Par 11-47 Page 11-496
SECTION 4. INSPECTION OF CIRCUIT-PROTECTION DEVICES
GENERAL. All electrical wires must be provided with some means of circuit protection. Electrical wire should be protected with circuit breakers or fuses located as close as possible to the electrical power source bus.
Normally, the manufacturer of electrical equipment will specify the fuse or breaker to be used when installing the respective equipment, or SAE publication, ARP 1199, may be referred to for recommended practices.
DETERMINATION OF CIRCUIT BREAKER RATINGS. Circuit protection devices must be sized to supply open circuit capability. A circuit breaker must be rated so that it will open before the current rating of the wire attached to it is exceeded, or before the cumulative rating of all loads connected to it are exceeded, whichever is lowest. A circuit breaker must always open before any component downstream can overheat and generate smoke or fire. Wires must be sized to carry continuous current in excess of the circuit protective device rating, including its time-current characteristics, and to avoid excessive voltage drop. Refer to section 5 for wire rating methods.
DC CIRCUIT PROTECTOR CHART. Table 11-3 may be used as a guide for the selection of circuit breaker and fuse rating to protect copper conductor wire. This chart was prepared for the conditions specified. If actual conditions deviate materially from those stated, ratings above or below the values recommended may be justified. For example, a wire run individually in the open air may possibly be protected by the circuit breaker of the next higher rating to that shown on the chart. In general, the chart is conservative for all ordinary aircraft electrical installations.
TABLE 11-3. DC wire and circuit protector chart.
Wire AN gauge copper
Circuit breaker amp.
Fuse amp.
22 20 18 16 14 12 10 8 6 4 2 1 0 5
7.5 10 15 20 30 40 50 80 100 125 5 5 10 10 15 20 30 50 70 70 100 150 150 Basis of chart:
(1) Wire bundles in 135 °F. ambient and altitudes up to 30,000 feet.
(2) Wire bundles of 15 or more wires, with wires carrying no more than 20 percent of the total current carrying capacity of the bundle as given in Specification MIL-W-5088 (ASG).
(3) Protectors in 75 to 85 °F. ambient.
(4) Copper wire Specification MIL-W-5088.
(5) Circuit breakers to S pecification MIL-C-5809 or equivalent.
(6) Fuses to Specification MIL-F-15160 or equivalent.
RESETTABLE CIRCUIT PROTECTION DEVICES.
All resettable type circuit breakers must open the circuit irrespective of the position of the operating control when an overload or circuit fault exists. Such circuit breakers are referred to as “trip free.”
Automatic reset circuit breakers, that automatically reset themselves periodically, are not recommended as circuit protection devices for aircraft.
CIRCUIT BREAKER USAGE. Circuit breakers are designed as circuit protection for the wire (see paragraph 11-48 and 11-49), not for protection of black boxes
Par 11-47 Page 11-497 or components, and are not recommended for use as switches. Use of a circuit breaker as a switch will decrease the life of the circuit breaker.
CIRCUIT BREAKER MAINTENANCE. Circuit breakers should be periodically cycled with no load to enhance contact performance by cleaning contaminants from the contact surfaces.
SWITCHES. In all circuits where a switch malfunction can be hazardous, a switch specifically designed for aircraft service should be used. These switches are of rugged construction and have sufficient contact capacity to break, make, and continuously carry the connected load current. The position of the switch should be checked with an electrical meter.
Electrical Switch Inspection. Special attention should be given to electrical circuit switches, especially the spring-loaded type, during the course of normal airworthiness inspection. An internal failure of the spring-loaded type may allow the switch to remain closed even though the toggle or button returns to the “off” position. During inspection, attention should also be given to the possibility that improper switch substitution may have been made.
With the power off suspect aircraft electrical switches should be checked in the on position for opens (high resistance) and in the off position for shorts (low resistance), with an ohmmeter.
Any abnormal side to side movement of the switch should be an alert to imminent failure even if the switch tested was shown to be acceptable with an ohmmeter.
Electromechanical Switches. Switches have electrical contacts and various types of switch actuators (i.e., toggle, plunger, push-button, knob, rocker).
Contacts designed for high-level loads must not be subsequently used for low-level applications, unless testing has been performed to establish this capability.
Switches are specifically selected based on the design for the aircraft service current ratings for lamp loads, inductive loads, and motor loads and must be replaced with identical make and model switches.
Proximity Switches. These switches are usually solidstate devices that detect the presence of a predetermined target without physical contact and are usually rated
0.5 amps or less.
Switch Rating. The nominal current rating of the conventional aircraft switch is usually stamped on the switch housing and represents the continuous current rating with the contacts closed. Switches should be derated from their nominal current rating for the following types of circuits:
Circuits containing incandescent lamps can draw an initial current that is 15 times greater than the continuous current. Contact burning or welding may occur when the switch is closed.
Inductive circuits have magnetic energy stored in solenoid or relay coils that is released when the control switch is opened and may appear as an arc.
Direct-current motors will draw several times their rated current during starting, and magnetic energy stored in their
Par 11-53 Page 11-498 armature and field coils is released when the control switch is opened.
Switch Selection. Switches for aircraft use should be selected with extreme cauti on. The contact ratings should be adequate for all load conditions and applicable voltages, at both sea level and the operational altitude. Consideration should be given to the variation in the electrical power characteristics, using MIL-STD-704 as a guide.
Derating Factors. Table 11-4 provides an approximate method for derating nominal ratings to obtain reasonable switch efficiency and service life under reactive load conditions.
WARNING: Do not use AC derated switches in DC circuits. AC switches will not carry the same amperage as a DC switch.
TABLE 11-4. Switch derating factors.
Nominal System Voltage Type of Load Derating Factor 28 VDC Lamp 8 28 VDC Inductive (relay-solenoid) 4 28 VDC Resistive (Heater) 2 28 VDC Motor 3 12 VDC Lamp 5 12 VDC Inductive (relay-solenoid) 2 12 VDC Resistive (Heater) 1 12 VDC Motor 2 NOTES:
1. To find the nominal rating of a switch required to operate a given device, multiply t he continuous current rating of the device by the derating factor corresponding to the voltage and type of load.
2. To find the continuous rating that a switch of a given nominal rating will handle efficiently, divide the switch nominal rating by the derating factor corresponding to the voltage and type of load.
Low Energy Loads. Switches rated for use at 28 VDC or more, and at 1.0 amp or more, generally have silver contacts. In general, silver contacts should not be used to control devices which have either a voltage less than 8 volts or a continuous current less than 0.5 amps unless the switch is specifically rated for use with low-energy loads. Table 11-5 provides general guidelines for selecting contact materials fo r low-energy loads, but is not applicable to hermetically sealed switches.
Typical logic load devices have a voltage of
0.5 volts to 28 volts and a continuous current of less than 0.5 amps. A suitable method of rating switches for use on logic load devices is specified in ANSI/EIA 5200000. (General specification for special use electromechanical switches of certified quality.)
TABLE 11-5. Selection of contact material.
NOTES:
1. If sulfide, moisture, or any form of contamination is present, a sealed switch should be used. The degree of sealing required (envir onmental or hermetic) is dependent upon the environment in which the switch is intended to be operated.
2. If particle contamination in any form is likely to reach the contacts, bifurcated contacts should be used.
3. Low-voltage high-current loads are difficult to predict and may result in a combined tendency of noncontact, sticking, and material transfer.
4. High-voltage high-current applications may require the use of Silver Nickel contacts.
Par 11-53 Page 11-499 Typical low-level load devices have a voltage of less than 0.5 volts and a continuous current of less than 0.5 amps. A suitable method of rating switches for use on logic load devices is specified in ANSI/EIA 5200000.
Shock and Vibration.
Electromechanical switches (toggle switches) are most susceptible to shock and vibration in the plane that is parallel to contact motion.
Under these conditions the switch contacts may momentarily separate.
ANSI/EIA 5200000 specifies that contact separations greater than 10 microseconds and that closing of open contacts in excess of 1 microsecond are failures. Repeated contact separations during high levels of vibration or shock may cause excessive electrical degradation of the contacts. These separations can also cause false signals to be registered by electronic data processors without proper buffering.
Although proximity switches do not have moving parts, the reliability of the internal electronic parts of the switch may be reduced.
Reliability and mean time between-failure (MTBF) calculations should reflect the applicable environment. Note that the mounting of both the proximity sensor and its target must be rigid enough to withstand shock or vibration to avoid creating false responses.
Electromagnetic/Radio Frequency Interference (EMI/RFI).
DC operated electromechanical switches are usually not susceptible to EMI/RFI. Proximity switches are susceptible to an EMI/RFI environment and must be evaluated in the application. Twisting lead wires, metal overbraids, lead wire routing, and the design of the proximity switch can minimize susceptibility.
The arcing of electromechanical switch contacts generates short duration EMI/RFI when controlling highly inductive electrical loads. Twisting lead wires, metal overbraids, and lead wire routing can reduce or eliminate generation problems when dealing with arcing loads. Proximity sensors generally use a relatively low-energy electromagnetic field to sense the target. Adequate spacing is required to prevent interference between adjacent proximity sensors or other devices susceptible to EMI/RFI. Refer to manufacturer’s instructions.
Temperature.
Electromechanical switches can withstand wide temperature ranges and rapid gradient shifts without damage. Most aircraft switches operate between -55 °C and 85 °C with designs available from -185 °C to 260 °C or more. Higher temperatures require more exotic materials, which can increase costs and limit life. It should be noted that o-ring seals and elastomer boot seals tend to stiffen in extreme cold. This can increase operating forces and reduce release forces or stop the switch from releasing.
Proximity sensors are normally designed for environments from -55 °C to 125 °C. During temperature excursions, the operating and release points may shift from 5 percent to 10 percent. Reliability of the proximity sensor will typically be highest at room temperature.
The reliability and MTBF estimates should be reduced for use under high temperatures or high thermal gradients.
Sealing.
NOTE: The materials used for sealing (o-rings, potting materials, etc.) should be compatible with any aircraft fluids to which the switch may be exposed.
Par 11-53 Page 11-500 Electromechanical switches range in sealing from partially sealed to hermetically sealed.
Use a sealed switch when the switch will be exposed to a dirty environment during storage, assembly, or operation. Use a higher level of sealing when the switch will not have an arcing load to self-clean the contacts.
Low-energy loads tend to be more susceptible to contamination.
Proximity switches for aircraft applications typically have a metal face and potting material surrounding any electronics and lead wire exits. The potting material should be compatible with the fluids the switch will be exposed to in the environment. The plastic sensing face of some proximity switches may be subject to absorption of water that may cause the operating point to shift should be protected.
Switch Installation. Hazardous errors in switch operation may be avoided by logical and consistent installation. “On-off” two-position switches should be mounted so that the “on” position is reached by an upward or forward movement of the toggle. When the switch controls movable airc raft elements, such as landing gear or flaps, the toggle should move in the same direction as the desired motion. Inadvertent operation of switches can be prevented by mounting suitable guards over the switches.
RELAYS. A relay is an electrically controlled device that opens and closes electrical contacts to effect the operation of other devices in the same or in another electrical circuit. The relay converts electrical energy into mechanical energy through various means, and through mechanical linkages, actuates electrical conductors (contacts) that control electrical circuits. Solid-state relays may also be used in electrical switching applications.
Use of Relays. Most relays are used as a switching device where a weight reduction can be achieved, or to simplify electrical controls. It should be remembered that the relay is an electrically operated switch, and therefore subject to dropout under low system voltage conditions.
Types of Connections. Relays are manufactured with various connective means from mechanical to plug-in devices. Installation procedures vary by the type of connection and should be followed to ensure proper operation of the relay.
Repair. Relays are complicat ed electromechanical assemblies and most are not repairable.
Relay Selection.
Contact ratings, as described on the relay case, describe the make, carry, and break capability for resistive currents only. Consult the appropriate specification to determine the derating factor to use for other types of current loads. (Ref. MIL-PRF-39016, MIL-PRF-5757, MIL-PRF-6016, MIL-PRF-835836.)
Operating a relay at less than nominal coil voltage may compromise its performance and should never be done without written manufacturer approval.
Relay Installation and Maintenance. For installation and maintenance, care should be taken to ensure proper placement of hardware, es pecially at electrical connections. The use of properly calibrated torque wrenches and following the manufacturer’s installation procedures is strongly recommended. This is especially important with hermetically sealed relays, since the glass-to-metal seal (used for
Par 11-53 Page 11-501 insulation of the electrically “live” components) is especially vulnerable to catastrophic failure as a result of overtorquing.
When replacing relays in alternating current (ac) applications, it is essential to maintain proper phase sequencing. For any application involving plug-in relays, proper engagement of their retaining mechanism is vital.
The proximity of certain magnetically permanent, magnet assisted, coil operated relays may cause them to have an impact on each other. Any manufacturer’s recommendations or precautions must be closely followed.
LOAD CONSIDERATIONS.
When switches or relays are to be used in applications where current or voltage is substantially lower than rated conditions, additional intermediate testing should be performed to ensure reliable operation. Contact the manufacturer on applications different from the rated conditions.
OPERATING CONDITIONS FOR SWITCHES AND RELAYS. Switches and relays should be compared to their specification rating to ensure that all contacts are made properly under all conditions of operation, including vibration equivalent to that in the area of the aircraft in which the switch or relay is to be installed.
11-57.11-65. [RESERVED.]
Par 11-66 Page 11-502
SECTION 5. ELECTRICAL WIRE RATING
GENERAL. Wires must be sized so that they:
have sufficient mechanical strength to allow for service conditions; do not exceed allowable voltage drop levels; are protected by system circuit protection devices; and meet circuit current carrying requirements.
Mechanical Strength of Wires. If it is desirable to use wire sizes smaller than #20, particular attention should be given to the mechanical strength and installation handling of these wires, e.g., vibration, flexing, and termination. Wire containing less than 19 strands must not be used. Consideration should be given to the use of high-strength alloy conductors in small gauge wires to increase mechanical strength. As a general practice, wires smaller than size #20 should be provided with additional clamps and be grouped with at least three other wires. They should also have additional support at terminations, such as connector grommets, strain relief clamps, shrinkable sleeving, or telescoping bushings. They should not be used in applications where they will be subjected to excessive vibration, repeated bending, or frequent disconnection from screw termination.
Voltage Drop in Wires. The voltage drop in the main power wires from the generation source or the battery to the bus should not exceed 2 percent of the regulated voltage when the generator is carrying rated current or the battery is being discharged at the 5-minute rate. The tabulation shown in table 11-6 defines the maximum acceptable voltage drop in the load circuits between the bus and the utilization equipment ground.
Resistance. The resistance of the current return path through the aircraft structure is generally considered negligible. However, this is based on the assumption that adequate TABLE 11-6. Tabulation chart (allowable voltage drop between bus and utilization equipment ground).
Nominal system voltage
Allowable voltage drop continuous operation
Intermittent operation
12 28 115 200
0.5 1 4 7
1 2 8 14
bonding to the structure or a special electric current return path has been provided that is capable of carrying the required electric current with a negligible voltage drop. To determine circuit resistance check the voltage drop across the circuit. If the voltage drop does not exceed the limit established by the aircraft or product manufacturer, the resistance value for the circuit may be considered satisfactory. Wh en checking a circuit, the input voltage should be maintained at a constant value.
Tables 11-7 and 11-8 show formulas that may be used to determine electrical resi stance in wires and some typical examples.
Resistance Calculation Methods. Figures 11-2 and 11-3 provide a convenient means of calculating maximum wire length for the given circuit current.
Values in tables 11-7 and 11-8 are for tinplated copper conductor wires. Because the resistance of tin-plated wire is slightly higher than that of nickel or silver-plated wire, maximum run lengths determined from these charts will be slightly less than the allowable limits for nickel or silver-plated copper wire and are therefore safe to use. Figures 11-2 and 11-3 can be used to derive slightly longer maximum run lengths for silver or nickelplated wires by multiplying the maximum run length by the ratio of resistance of tin-plated wire, divided by the resistance of silver or nickel-plated wire.
TABLE 11-7. Examples of determining required wire size using figure 11-2.
Voltage drop
Run Lengths
Circuit Current Wire size from chart
Checkcalculated voltage drop = (Resistance /Ft) (Length) (Current) = 1 volt
100 feet
20 amps No. 6
(.000445 ohms/ft)
0.5 volt
4 volt
50 feet
100 feet
40 amps
No. 2
No. 12 (100 ft) (20 amps)= .89 volts
(.000183 ohms/ft) (50 ft) (40 amps)= .366 volts
(.00202
Par 11-66 Page 11-503
7 volt
100 feet amps
20 amps
No. 14 See Note 1 ohms/ft) (100 ft) (20 amps)=
4.04 volts
(.00304 ohms/ft) (100 ft) (20 amps)=
6.12 volts
Note #1: #14 Wire should be routed separately for this current rating.
TABLE 11-8. Examples of determining maximum run length using figure 11-3.
Maximum Voltage drop
Wire Size
Circuit Current Maximum Wire Run Length Check-calculated voltage drop = (Resistance/Ft) (Length)(Current)= 1 volt
0.5 volt
4 volt
7 volt No. 10
- - - -
- - - -
- - - - 20 amps
- -
- -
- - 39 feet
19.5 feet
156 feet
273 feet
(.00126 ohms/ft) (39 ft)(20 amps)= .98 volts
(.00126 ohms/ft)
(19.5 ft) (20 amps)= .366 volts
(.00126 ohms/ft) (156 ft) (20 amps)=
3.93 volts
(.00126 ohms/ft) (273 ft) (20 amps)=
6.88 volts As an alternative method or a means of checking results from figure 11-2, resistance for a given wire size can be read from table 11-9 and multiplied by wire run length and circuit current.
Voltage drop calculations for aluminum wires can be accomplished by multiplying the resistance for a given wire size, defined in table 11-10, by the wire run length and circuit current.
When the estimated or measured conductor temperature (T 2) exceeds 20 °C, such as in areas having elevated ambient temperatures or in fully loaded power-feed wires, the maximum allowable run length (L 2), must be shortened from L 1 (the 20 °C value) using the following formula for copper conductor wire:
L CLCT2 1 2254 5234 5= °°+(.) ( )(.) ( )
For aluminum conductor wire, the formula is:
L C LCT2 1 2258 1238 1= °°+(. ) ( )(. ) ( )
These formulas use the reciprocal of each material’s resistivity temperature coefficient to take into account increased conductor resist ance resulting from operation at elevated temperatures.
To determine T 2 for wires carrying a high percentage of their current carrying capability at elevated temperatures, laboratory testing using a load bank and a high-temperature chamber is recommended. Such tests should be run at anticipated worse case ambient temperature and maximum current-loading combinations.
1 1/0 2/0 3/0 4/0 LENGTH (LI) IS BASED ON CONDUCTOR TEMPERATURES OF 20 °C TO DETERMINE LENGTH (L2) AT HIGHER CONDUCTOR TEMPERATURE USE FORMULA WHERE T2 = ESTIMATED L2= (254.5) (LI) (234.5) + (12) AMPERES 10 12 NOTE VOLTAGE DROP CHART CONTINOUS FLOW AT 20° CONDUCTOR TEMPERATURE °C TIN-PLATED MIL-W-27759 18 16 WIRE SIZE CONDUCTOR 20 EXAMPLE # 1 EXAMPLE # 2 NO 8 WIRE AT 20 AMPS NO 12 WIRE AT 20 AMPS NO 14 WIRE AT 20 AMPS CONTINOUS CIRCUIT VOLTAGE 30 20 001 910010 1O.5 VOLTAGE DROP
WIRE LENGTH IN FEET FIGURE 11-2. Conductor chart, continuous flow.
Par 11-66 Page 11-504
AMPERES 1 1/0 2/0 3/0 4/0 NOTE LENGTH (LI) IS BASED ON CONDUCTOR TEMPERATURES OF 20° C TO DETERMINE LENGTH (L2) AT HIGHER CONDUCTOR TEMPERATURE USE FORMULA WHERE T2 = ESTIMATED CONDUCTOR TEMPERATURE °C (254.5) (LI) L2= (234.5) + (T2) 18 16 20 VOLTAGE DROP CHART INTERMITTENT FLOW AT 20° TIN-PLATED MIL-W-27759
WIRE SIZE CONDUCTOR 00 CIRCUIT VOLTAGE 115 240 112 48 8 2 VOLTAGE DROP
WIRE LENGTH IN FEET FIGURE 11-3. Conductor chart, intermittent flow.
Par 11-66 Page 11-505
Par 11-66 Page 11-506 TABLE 11-9. Current carrying capacity and resistance of copper wire.
Wire Size Continuous duty current (amps)-Wires in bundles, groups, harnesses, or conduits. (See Note #1) Max. resistance ohms/1000ft@20° C Nominal conductor Wire Conductor Temperature Rating tin plated conductor area - 105°C 150°C 200°C (See Note #2) circ.mils 24 22 20 18 16 14 12 10 8 6 4 2 1 0 00 000 0000
2.5 3 4 6 7 10 13 17 38 50 68 95 113 128 147 172 204 4 5 7 9 11 14 19 26 57 76 103 141 166 192 222 262 310 5 6 9 12 14 18 25 32 71 97 133 179 210 243 285 335 395
28.40
16.20
9.88
6.23
4.81
3.06
2.02
1.26
0.70
0.44
0.28
0.18
0.15
0.12
0.09
0.07
0.06 475 755 1,216 1,900 2,426 3,831 5,874 9,354 16,983 26,818 42,615 66,500 81,700 104,500 133,000 166,500 210,900 Note #1: Rating is for 70°C ambient, 33 or more wires in the bundle for sizes 24 through 10, and 9 wires for size 8 and larger, with no more t han 20 percent of harness current carrying capacity being used, at an operating altitude of 60,000 feet. For rating of wires under other conditions or configurations see paragraph 11-69.
Note #2: For resistance of silver or nicke l-plated conductors see wire specifications.
TABLE 11-10. Current carrying capacity and resistance of aluminum wire.
Wire Size Continuous duty current (amps) Wires in bundles, groups or harnesses or conduits (See table 11-9 Note #1)
Max. resistance ohms/1000ft Wire conductor temperature rating @20°C 105°C 150 °C 8 6 4 2 1 0 00 000 0000 30 40 54 76 90 102 117 138 163 45 61 82 113 133 153 178 209 248
1.093
0.641
0.427
0.268
0.214
0.169
0.133
0.109
0.085 Note: Observe design practices described in paragraph 11-67 for aluminum conductor
Par 11-66 Page 11-507 Approximate T 2 can be estimated using the following formula:
TT TT I I R21 1=+ − () ( / max2 )
Where:
T1 = Ambient Temperature
T2 = Estimated Conductor Temperature
TR = Conductor Temperature Rating
I2 = Circuit Current (A=Amps)
Imax = Maximum Allowable Current
(A=Amps) at TR
This formula is quite conservative and will typically yield somewhat higher estimat ed temperatures than are likely to be encountered under actual operating conditions.
METHODS FOR DETERMINING CURRENT CARRYING CAPACITY OF WIRES. This paragraph contains methods for determining the current carrying capacity of electrical wire, both as a single wire in free air and when bundled into a harness. It presents derating factors for altitude correction and examples showing how to use the graphical and tabular data provided for this purpose. In some instances, the wire may be capable of carrying more current than is recommended for the contacts of the related connector. In this instance, it is the contact rating that dictates the maximum current to be carried by a wire.
Wires of larger gauge may need to be used to fit within the crimp range of connector contacts that are adequately rated for the current being carried. Figure 11-5 gives a family of curves whereby the bundle derating factor may be obtained.
Effects of Heat Aging on Wire Insulation. Since electrical wire may be installed in areas where inspection is infrequent over extended periods of time, it is necessary to give speci al consideration to heataging characteristics in the selection of wire.
Resistance to heat is of primary importance in the selection of wire for aircraft use, as it is the basic factor in wire rating. Where wire may be required to operate at higher temperatures due either to high ambient temperatures, high-current loading, or a combination of the two, selection should be made on the basis of satisfactory performance under the most severe operating conditions.
Maximum Operating Temperature. The current that causes a temperature steady st ate condition equal to the rated temperature of the wi re should not be exceeded.
Rated temperature of the wire may be based upon the ability of either the conduc tor or the insulation to withstand continuous operation without degradation.
Single Wire in Free Air. Determining a wiring system’s current carryi ng capacity begins with determining the maximum current that a given-sized wire can carry without exceeding the allowable temperature difference (wire rating minus ambient °C).
The curves are based upon a single copper wire in free air. (See figures 11-4a and 11-4b.)
Wires in a Harness. When wires are bundled into harnesses, the current derived for a single wire must be reduced as shown in figure 11-5. The amount of current derating is a function of the number of wires in the bundle and the percentage of the total wire bundle capacity that is being used.