop.
To determine. that the conductor size is sufficient to preclude overheating, disregard both the numbers along the left side of the chart and the horizontal lines. Assume that the conductor is to be a single wire in free air carrying continuous current.
Place a pointer at the top of the chart on the diagonal line numbered 20 amperes. Follow this line until the pointer intersects the diagonal line marked "curve 2." Drop the pointer straight downward to the bottom of the chart. This point is between numbers 16 and 18. The larger size, No. 16, should be selected. This is the smallest size conductor acceptable for carrying 20-ampere current in a single wire in free air without overheating.
If the installation is for equipment having only an intermittent (max. 2 min.) requirement for power, the chart in figure 11-8 is used in the same manner.
Conductor insulation Two fundamental properties of insulation materials (for example, rubber, glass, asbestos, or plastic) are insulation resistance and dielectric strength.
These are entirely different and distinct properties.
Insulation resistance is the resistance to current leakage through and over the surface of insulation materials. Insulation resistance can be measured with a megger without damaging the insulation, and data so obtained serves as a useful guide in determining the general condition of the insulation.
However, the data obtained in this manner may not give a true picture of the condition of the insulation. Clean, dry insulation having cracks or other faults might show a high value of insulation resistance but would not be suitable for use.
Dielectric strength is the ability of the insulator to withstand potential difference and is usually expressed in terms of the voltage at which the insulation fails because of the electrostatic stress. Maximum dielectric strength values can be measured by raising the voltage of a test sample until the insulation breaks down.
Because of the expense of insulation and its stiffening effect, together with the great variety of physical and electrical conditions under which the conductors are operated, only the necessary minimum insulation is applied for any particular type of cable designed to do a specific job.
The type of conductor insulation material varies with the type of installation. Such types of insulation as rubber, silk, and paper are no longer used extensively in aircraft systems. More common today are such materials as vinyl, cotton, nylon, Teflon, and Rockbestos.
Identifying Wire and Cable Aircraft electrical system wiring and cable may be marked with a combination of letters and numbers to identify the wire, the circuit it belongs to, 2229 the gage number, and other information necessary to relate the wire or cable to a wiring diagram.
Such markings are called the identification code.
There is no standard procedure for marking and identifying wiring; each manufacturer normally develops his own identification code. One identification system (figure 11-9) shows the usual spacing 439
in marking a wire. The number 22 in the code refers to the system in which the wire is installed,
e.g., the VHF system. The next set of numbers, .013, is the wire number, and the 18 indicates the wire size.
22.013-18
22.013-18 TB-1 FIGURE 11-9. Wire identification code.
FICURE 11-10. Wire identification at a terminal block.
Some system components, especially plugs and jacks, are identified by a letter or group of letters and numbers added to the basic identification number. These letters and numbers may indicate the location of the component in the system. Interconnected cables are also marked in some systems to indicate location, proper termination, and use.
In any system, the marking should be legible, and the stamping color should contrast with the color of the wire insulation. For example, black stamping should be used with light-colored backgrounds, or white stamping on dark-colored backgrounds.
Wires are usually marked at intervals of not more than 15 in. lengthwise and within 3 in. of each junction or terminating point. Figure 11-10 shows wire identification at a terminal block.
Coaxial cable and wires at terminal blocks and junction boxes are often identified by marking or stamping a wiring sleeve rather than the wire itself.
For general purpose wiring, a flexible vinyl sleeving, either clear or white opaque, is commonly used.
For high-temperature applications, silicone rubber or silicone fiber glass sleeving is recommended.
Where resistance to synthetic hydraulic fluids or other solvents is necessary, either clear or white opaque nylon sleeving can be used.
While the preferred method is to stamp the identification marking directly on the wire or on the sleeving, other methods are often employed. Figure 11-11 shows two alternate methods: one method uses Pressuresensitive tape XYZ15 Sleeve marker tied in place XYZ 15 FIcURE 11-11. Alternate methods of identifying wire bundles.
a marked sleeve tied in place; the other uses a pressure-sensitive tape.
Electrical Wiring Installation The following recommended procedures for installing aircraft electrical wiring are typical of those used on most aircraft. For purposes of this discussion, the following definitions are applicable:
(1) Open wiring-any wire, wire group, or wire bundle not enclosed in conduit.
(2) Wire group-two or more wires going to the same location tied together to retain identity of the group.
(3) Wire bundle-two or more wire groups tied together because they are going in the same direction at the point where the tie is located.
(4) Electrically protected wiring-wires which include (in the circuit) protection against overloading, such as fuses, circuit breakers, or other limiting devices.
(5) Electrically unprotected wiring-wires (generally from generators to main bus distribution points) which do not have protection, such as fuses, circuit breakers, or other current-limiting devices.
Wire Groups and Bundles Grouping or bundling certain wires, such as electrically unprotected power wiring and wiring going to duplicate vital equipment, should be avoided.
Wire bundles should generally be less than 75 wires, or 1-1/2 to 2 in. in diameter where practicable. When several wires are grouped at junction boxes, terminal blocks, panels, etc., identity of the group within a bundle (figure 11-12) can be re- Bundle tie Group tie Bundle tie FIGURE 11-12. Group and bundle ties.
Twisting Wires When specified on the engineering drawing, or when accomplished as a local practice, parallel wires must sometimes be twisted. The following are the most common examples:
(1) Wiring in the vicinity of magnetic compass or flux valve.
(2) Three-phase distribution wiring.
(3) Certain other wires (usually radio wiring) as specified on engineering drawings.
Twist the wires so that they will lie snugly against each other, making approximately the number of twists per foot as shown in figure 11-13. Always check wire insulation for damage after twisting. If the insulation is torn or frayed, replace the wire.
FIcURE 11-13. Recommended number of twists per foot.
Wire Size #22 #20 #14 #12 #8 #6 #1 2 Wires 3 Wires 10| 10 6½ 6 5½ 5 4 3 Spliced Connections in Wire Bundles Spliced connections in wire groups or bundles should be located so that they can be easily inspected. Splices should also be staggered (figure 11-14) so that the bundle does not become exces.
sively enlarged. All noninsulated splices should be covered with plastic, securely tied at both ends.
Slack in Wiring Bundles Single wires or wire bundles should not be installed with excessive slack. Slack between supports should normally not exceed a maximum of 1/2 in.
deflection with normal hand force (figure 11-15).
However, this may be exceeded if the wire bundle is thin and the clamps are far apart. Slack should never be so great that the wire bundle could abrade against any surface. A sufficient amount of slack should be allowed near each end of a bundle to:
(1) Permit easy maintenance.
(2) Allow replacement of terminals.
(3) Prevent mechanical strain on the wires, wire junctions, and supports.
(4) Permit free movement of shock and vibration-mounted equipment.
(5) Permit shifting of equipment for purposes of maintenance.
Bend Radii Bends in wire groups or bundles should be not less than 10 times the outside diameter of the wire group or bundle. However, at terminal strips, where wire is suitably supported at each end of the bend, a minimum radius of three times the outside diameter of the wire, or wire bundle, is normally acceptable. There are, of course, exceptions to these guidelines in the case of certain types of cable; for 441
FIGURE 11-14. Staggered splices in a wire bundle.
example, coaxial cable should never be bent to a Protection against High Temperature smaller radius than ten times the outside diameter.
Routing and Installations All wiring should be installed so that it is mechanically and electrically sound and neat in appearance. Whenever practicable, wires and bundles should be routed parallel with, or at right angles to, the stringers or ribs of the area involved. An exception to this general rule is coaxial cable, which is routed as directly as possible.
The wiring must be adequately supported throughout its length. A sufficient number of supports must be provided to prevent undue vibration of the unsupported lengths. All wires and wire groups should be routed and installed to protect them from:
(1) Chafing or abrasion.
(2) High temperature.
(3) Being used as handholds, or as support for personal belongings and equipment.
(4) Damage by personnel moving within the
(5) Damage from cargo stowage or shifting.
(6) Damage from battery acid fumes, spray, To prevent insulation deterioration, wires should be kept separate from high-temperature equipment, such as resistors, exhaust stacks, or heating ducts.
The amount of separation is normally specified by engineering drawings. Some wires must invariably be run through hot areas. These wires must be insulated with high-temperature material such as asbestos, fiber glass, or Tefion. Additional protection is also often requred in the form of conduits.
A low-temperature insulation wire should never be used to replace a high-temperature insulation wire.
Many coaxial cables have soft plastic insulation, such as polyethylene, which is especially subject to deformation and deterioration at elevated temperatures. All high-temperature areas should be avoided when installing these cables insulated with plastic or polyethylene.
Additional abrasion protection should be given to asbestos wires enclosed in conduit. Either conduit with a high-temperature rubber liner should be used, or asbestos wires can be enclosed individually in high-temperature plastic tubes before being installed in the conduit.
(7) Damage from solvents and fluids.
Protection Against Chafing Wires and wire groups should be protected against chafing or abrasion in those locations where contact with sharp surfaces or other wires would damage the insulation. Damage to the insulation can cause short circuits, malfunction, or inadvertent operation of equipment. Cable clamps should be used to support wire bundles at each hole through a bulkhead (figure 11-16). If wires come closer than 1/4 in. to the edge of the hole, a suitable grommet is used in the hole as shown in figure 11-17.
Sometimes it is necessary to cut nylon or rubber grommets to facilitate installation. In these instances, after insertion, the grommet can be secured in place with general-purpose cement. The cut should be at the top of the hole, and made at an angle of 45° to the axis of the wire bundle hole.
Protection Against Solvents and Fluids Wires should not be installed in areas where they will be subjected to damage from fluids or in the lowest 4 in. of an aircraft fuselage, except those that must terminate in that area. If there is a possibility that wire may be soaked with fluids, plastic tubing should be used to protect the wire. This tubing should extend past the exposure area in both directions and should be tied at each end. If the wire has a low point between the tubing ends, provide a ⅛-in. drain hole, as shown in figure 11-18.
This hole should be punched into the tubing after the installation is complete and the low point definitely established by using a hole punch to cut a half circle. Care should be taken not to damage any wires inside the tubing when using the punch.
Wire should never be routed below an aircraft 442
1/2 Inch Maximum with normal hand pressure FIcURE 11-15. Slack in wire bundle between supports.
Cable clamp Clearance 1/4" minimum Angle bracket with two point fastening FIcURE 11-16. Cable clamp at bulkhead hole.
battery. All wires in the vicinity of an aircraft battery should be inspected frequently and wires discolored by battery fumes should be replaced.
Protection of Wires in Wheel Well Area Wires located in wheel wells are subject to many additional hazards, such as exposure to fluids, pinching, and severe flexing in service. All wire bundles should be protected by sleeves of flexible tubing securely held at each end, and there should be no relative movement at points where flexible tubing is secured. These wires and the insulating tubing should be inspected carefully at frequent intervals, and wires or tubing should be replaced at the first sign of wear. There should be no strain on attachments when parts are fully extended, but slack should not be excessive.
Routing Precautions When wiring must be routed parallel to combustible fluid or oxygen lines for short distances, as much fixed separation as possible should be maintained. The wires should be on a level with, or above, the plumbing lines. Clamps should be spaced so that if a wire is broken at a clamp it will not contact the line. Where a 6-in. separation is not possible, both the wire bundle and the plumbing line can be clamped to the same structure to prevent any relative motion. If the separation is less than 2 in. but more than 1/2 in., a polyethylene sleeve may be used over the wire bundle to give further protection. Also two cable clamps back-to-back, as shown in figure 11-19, can be used to maintain a rigid separation only, and not for support of the bundle.
No wire should be routed so that it is located nearer than 1/2 in. to a plumbing line. Neither should a wire or wire bundle be supported from a Wires less than 1/4-inch from hole edge Approved grommet 27220 FIGURE 11-17. Cable clamp and grommet at bulkhead hole.
Drainage hole 1/8-inch diameter at lowest point in tubing. Make the hole after installation is complete and lowest point is firmly established FIcURE 11-18. Drain hole in low point of tubing. FicURE 11-19. Separation of wires from plumbing lines.
plumbing line that carries flammable fluids or oxy.
gen.
Wiring should be routed to maintain a minimum clearance of at least 3 in. from control cables. If this cannot be accomplished, mechanical guards should be installed to prevent contact between wiring and control cables.
Installation of Cable Clamps Cable clamps should be installed with regard to the proper angle, as shown in figure 11-20. The mounting screw should be above the wire bundle. It is also desirable that the back of the cable clamp rest against a structural member where practicable.
Figure 11-21 shows some typical mounting hardware used in installing cable clamps.
Care should be taken that wires are not pinched in cable clamps. Where possible, mount the cables directly to structural members, as shown in figure
11-22.
Clamps can be used with rubber cushions to secure wire bundles to tubular structures as shown in figure 11-23. Such clamps must fit tightly, but should not be deformed when locked in place.
LACING AND TYING WIRE BUNDLES Wire groups and bundles are laced or tied with cord to provide ease of installation, maintenance, and inspection. This section describes and illustrates recommended procedures for lacing and tying wires with knots which will hold tightly under all conditions. For the purposes of this discussion, the following terms are defined:
(1) Tying is the securing together of a group Safe angles FIGURE 11-20. Proper mounting angles for cable clamps.
or bundle of wires by individual pieces of cord tied around the group or bundle at regular intervals.
(2) Lacing is the securing together of a group or bundle of wires by a continuous piece of cord forming loops at regular intervals around the group or bundle.
(3) A wire group is two or more wires tied or laced together to give identity to an individual system.
(4) A wire bundle is two or more wires or groups tied or laced together to facilitate maintenance.
The material used for lacing and tying is either cotton or nylon cord. Nylon cord is moisture- and fungus-resistant, but cotton cord must be waxed before using to give it these necessary protective characteristics.
Single-Cord Lacing Figure 11-24 shows the step in lacing a wire bundle with a single cord. The lacing procedure is started at the thick end of the wire group or bundle with a knot consisting of a clove hitch with an extra loop. The lacing is then continued at regular inter- Screw vals with half hitches along the wire group or bundle and at each point where a wire or wire group branches off. The half hitches should be spaced so that the bundle is neat and secure. The lacing is ended by tying a knot consisting of a clove hitch with an extra loop. After the knot is tied, the free Cable clamp
- Plain
• Plain washer washer Lockwasher Nut Lockwasher split Nut extemal teeth self-locking FIGURE 11-21. Typical mounting hardware for cable clamps.
MS 21919 Cable clamps ends of the lacing cord should be trimmed to approximately 3/8 in.
Double-Cord Lacing Figure 11-25 illustrates the procedure for dou- Tubular structure "Angle" member "Z" member Correct Angle bracket
• Clamp Cable clamp Wire is pinched in clamp Incorrect FICURE 11-23. Installing cable clamp to FIGURE 11-22. Mounting cable clamp to structure. tubular structure.
Pull here until tight before finishing knot Cord crosses under loop Step A-Starting knot Final knot Starting knot tightened First part of final knot Step B-Intermediate half hitches tightened Step C-(Part I) Trim to 3/8" min.
Step C-(Part II) Final knot FIGURE 11-24. Single-cord lacing.
ble-cord lacing. The lacing is started at the thick end of the wire group or bundle with a bowline-ona-bight knot (A of figure 11-25). At regular intervals along the wire group or bundle, and at each point where a wire branches off, the lacing is continued using half hitches, with both cords held firmly together. The half hitches should be spaced so that the group or bundle is neat and secure. The lacing is ended with a knot consisting of a half hitch, continuing one of the cords clockwise and the other counterclockwise and then tying the cord ends with a square knot. The free ends of the lacing cord should be trimmed to approximately 3/8 in.
Lacing Branch-Offs Figure 11-26 illustrates a recommended procedure for lacing a wire group that branches off the main wire bundle. The branch-off lacing is started with a knot located on the main bundle just past the branch-off point. Continue the lacing along the branched-off wire group, using regularly spaced half hitches. If a double cord is used, both cords should be held snugly together. The half hitches should be spaced to lace the bundle neatly and securely. The lacing is ended with the regular terminal knot used in single- or double-cord lacing. The free ends of the lacing cord should be neatly trimmed.
Tying All wire groups or bundles should be tied where supports are more than 12 in. apart, Figure 11-28 illustrates a recommended procedure for tying a wire group or bundle. The tie is started by wrapping the cord around the wire group to tie a clovehitch knot. Then a square knot with an extra loop is tied, and the free ends of the cord are trimmed.
Temporary ties are sometimes used in making up and installing wire groups and bundles. Colored cord is normally used to make temporary ties, since they are removed when the installation is complete.
Whether laced or tied, bundles should be secured to prevent slipping, but not so tightly that the cord cuts into or deforms the insulation. This applies especially to coaxial cable, which has a soft dielectric insulation between the inner and outer conductor.
The part of a wire group or bundle located inside a conduit is not tied or laced, but wire groups or bundles inside enclosures, such as junction boxes, should be laced only.
CUTTING WIRE AND CABLE To make installation, maintenance, and repair easier, wire and cable runs in aircraft are broken at specified locations by junctions, such as connectors, 446
Starting knot for branch off Main bundle lacing Branch-off lacing Starting knot-Bowline-on a bight Half-hitch required at branch-off point Starting knot tightened FICURE 11-26. Lacing a branch-off.
Intermediate half hitches Stripping Wire and Cable Before wire can be assembled to connectors, terminals, splices, etc., the insulation must be stripped 2020# from connecting ends to expose the bare conductor.
Copper wire can be stripped in a number of ways depending on the size and insulation. Figure 11-27 lists some types of stripping tools recommended for various wire sizes and types of insulation.
FIGURE 11-27. Wire strippers for copper wire.
Stripper Wire Size Insulations Final knot C Hot-blade 426-44 All except asbestos Rotary, electric #26#4 All FICURE 11-25. Double-cord lacing. Bench #20-46 AII Hand pliers #26-78 All Knife #2 -#0000 AIl terminal blocks, or buses. Before assembly to these junctions, wires and cables must be cut to length. Aluminum wire must be stripped very carefully, All wires and cables should be cut to the lengths using extreme care, since individual strands will specified on drawings and wiring diagrams. The cut break very easily after being nicked.
should be made clean and square, and the wire or The following general precautions are recomcable should not be deformed. If necessary, large- mended when stripping any type of wire:
diameter wire should be re-shaped after cutting. (1) When using any type of wire stripper, Good cuts can be made only if the blades of cutting hold the wire so that it is perpendicular to tools are sharp and free from nicks. A dull blade cutting blades.
will deform and extrude wire ends. (2) Adjust automatic stripping tools care- 447