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.
Par 11-174 Page 11-77
c. Terminal Lugs. Wire terminal lugs should be used to connect wiring to terminal block studs or equipment terminal studs. No more than four terminal lugs or three terminal lugs and a bus bar should be connected to any one stud. Total number of terminal lugs per stud includes a common bus bar joining adjacent studs. Four terminal lugs plus a common bus bar thus are not permitted on one stud. Terminal lugs should be selected with a stud hole diameter that matches the diameter of the stud. However, when the terminal lugs attached to a stud vary in diameter, the greatest diameter should be placed on the bottom and the smallest diameter on top. Tightening terminal connections should not deform the terminal lugs or the studs. Terminal lugs should be so positioned that bending of the terminal lug is not required to remove the fastening screw or nut, and movement of the terminal lugs will tend to tighten the connection.
d. Copper Terminal Lugs. Solderless crimp style, copper wire terminal lugs should be used and conform to MIL-T-7928. Spacers or washers should not be used between the tongues of terminal lugs.
e. Aluminum Terminal Lugs. The aluminum terminal lugs conforming to AS70991 (AS25435, AS25436, and AS25438) should be crimped to aluminum wire only. The tongue of the aluminum terminal lugs, or the total number of tongues of aluminum terminal lugs when stacked, should be sandwiched between two MS25440-5 flat washers when terminated on terminal studs. Spacers or washers should not be used between the tongues of terminal lugs.
Special attention should be given to aluminum wire and cable installations to guard against conditions that would result in excessive voltage drop and high resistance at junctions that may ultimately lead to failure of the junction. Examples of such conditions are improper installation of terminals and washers, improper torsion (“torquing” of nuts), and inadequate terminal contact areas.
f. Class 2 Terminal Lugs. The Class 2 terminal lugs conforming to MIL-T-7928 may be used for installation, provided that in such installations, Class 1 terminal lugs are adequate for replacement without rework of installation or terminal lugs. Class 2 terminal lugs should be the insulated type, unless the conductor temperature exceeds 105 °C. In that case, uninsulated terminal lugs should be used. Parts lists should indicate the appropriate Class 1 terminal lugs to be used for service replacement of any Class 2 terminal lugs installed.
g. Termination of Shielded Wire. For termination of shielded wire, refer to MIL-DTL-27500.
11-175. ATTACHMENT OF TERMINALS TO STUDS. Connectors and terminals in aircraft require special attention to ensure a safe and satisfactory installation. Every possibility of short circuits, due to improper installation, poor maintenance, and service life, should be addressed in the design. Electrical equipment malfunction has frequently been traced to poor terminal connections at terminal boards. Loose, dirty, or corroded contact surfaces can produce localized heating that may ignite nearby combustible materials or overheat adjacent wire insulation. (See paragraph 11-178.)
11-176. STUDS AND INSULATORS. The following recommendations concerning studs also apply to other feed-through conductors.
Par 11-176 Page 11-78
a. Current Carrying Stud Resistance. Due to heat loss arising from wire-to-lug and lug-to-stud voltage drop, the resistance per unit length of a current carrying stud should not be greater than that of the wire.
b. Size of Studs. In designing the stud for a feed-through connection, attention should be given to the higher resistance of brass, as compared to copper. A suggested method of determining the size is to use a current density in the stud equivalent to that of the wire, compensating for the difference of resistance of the metals. Consideration should also be given to mechanical strength.
c. Support for Studs. The main stud support in the feed-through insulation should be independent of the attachment of the lugs to the stud. Therefore, loosening of the insulation support of the stud will not affect the electric contact efficiency. In other words, the contact pressure on the wire lugs should not in any way be affected by the loosening of the stud in the insulator.
d. Support of Wire at Studs. Unless some other positive locking action is provided, the lug or wire should be supported next to the stud to prevent loosening the connection with a side pull on the wire. Torque recommendations for attaching electrical wiring devices to terminal boards or blocks, studs, posts, etc., are normally found in the manufacturer’s maintenance instruction manual.
e. Feed-Through Insulator and Stud Design. Feed-through insulator design should be such as to prevent a loose insulator from failing to provide circuit isolation. It should not be able to move from between the stud and the structure, thus allowing the two to come into contact. The assembly should be so designed that it is impossible to inadvertently misassemble the parts so that faults will result. Also, it is desirable to provide means to prevent the feed-through stud from turning while tightening the connection.
11-177. WIRE TERMINALS AND BINDING POSTS. All wire terminals in or on electrical equipment, except case ground, must be firmly held together with two nuts or suitable locking provisions, or should be secured in a positive manner to equipment in such a way that no insulation material is involved in maintaining physical pressure between the various current carrying members of an electrical connection. Terminal studs or binding posts should be of a size that is entirely adequate for the current requirements of the equipment and have sufficient mechanical strength to withstand the torque required to attach the cable to the equipment. All terminals on equipment should have barriers and covers provided by equipment manufacturers.
11-178. CRIMP ON TERMINAL LUGS AND SPLICES (pre-insulated crimp type). The crimp on terminal lugs and splices must be installed using a high-quality, ratchet-type crimping tool. We recommend the use of the proper calibrated tool. Aircraft quality crimp tools are manufactured to standards. Such tools are provided with positioners for the wire size and are adjusted for each wire size. It is essential that the crimp depth be appropriate for each wire size.
If the crimp is too deep or not deep enough, it may break or cut individual strands, or it may not be tight enough to retain the wire in the terminal or connector. Crimps that are not tight enough are also susceptible to high resistance due to corrosion build-up between the crimped terminal
Par 11-178 Page 11-79 and the wire. MIL-C-22520/2 or MIL-DTL-2250 specification covers in detail the general requirement for crimp tools, inspection gages, and tool kits.
a. Hand, portable, and stationary power tools are available for crimping terminal lugs.
These tools crimp the barrel to the conductor, and simultaneously from the insulation support to the wire insulation.
b. Crimp tools must be carefully inspected:
(1) Ensure that the full cycle ratchet mechanism is tamper-proof so that it cannot be disengaged prior to or during the crimp cycle.
(2) If the tool does not function or faults are found, reject the tool and send the tool to be repaired.
(3) The tool calibration and adjustments are made only by the manufacturer or an approved calibration laboratory.
(4) Suitable gages of the Go/No Go type are available and shall be used prior to any crimping operation and whenever possible during operation to ensure crimp dimensions.
11-179. LOCK WASHERS FOR TERMINALS ON EQUIPMENT. Where locknuts are used to ensure binding and locking of electrical terminals, they should be of the all metal type. In addition, a spring lock washer of suitable thickness may be installed under the nut to ensure good contact pressure. A plain washer should be used between the spring washer and the terminal to prevent galling. A plain nut with a spring lock washer and a plain washer may be used to provide binding and contact pressure.
11-180. –11- 184. [RESERVED.]
Par 11-185 Page 11-80 SECTION 15. GROUNDING AND BONDING
11-185. GENERAL. One of the more important factors in the design and maintenance of aircraft electrical systems is proper bonding and grounding. Inadequate bonding or grounding can lead to unreliable operation of systems, e.g., EMI, electrostatic discharge damage to sensitive electronics, personnel shock hazard, or damage from lightning strike. This section provides an overview of the principles involved in the design and maintenance of electrical bonding and grounding. SAE ARP1870 provides for more complete detailed information on grounding and bonding, and the application of related hardware.
11-186. GROUNDING. Grounding is the process of electrically connecting conductive objects to either a conductive structure or some other conductive return path for the purpose of safely completing either a normal or fault circuit.
a. Types of Grounding. If wires carrying return currents from different types of sources, such as signals of DC and AC generators, are connected to the same ground point or have a common connection in the return paths, an interaction of the currents will occur. Mixing return currents from various sources should be avoided because noise will be coupled from one source to another and can be a major problem for digital systems. To minimize the interaction between various return currents, different types of grounds should be identified and used. As a minimum, the design should use three ground types: (1) AC returns, (2) DC returns, and (3) all others. For distributed power systems, the power return point for an alternative power source would be separated. For example, in a two-AC generator (one on the right side and the other on the left side) system, if the right AC generator were supplying backup power to equipment located in the left side, (left equipment rack) the backup AC ground return should be labeled “AC Right.” The return currents for the left generator should be connected to a ground point labeled “AC Left.”
b. Current Return Paths. The design of the ground return circuit should be given as much attention as the other leads of a circuit. A requirement for proper ground connections is that they maintain an impedance that is essentially constant. Ground return circuits should have a current rating and voltage drop adequate for satisfactory operation of the connected electrical and electronic equipment. EMI problems, that can be caused by a system’s power wire, can be reduced substantially by locating the associated ground return near the origin of the power wiring
(e.g., circuit breaker panel) and routing the power wire and its ground return in a twisted pair.
Special care should be exercised to ensure replacement on ground return leads. The use of numbered insulated wire leads instead of bare grounding jumpers may aid in this respect. In general, equipment items should have an external ground connection, even when internally grounded. Direct connections to a magnesium (which may create a fire hazard) structure must not be used for ground return.
c. Heavy-Current Grounds. Power ground connections, for generators, transformer rectifiers, batteries, external power receptacles, and other heavy-current, loads must be attached to individual grounding brackets that are attached to aircraft structure with a proper metal-to-metal bonding attachment. This attachment and the surrounding structure must provide adequate conductivity to accommodate normal and fault currents of the system without creating excessive voltage drop or damage to the structure. At least three fasteners, located in a triangular or rectangular pattern, must be used to secure such brackets in order to minimize susceptibility to