Par 11-139 Page 11-65
c. Service Loop Routing. The service loop harness should be routed directly from the breakout point to the component. The harness should not contact moving mechanical components or linkage, and should not be wrapped or tangled with other service loop harnesses.
d. Service Loop Harness Termination. Strain relief should be provided at the service loop harness termination, and is normally provided by the connector manufacturer’s back shell, heat-shrinkable boot, or tubing.
11-140.–11-145. [RESERVED.]
Par 11-146 Page 11-66 SECTION 11. CLAMPING
11-146. GENERAL. Wires and wire bundles must be supported by using clamps meeting AS21919, or plastic cable straps in accessible areas if correctly applied within the restrictions of paragraph 11-158. Clamps and other primary support devices must be constructed of materials that are compatible with their installation and environment, in terms of temperature, fluid resistance, exposure to ultraviolet (UV) light, and wire bundle mechanical loads. They should be spaced at intervals not exceeding 24 inches. Clamps on wire bundles should be selected so that they have a snug fit without pinching wires, as shown in Figures 11-11 through 11-13.
CAUTION: The use of metal clamps on coaxial RF cables may cause problems if clamp fit is such that RF cable’s original cross-section is distorted.
a. Clamps on wire bundles should not allow the bundle to move through the clamp when a slight axial pull is applied. Clamps on RF cables must fit without crushing and must be snug enough to prevent the cable from moving freely through the clamp, but may allow the cable to slide through the clamp when a light axial pull is applied. The cable or wire bundle may be wrapped with one or more turns of electrical tape when required to achieve this fit. Plastic clamps or cable ties must not be used where their failure could result in interference with movable controls, wire bundle contact with movable equipment, or chafing damage to essential or unprotected wiring. They must not be used on vertical runs where inadvertent slack migration could result in chafing or other damage. Clamps must be installed with their attachment hardware positioned above them, wherever practicable, so that they are unlikely to rotate as the result of wire bundle weight or wire bundle chafing. (See Figure 11-11.)
b. C lamps lined with nonmetallic material should be used to support the wire bundle along the run. Tying may be used between clamps, but should not be considered as a substitute for adequate clamping. Adhesive tapes are subject to age deterioration and, therefore, are not acceptable as a clamping means.
c. The back of the clamp, whenever practical, should be rested against a structural member. Stand-offs should be used to maintain clearance between the wires and the structure.
Clamps must be installed in such a manner that the electrical wires 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 terminal and to minimize adverse effects on shock mounted equipment. Where wires or wire bundles pass through bulkheads or other structural members, a grommet or suitable clamp should be provided to prevent abrasion.
d. When wire bundle is clamped into position, if there is less than 3/8-inch clearance between the bulkhead cutout and the wire bundle, a suitable grommet should be installed as indicated in Figure 11-14. The grommet may be cut at a 45-degree angle to facilitate installation, provided it is cemented in place and the slot is located at the top of the cutout.
11-147. WIRE AND CABLE CLAMPS INSPECTION. Inspect wire and cable clamps for proper tightness. Where cables pass through structure or bulkheads, inspect for proper clamping and grommets. Inspect for sufficient slack between the last clamp and the electronic equipment
to prevent strain at the cable terminals and to minimize adverse effects on shock-mounted equipment.
FIGURE 11-11. SAFE ANGLE FOR CABLE CLAMPS
DANGEROUS ANGLES
SAFE ANGLES FIGURE 11-12. TYPICAL MOUNTING HARDWARE FOR AS21919 CABLE CLAMPS A5-21911
CABLE CLAMI OR
EQUIVALENT MS-27009/NAS 603 SCREW AN 960 AN 960 WASHER,
WASHER, PLAIN PLAIN AN 9368
LOCKWASHER, EXTERNAL TEETH MS 21044 NUT, MS 35338
SELF-LOCKING LOCKWASHER, AN 340 NUT SPLIT Par 11-147 Page 11-67
Par 11-147 Page 11-68 FIGURE 11-13. INSTALLING CABLE CLAMP TO STRUCTURE
FIGURE 11-14. CLAMPING AT A BULKHEAD HOLE A. CUSHION CLAMP AT MS21919 BULKHEAD HOLE.
CABLE CLAMP
CLEARANCE 3/8" MINIMUM WIRES LESS THAN 3/8 INCH
FROM HOLE EDGE
GROMMET
ANGLE BRACKET WITH
TWO POINT FASTENING MS21919
CABLE CLAMP B. CUSHION CLAMP AT
BULKHEAD HOLE WITH MS35489 GROMMET.
C. CUSHION CLAMP AT
BULKHEAD HOLE WITH MS21266 GROMMET.
ANGLE BRACKET WITH
TWO POINT FASTENING
11-148.-11-154. [RESERVED.] Par 11-147 Page 11-69
Par 11-155 Page 11-70 SECTION 12. WIRE INSULATION AND LACING STRING TIE
11-155. GENERAL. Insulation of wires should be appropriately chosen in accordance with the environmental characteristics of wire routing areas. Routing of wires with dissimilar insulation, within the same bundle, is not recommended, particularly when relative motion and abrasion between wires having dissimilar insulation can occur. Soft insulating tubing (spaghetti) cannot be considered as mechanical protection against external abrasion of wire, since at best, it provides only a delaying action. Conduit or ducting should be used when mechanical protection is needed.
11-156. INSULATION MATERIALS. Insulating materials should be selected for the best combination of characteristics in the following categories:
a. Abrasion resistance.
b. Arc resistance (noncarbon tracking).
c. Corrosion resistance.
d. Cut-through strength.
e. Dielectric strength.
f. Flame resistance.
g. Heat distortion temperature.
h. Impact strength.
i. Mechanical strength.
j. Resistance to fluids.
k. Resistance to notch propagation.
l. Smoke emission.
m. Special properties unique to the aircraft.
n. For a more complete selection of insulated wires refer to SAE AS4372, Performance Requirements for Wire, Electric, Insulated Copper or Copper Alloy, and SAE AS4373, Test Methods for Insulated Electric Wire.
11-157. STRIPPING INSULATION. Attachment of wire, to connectors or terminals, requires the removal of insulation to expose the conductors. This practice is commonly known as stripping. Stripping may be accomplished in many ways; however, the following basic principles should be practiced.
a. Make sure all cutting tools used for stripping are sharp.
Par 11-157 Page 11-71
b. When using special wire stripping tools, adjust the tool to avoid nicking, cutting, or otherwise damaging the strands.
c. Damage to wires should not exceed the limits specified in Table 11-15.
d. When performing the stripping operation, remove no more insulation than is necessary.
TABLE 11-15. ALLOWABLE NICKED OR BROKEN STRANDS Maximum Allowable Nicked and Broken Strands Wire Size Conductor Material Number of Strands Per Conductor Total Allowable Nicked and Broken Strands 24-14 Copper 19 2 nicked, none broken 12-10 or 37 4 nicked, none broken 8-4 Copper Alloy 133 6 nicked, 6 broken 2-1 665-817 6 nicked, 6 broken 0-00 1,045-1,330 6 nicked, 6 broken 000 1,665- 6 nicked, 6 broken 0000 2,109- 6 nicked, 6 broken 8-000 Aluminum All numbers of strands None, None
11-158. LACING AND TIES. Ties, lacing, and straps are used to secure wire groups or bundles to provide ease of maintenance, inspection, and installation. Braided lacing tape per A-A-52080 is suitable for lacing and tying wires. In lieu of applying ties, straps meeting MS17821-4-9 or MS17822-1-9B may be used in areas where the temperature does not exceed 120 °C. Straps may not be used in areas of SWAMP such as wheel wells, near wing flaps, or wing folds. They may not be used in high-vibration areas, where failure of the strap would permit wiring to move against parts which could damage the insulation and foul mechanical linkages or other moving mechanical parts. They also may not be used where they could be exposed to UV light, unless the straps are resistant to such exposure.
a. Lacing. Lace wire groups or bundles inside junction boxes or other enclosures. Single cord-lacing method, shown in Figure 11-15, and tying tape, meeting specification A-A-52080, may be used for wire groups of bundles 1 inch in diameter or less. The recommended knot for starting the single cord-lacing method is a clove hitch secured by a double-looped overhand knot as shown in Figure 11-15, step a. Use the double cord-lacing method on wire bundles 1-inch in diameter or larger as shown in Figure 11-16. When using the double cord-lacing method, employ a bowline on a bight as the starting knot.
b. Tying. Use wire group or bundle ties where the supports for the wire are more than 12 inches apart. A tie consists of a clove hitch, around the wire group or bundle, secured by a square knot as shown in Figure 11-17.
c. Plastic Ties. See paragraph 11-220 and Table 11-22.
FIGURE 11-15. SINGLE CORD LACING
PULL HERE UNTIL TIGHT
BEFORE FINISHING KNOT
CORD CROSSES
UNDER LOOP STEP a - STARTING KNOT
FINAL KNO
STARTING KNOT—
TIGHTENED
STEP & - INTERMEDIATE HALF HITCHES FIRST PART OF FINAL STEP c - (PART I) KNOT TIGHTENED TRIM TO 3/8" MIN STEP c - (PART II) FINAL KNOT Par 11-158 Page 11-72
Par 11-158 Page 11-73 FIGURE 11-16. DOUBLE CORD LACING
Par 11-158 Page 11-74 FIGURE 11-17. MAKING TIES
11-159. INSULATION TAPE. Insulation tape should be of a type suitable for the application, or as specified for that particular use. Tape should be used primarily as a filler under clamps and as secondary support. Non-adhesive tape may be used to wrap around wiring for additional protection, such as in wheel wells. All tape should have the ends tied or otherwise suitably secured to prevent unwinding. Tape used for protection should be applied so that overlapping layers shed liquids. Drainage holes should be provided at all trap points and at each low point between clamps. Plastic tapes, that absorb moisture or have volatile plasticizers that produce chemical reactions with other wiring, should not be used. (Refer to AS50881.)
11-160.–11-166. [RESERVED.]
Par 11-167 Page 11-75 SECTION 13. SPLICING
11-167. GENERAL. Splicing is permitted on wiring as long as it does not affect the reliability and the electromechanical characteristics of the wiring. Splicing of power wires, coaxial cables, multiplex bus, and large gauge wire must have approved data.
a. Splicing of electrical wire should be kept to a minimum and avoided entirely in locations subject to extreme vibrations. Splicing of individual wires in a group or bundle should have engineering approval and the splice(s) should be located to allow periodic inspection.
b. Many types of aircraft splice connectors are available for use when splicing individual wires. Use of a self-insulated splice connector is preferred; however, an uninsulated splice connector may be used, provided the splice is covered with plastic sleeving that is secured at both ends. Environmentally sealed splices that conform to MIL-T-7928 provide a reliable means of splicing in SWAMP areas. However, an uninsulated splice connector may be used, provided the splice is covered with dual wall shrink sleeving of a suitable material.
c. There should not be more than one splice in any one wire segment between any two connectors or other disconnect points, except: when attaching to the spare pigtail lead of a potted connector, to splice multiple wires to a single wire, to adjust wire size to fit connector contact crimp barrel size, and to make an approved repair. (Refer to MIL-W-5088, now SAE AS50881A, and NAVAIR 01-1A-505.)
d. Spl ices in bundles must be staggered so as to minimize any increase in the size of the bundle, preventing the bundle from fitting into its designated space, or cause congestion that will adversely affect maintenance. (See Figure 11-18.)
e. Splices should not be used within 12 inches of a termination device, except for paragraph f below.
f. Splices may be used within 12 inches of a termination device when attaching to the pigtail spare lead of a potted termination device, or to splice multiple wires to a single wire, or to adjust the wire sizes so that they are compatible with the contact crimp barrel sizes.
g. Selection of proper crimping tool, see paragraph 11-178.
FIGURE 11-18. STAGGERED SPLICES IN WIRE BUNDLE 11 -168.–11-173. [RESERVED.]
Par 11-174 Page 11-76 SECTION 14. TERMINAL REPAIRS
11-174. GENERAL. Terminals are attached to the ends of electrical wires to facilitate connection of the wires to terminal strips or items of equipment. The tensile strength of the wire-to-terminal joint should be at least equivalent to the tensile strength of the wire itself, and its resistance negligible relative to the normal resistance of the wire.
a. Selection of Wire Terminals. The following should be considered in the selection of wire terminals.
(1) Current rating.
(2) Wire size (gauge) and insulation diameter.
(3) Conductor material compatibility.
(4) Stud size.
(5) Insulation material compatibility.
(6) Application environment.
(7) Solder/solderless.
(8) Pre-insulated crimp-type ring-tongue terminals are preferred. The strength, size, and supporting means of studs and binding posts, as well as the wire size, should be considered when determining the number of terminals to be attached to any one post. In high-temperature applications, the terminal temperature rating must be greater than the ambient temperature plus current related temperature rise. Use of nickel-plated terminals and of uninsulated terminals with high-temperature insulating sleeves should be considered. Terminal blocks should be provided with adequate electrical clearance or insulation strips between mounting hardware and conductive parts.
b. Terminal Strips. Wires are usually joined at terminal strips. A terminal strip fitted with barriers should be used to prevent the terminals on adjacent studs from contacting each other.
Studs should be anchored against rotation. When more than four terminals are to be connected together, a small metal bus should be mounted across two or more adjacent studs. In all cases, the current should be carried by the terminal contact surfaces and not by the stud itself. Defective studs should be replaced with studs of the same size and material since terminal strip studs of the smaller sizes may shear due to overtightening the nut. The replacement stud should be securely mounted in the terminal strip, and the terminal securing nut should be tight. Terminal strips should be mounted in such a manner that loose 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. Terminal strips that provide connection of radio and electronic systems to the aircraft electrical system should be inspected for loose connections, metallic objects that may have fallen across the terminal strip, dirt and grease accumulation, etc. These type conditions can cause arcing which may result in a fire, or system failures.