Par 11-36 Page 11-15 electrical system should have accurate and up-to-date data concerning the capacity of the installed electrical power source(s) and the load(s) imposed by installed electrical power-consuming devices. Such data should provide a true picture of the status of the electrical system. New or additional electrical devices should not be installed in an aircraft or the capacity changed of any power source, until the status of the electrical system in the aircraft has been determined accurately and found not to adversely affect the integrity of the electrical system.
11-37. JUNCTION BOX CONSTRUCTION. Replacement junction boxes should be fabricated using the same material as the original or from a fire-resistant, nonabsorbent material, such as aluminum, or an acceptable plastic material. Where fire-proofing is necessary, a stainless steel junction box is recommended. Rigid construction will prevent “oil-canning” of the box sides that could result in internal short circuits. In all cases, drain holes should be provided in the lowest portion of the box. Cases of electrical power equipment must be insulated from metallic structure to avoid ground fault related fires. (See paragraph 11-7.)
a. Internal Arrangement. The junction box arrangement should permit easy access to any installed items of equipment, terminals, and wires. Where marginal clearances are unavoidable, an insulating material should be inserted between current carrying parts and any grounded surface. It is not good practice to mount equipment on the covers or doors of junction boxes, since inspection for internal clearance is impossible when the door or cover is in the closed position.
b. Installation. Junction boxes should be securely mounted to the aircraft structure in such a manner that the contents are readily accessible for inspection. When possible, the open side should face downward or at an angle so that loose metallic objects, such as washers or nuts, will tend to fall out of the junction box rather than wedge between terminals.
c. Wi ring. Junction box layouts should take into consideration the necessity for adequate wiring space and possible future additions. Electrical wire bundles should be laced or clamped inside the box so that cables do not touch other components, prevent ready access, or obscure markings or labels. Cables at entrance openings should be protected against chafing by using grommets or other suitable means.
11-38.–11-46. [RESERVED.]
Par 11-47 Page 11-16 SECTION 4. INSPECTION OF CIRCUIT-PROTECTION DEVICES
11-47. 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 ARP1199 may be referred to for recommended practices.
11-48. 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. See Section 5 for wire rating methods.
11-49. DC CIRCUIT PROTECTOR CHART. Table 11-4 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.
Par 11-49 Page 11-17 TABLE 11-4. DC WIRE AND CIRCUIT PROTECTOR CHART Wire AN Gauge Copper Circuit Breaker amp. Fuse amp.
22 5 5 20 7.5 5 18 10 10 16 15 10 14 20 15 12 30 20 10 40 30 8 50 50 6 80 70 4 100 70 2 125 100 1 150 0 150 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 AS50881 (ASG).
(3) Protectors in 75 to 85 °F ambient.
(4) Copper wire AS50881.
(5) Circuit breakers to AS5809 or equivalent.
(6) Fuses to MIL-PRF-15160 or equivalent.
11-50. RESETTABLE CIRCUIT PROTECTION DEVICES.
a. 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.”
b. Automatic reset circuit breakers, that automatically reset themselves periodically, are not recommended as circuit protection devices for aircraft.
11-51. CIRCUIT BREAKER USAGE. Circuit breakers are designed as circuit protection for the wire (see paragraphs 11-48 and 11-49), not for protection of black boxes or components. Use of a circuit breaker as a switch is not recommended. Use of a circuit breaker as a switch will decrease the life of the circuit breaker.
Par 11-52 Page 11-18
11-52. CIRCUIT BREAKER MAINTENANCE. Circuit breakers should be periodically cycled with no load to enhance contact performance by cleaning contaminants from the contact surfaces. Also periodically verify legibility of rating and location markings.
11-53. 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.
a. 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.
(1) 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.
(2) 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.
b. Electromechanical Switches. Switches have electrical contacts and various types of switch actuators (i.e., toggle, plunger, push-button, knob, rocker).
(1) Contacts designed for high-level loads must not be subsequently used for low-level applications, unless testing has been performed to establish this capability.
(2) 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.
c. Proximity Switches. These switches are usually solid-state devices that detect the presence of a predetermined target without physical contact and are usually rated 0.5 amps or less.
d. 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:
(1) 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.
(2) 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.
Par 11-53 Page 11-19
(3) Direct-current motors will draw several times their rated current during starting, and magnetic energy stored in their armature and field coils is released when the control switch is opened.
e. Switch Selection. Switches for aircraft use should be selected with extreme caution.
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.
f. Derating Factors. Table 11-5 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 11-5. 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 the 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.
g. 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 11-6 provides general guidelines for selecting contact materials for low-energy loads, but is not applicable to hermetically sealed switches.
TABLE 11-6. SELECTION OF CONTACT MATERIAL Supply Voltage (Volts) silver 30 gold or silver 8 gold or bifurcated gold silver 1 bifurcated gold
0.001 0.01 0.5 1.00 Current (Amperes) NOTES:
1. If sulfide, moisture, or any form of contamination is present, a sealed switch should be used. The degree of sealing required (environmental 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.
(1) 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, Generic Specification for Special-Use Electromechanical Switches of Certified Quality.
(2) 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.
Par 11-53 Page 11-20
Par 11-53 Page 11-21
h. Shock and Vibration.
(1) 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.
(2) 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.
i. Electromagnetic/Radio-Frequency Interference (EMI/RFI).
(1) 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.
(2) 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.
j. Temperature.
(1) 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.
(2) 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.
Par 11-53 Page 11-22
k. 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.
(1) 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.
(2) 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.
l. 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 aircraft 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.
11-54. 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.
a. 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.
b. 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.
c. Repair. Relays are complicated electromechanical assemblies and most are not repairable.
d. Relay Selection.
(1) 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
Par 11-54 Page 11-23 derating factor to use for other types of current loads. (Refer to MIL-PRF-39016, MIL-PRF-5757, MIL-PRF-6106, MIL-PRF-83536.)
(2) Operating a relay at less than nominal coil voltage may compromise its performance and should never be done without written manufacturer approval.
e. Relay Installation and Maintenance. For installation and maintenance, care should be taken to ensure proper placement of hardware, especially 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 insulation of the electrically “live” components) is especially vulnerable to catastrophic failure as a result of over-torquing.
(1) 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.
(2) 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.
11-55. 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.
11-56. 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 11-24 SECTION 5. ELECTRICAL WIRE RATING
11-66. 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.
a. 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.
b. 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 11-7 defines the maximum acceptable voltage drop in the load circuits between the bus and the utilization equipment ground.
TABLE 11-7. TABULATION CHART (ALLOWABLE VOLTAGE DROP BETWEEN
BUS AND UTILIZATION EQUIPMENT GROUND) Nominal System Voltage Allowable Voltage Drop Continuous Operation Intermittent Operation 14 0.5 1 28 1 2 115 4 8 200 7 14
c. 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 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. When checking a circuit, the input voltage should be maintained at a constant value. Tables 11-8 and 11-9 show formulas that may be used to determine electrical resistance in wires and some typical examples.
Par 11-66 Page 11-25 TABLE 11-8. EXAMPLES OF DETERMINING REQUIRED TIN-PLATED COPPER WIRE SIZE AND CHECKING VOLTAGE DROP USING FIGURE 11-2 Voltage Drop Run Lengths (Feet) Circuit Current (Amps) Wire Size From Chart Check-Calculated Voltage Drop (VD)= (Resistance/Ft) (Length) (Current) 1 107 20 No. 6 VD = (.00044 ohms/ft.) (107)(20) = 0.942
0.5 90 20 No. 4 VD = (.00028 ohms/ft.) (90)(20) = 0.504 4 88 20 No. 12 VD = (.00202 ohms/ft.) (88)(20) = 3.60 7 100 20 No. 14 VD = (.00306 ohms/ft.) (100)(20) = 6.12 TABLE 11-9. EXAMPLES OF DETERMINING MAXIMUM TIN-PLATED COPPER WIRE LENGTH AND CHECKING VOLTAGE DROP USING FIGURE 11-2 Maximum Voltage Drop Wire Size Circuit Current (Amps) Maximum Wire Run Length (Feet) Check-Calculated Voltage Drop (VD) = (Resistance/Ft) (Length) (Current) 1 No. 10 20 39 VD = (.00126 ohms/ft.) (39)(20) = .98
0.5 — 19.5 VD = (.00126 ohms/ft.) (19.5)(20) =.366 4 — 156 VD = (.00126 ohms/ft.) (156)(20) = 3.93 7 — 273 VD = (.00126 ohms/ft.) (273)(20) =6.88
d. Resistance Calculation Methods. Figures 11-2 and 11-3 provide a convenient means of calculating maximum wire length for the given circuit current.
(1) Values in Tables 11-8 and 11-9 are for tin-plated 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 nickel-plated 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.
(2) As an alternative method or a means of checking results from Figure 11-2, continuous flow resistance for a given wire size can be read from Table 11-10 and multiplied by the wire run length and the circuit current. For intermittent flow, use Figure 11-3.
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(3) Voltage drop calculations for aluminum wires can be accomplished by multiplying the resistance for a given wire size, defined in Table 11-11, by the wire run length and circuit current.
(4) When the estimated or measured conductor temperature (T2) exceeds 20 °C, such as in areas having elevated ambient temperatures or in fully loaded power-feed wires, the maximum allowable run length (L2), must be shortened from L1 (the 20 °C value) using one of the following formulas:
(a) For copper conductor wire:
(b) For aluminum conductor wire:
(c) T hese formulas use the reciprocal of each material’s resistivity temperature coefficient to take into account increased conductor resistance resulting from operation at elevated temperatures.
(5) To determine T2 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.
(6) Approximate T2 can be estimated using the following formula:
Th is formula is quite conservative and will typically yield somewhat higher estimated temperatures than are likely to be encountered under actual operating conditions.
NOTE: Aluminum Wire: From Tables 11-10 and 11-11, note that the co nductor resistance of aluminum wire and that of copper wire (two numbers higher) are similar. Accordingly, the electric wire current in