dry chemical fire extinguisher should not be used in the cockpit due to possible interference with visibility and the collection of nonconductive powder on electrical contacts of surrounding equipment.
The extinguisher is equipped with a fixed nozzle which is directed toward the fire source to smother the fire. The trigger is also lockwired but can be broken by a sharp squeeze of the trigger.
(4) The development of halogenated hydrocarbons (freons) as fire extinguishing agents with low toxicity for airborne fire extinguishing protection systems logically directed attention to its use in hand type fire extinguishers.
Bromotrifluoromethane (Halon 1301) having a rating of 6 on the toxicity scale is the logical successor to CO2 as a hand type fire extinguisher agent. It is effective on fires in lower concentrations. Halon 1301 can extinguish a fire with a concentration of 2% by volume. This compares with about 40% by volume concentration required for CO2 to extinguish the same fire.
This quality allows Halon 1301 to be used in occupied personnel compartments without depriving people of the oxygen they require. Another advantage is that no residue or deposit remains after use. Halogen 1301 is the ideal agent to use in airborne hand held fire extinguishers because:
(1) its low concentration is very effective, (2) it may be used in occupied personnel compartments, (3) it is effective on all 3 type fires, and
(4) no residue remaining after its use.
Extinguishers Unsuitable as Cabin or Cockpit Equipment The common aerosol can type extinguishers are definitely not acceptable as airborne hand type extinguishers. In one instance, an aerosol type foam extinguisher located in the pilot's seat back pocket exploded and tore the upholstery from the seat. The interior of the aircraft was damaged by the foam. This occurred when the aircraft was on the ground and the outside air temperature was 90° F. In addition to the danger from explosion, the size is inadequate to combat even the smallest fire.
A dry chemical extinguisher was mounted near a heater vent on the floor. For an unknown reason, the position of the unit was reversed. This placed the extinguisher directly in front of the heater vent.
During flight, with the heater in operation, the extinguisher became overheated and exploded filling the compartment with dry chemical powder. The proximity of heater vents should be considered • hand fire coco.
when selecting a location for a hand fire extinguisher.
Additional information relative to airborne hand fire extinguishers may be obtained from the local FAA District Office and from the National Fire Protection Association, 470 Atlantic Ave., Boston, MA 02210.
SMOKE DETECTION SYSTEMS A smoke detection system monitors the cargo and baggage compartments for the presance of smoke, which is indicative of a fire condition. Smoke detection instruments, which collect air for sampling, are mounted in the compartments in strategic locations.
A smoke detection system is used where the type of fire anticipated is expected to generate a substantial amount of smoke before temperature changes are sufficient to actuate a heat detection system.
Smoke detection instruments are classified by method of detection as follows: Type 1 - Measure.
ment of carbon monoxide gas (CO detectors), Type II - Measurement of light transmissibility in air (photoelectric devices), and Type IIII - Visual detection of the presence of smoke by directly viewing air samples (visual devices).
To be reliable, smoke detectors must be maintained so that smoke in a compartment will be indicated as soon as it begins to accumulate. Smoke detector louvers, vents, and ducts must not be obstructed.
Carbon Monoxide Detectors The CO detectors, which detect concentrations of carbon monoxide gas, are rarely used to monitor cargo and baggage compartments. However, they have gained widespread use in conducting tests for the presence of carbon monoxide gas in aircraft cabins and cockpits.
Carbon monoxide is a colorless, odorless, tasteless, non-irritating gas. It is the byproduct of incomplete combustion, and is found in varying degrees in all smoke and fumes from burning carbonaceous substances. Exceedingly small amounts of the gas are dangerous. A concentration of .02% (2 parts in 10,000) may produce headache, mental dullness, and physical loginess within a few hours.
There are several types of portable testers (sniffers) in use. One type has a replaceable indicator tube which contains a yellow silica gel, impregnated with a complex silico-molybdate compound and is catalyzed using palladium sulfate.
• In use, a sample of air is drawn through the detector tube. When the air sample contains carbon monoxide, the yellow silica gel turns to a shade of 430
green. The intensity of the green color is proportional to the concentration of carbon monoxide in the air sample at the time and location of the tests.
Another type indicator may be worn as a badge or installed on the instrument panel or cockpit A functional check of the detector should be made after installation, and at frequent intervals thereafter.
wall. It is a button using a tablet which changes from a normal tan color to progressively darker shades of gray to black. The transition time required is relative to the concentration of CO. At 28 v.d.c. Test switch a concentration of 50 ppm CO (0.005%), the indication will be apparent within 15 to 30 minutes.
A concentration of 100 ppm CO (0.01%) will change color of the tablet from tan to gray in 2-5 minutes, from tan to dark gray in 15 to 20 minutes.
Detector 7 Photoelectric Smoke Detectors Beacon This type of detector consists of a photoelectric lamp cell, a beacon lamp, a test lamp, and a light trap, all mounted on a labyrinth. An accumulation of 10% smoke in the air causes the photoelectric cell to | To amplifier and conduct electric current. Figure 10-29 shows the fire warning circuit details of the smoke detector, and indicates how the smoke particles refract the light to the photoelectric cell. When activated by smoke, the detector supplies Test relay a signal to the smoke detector amplifier. The ampli- Test fier signal activates a warning light and bell. lamp FIGURE 10-30. Smoke detector test circuit.
Photoelectric cell Beacon lamp
• Visual Smoke Detectors On a few aircraft visual smoke detectors provide Light retracted to photoelectric cell particles Smoke the only means of smoke detection. Indication is by smoke particles provided by drawing smoke through a line into the Labrynth indicator, using either a suitable suction device or cabin pressurization.
• 00 When smoke is present a lamp within the indicator is illuminated automatically by the smoke detec- Light -Test lamp rendered visible in the appropriate window of the tor. The light is scattered so that the smoke is FIGURE 10-29. Photoelectric smoke detector. indicator. If no smoke is present the lamp will not be illuminated. A switch is provided to illuminate the lamp for test purposes. A device is also pro- A test switch (figure 10-30) permits checking the vided in the indicator to show that the necessary operation of the smoke detector. Closing the switch airflow is passing through the indicator.
connects 28 v. d.c. to the test relay. When the test relay energizes, voltage is applied through the beacon lamp and test lamp in series to ground. A fire indication will be observed only if the beacon and test lamp, the photoelectric cell, the smoke detector amplifier, and associated circuits are operable.
The efficiency of any detection system depends on the positioning and serviceability of all the components of the system. The foregoing information is intended to provide familiarization with the various systems. For details of a particular installation, refer to the relevant manuals for the aircraft concerned.
The maximum allowable concentration, under (parts per million) which is equal to 0.005% of Federal Law, for continuing exposure is 50 ppm carbon monoxide. (See figure 10-31.)
The maximum allowable concentration under Federal Law for continuing exposure is 50 ppm (parts per million) which is equal to 0.005% of carbon monoxide.
Parts Per Million Percentage Reaction 50 0.005% Maximum allowable concentration under Federal Law.
100 0.01 % Tiredness, mild dizziness.
200 0.02 % Headaches, tiredness, dizziness, nausea after 2 or 3 hours.
800 0.08 % Unconsciousness in 1 hour and death in 2 to 3 hours.
2,000 0.20 % Death after 1 hour.
3,000 0,30 % Death within 30 minutes.
10,000 1.00 % Instantaneous death.
FIGURE 10-31. Human reactions to carbon monoxide poisoning.
GENERAL The satisfactory performance of any modern aircraft depends to a very great degree on the continuing reliability of electrical systems and subsystems.
Improperly or carelessly installed wiring or improperly or carelessly maintained wiring can be a source of both immediate and potential danger. The continued proper performance of electrical systems depends on the knowledge and techniques of the mechanic who installs, inspects, and maintains the electrical system wires and cables.
Procedures and practices outlined in this section are general recommendations and are not intended to replace the manufacturer's instructions and approved practices.
For the purpose of this discussion, a wire is described as a single, solid conductor, or as a stranded conductor covered with an insulating material. figure 11-1 illustrates these two definitions of a wire.
Conductors Wire single solid conductor Solid conductor. Stranded condactor 002 1 8 19 1 FICURE 11-1. Two types of aircraft wire.
The term cable, as used in aircraft electrical installations, includes:
(1) Two or more separately insulated conductors in the same jacket (multi-conductor cable).
CHAPTER 11
AIRCRAFT ELECTRICAL SYSTEMS
(2) Two or more separately insulated conductors twisted together (twisted pair).
(3) One or more insulated conductors, covered with a metallic braided shield (shielded cable).
(4) A single insulated center conductor with a metallic braided outer conductor (radio frequency cable). The concentricity of the center conductor and the outer conductor is carefully controlled during manufacture to ensure that they are coaxial.
Wire Size Wire is manufactured in sizes according to a standard known as the AWG (American wire gage). As shown in figure 11-2, the wire diameters become smaller as the gage numbers become larger.
The largest wire size shown in figure 11-2 is number 0000, and the smallest is number 40. Larger and smaller sizes are manufactured but are not commonly used.
A wire gage is shown in figure 11-3. This type of gage will measure wires ranging in size from number zero to number 36. The wire to be measured is inserted in the smallest slot that will just accommodate the bare wire. The gage number corresponding to that slot indicates the wire size. The slot has parallel sides and should not be confused with the semicircular opening at the end of the slot. The opening simply permits the free movement of the wire all the way through the slot.
Gage numbers are useful in comparing the diameter of wires, but not all types of wire or cable can be accurately measured with a gage. Large wires are usually stranded to increase their flexibility. In such cases, the total area can be determined by multiplying the area of one strand (usually computed in circular mils when diameter or gage number is known) by the number of strands in the wire or cable.
Factors Affecting the Selection of Wire Size Several factors must be considered in selecting the size of wire for transmitting and distributing electric power.
Cross section Ohms per 1,000 ft.
Gage Diameter num- (mils) Circular Square 25°C. 65°C.
ber mils inches (=770 F.) (=149°F.)
0000 460.0 212,000.0 0.166 0.0500 0.0577 000 410.0 168,000.0 .132 .0630 .0727 00 365.0 133,000.0 .105 .0795 .0917
325.0 106,000.0 .0829 .100 .116 1 289.0 83,700.0 .0657 .126 .146
258.0 66,400.0 .0521 .159 .184 3 229.0 52,600.0 .0413 .201 .232 4 204.0 41,700.0 .0328 .253 .292
182.0 33,100.0 .0260 .319 ,369 6 162.0 26,300.0 .0206 .403 .465 7 144.0 20,800.0 .0164 .508 .586
128.0 16,500.0 0130 .641 .739 9 114.0 13,100.0 .0103 .808 .932 10 102.0 10,400.0 .00815 1.02 1.18 11 91.0 8,230.0 .00647 1.28 1.48 12 81.0 6,530.0 .00513 1.62 1.87 13 72.0 5,180.0 .00407 2.04 2.36 14 64.0 4,110.0 .00323 2.58 2.97 15 57.0 3,260.0 .00256 3.25 3.75 16 51.0 2,580.0 .00203 4.09 4.73 17 45.0 2,050.0 .00161 5.16 5.96 18 40.0 1,620.0 .00128 6.51 7.51 19 36.0 1,290.C .00101 8.21 9.48 20 32.0 1,020.0 .000802 10.4 11.9 21 28.5 810.0 .000636 13.1 15.1 22 25.3 642.0 .000505 16.5 19.0 23 22.6 509.0 000400 20.8 24.0 24 20.1 404.0 .000317 26.2 30.2 25 17.9 320.0 .000252 33.0 38.1 26 15.9 254.0 ,000200 41.6 48.0 27 14.2 202.0 .000158 52.5 60.6 28 12.6 160.0 .000126 66.2 76.4 29 11.3 127.0 .0000995 83.4 96.3 30 10.0 101.0 .0000789 105.0 121.0 31 8.9 79.7 .0000626 133.0 153.0 32 8.0 63.2 .0000496 167.0 193.0 33 7.1 50.1 .0000394 211.0 243.0 34 6.3 39.8 .0000312 266.0 307.0 35 5.6 31.5 .0000248 335.0 387.0 36 5.0 25.0 .0000196 423.0 488.0 37 4.5 19.8 .0000156 533.0 616.0 38 4.0 15.7 .0000123 673.0 776.0 39 3.5 12.5 .0000098 848.0 979.0 40 3.1 9.9 .0000078 1,070.0 1,230.0 FICURE 11-2. American wire gage for standard annealed solid copper wire.
Wire FICURE 11-3. A wire gage.
One factor is the allowable power loss (IR loss) in the line. This loss represents electrical energy converted into heat. The use of large conductors will reduce the resistance and therefore the IR loss. However, large conductors are more expensive initially than small ones; they are heavier and require more substantial supports.
A second factor is the permissible voltage drop (IR drop) in the line. If the source maintains a constant voltage at the input to the lines, any variation in the load on the line will cause a variation in line current and a consequent variation in the IR drop in the line. A wide variation in the IR drop in the line causes poor voltage regulation at the load.
The obvious remedy is to reduce either current or resistance. A reduction in load current lowers the amount off power being transmitted, whereas a reduction in line resistance increases the size and weight of conductors required. A compromise is generally reached whereby the voltage variation at the load is within tolerable limits and the weight of line conductors is not excessive.
A third factor is the current-carrying ability of the conductor. When current is drawn through the conductor, heat is generated. The temperature of the wire will rise until the heat radiated, or otherwise dissipated, is equal to the beat generated by the passage of current through the line. If the conductor is insulated, the heat generated in the conductor is not so readily removed as it would be if the conductor were not insulated. Thus, to protect the insulation from too much heat, the current through the conductor must be maintained below a certain value.
When electrical conductors are installed in locations where the ambient temperature is relatively high, the heat generated by external sources constitutes an appreciable part of the total conductor heating. Allowance must be made for the influence of external heating on the allowable conductor current, and each case has its own specific limitations.
The maximum allowable operating temperature of insulated conductors varies with the type of conductor insulation being used.
Tables are available that list the safe current ratings for various sizes and types of conductors covered with various types of insulation. Figure 11-5 shows the current-carrying capacity, in amperes, of single copper conductors at an ambient temperature of below 30°C. This example provides measurements for only a limited range of wire sizes.
Factors Affecting Selection of Conductor Material Although silver is the best conductor, its cost limits its use to special circuits where a substance with high conductivity is needed.
The two most generally used conductors are copper and aluminum. Each has characteristics that make its use advantageous under certain circumstances. Also, each has certain disadvantages Copper has a higher conductivity; it is more ductile (can be drawn), has relatively high tensile strength, and can be easily soldered. It is more expensive and heavier than aluminum.
Although aluminum has only about 60% of the conductivity of copper, it is used extensively. Its lightness makes possible long spans, and its relatively large diameter for a given conductivity reduces corona (the discharge of electricity from the wire when it has a high potential). The discharge is greater when small diameter wire is used than when large diameter wire is used. Some bus bars are made of aluminum instead of copper where there is a greater radiating surface for the same conductance. The characteristics of copper and aluminum are compared in figure 11-4.
FIGURE 11-4. Characteristics of copper and aluminum.
Characteristic Copper Aluminum Tensile strength (lb./in.)..-. 55,000 25,000 Tensile strength for same con.
ductivity (Ib.) -_ 55,000 40,000 Weight for same conductivity (lb.) 100 48 Cross section for same corductivity (C. M.).-- 100 160 Specific resistance (Q/mil ft.). 10.6 17 435
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