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9/8/98 AC 43.13-1B 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 11-4 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. g. Low Energy Loads. Switches rated foruseat28VDCormore,andat1.0ampor more, generally have silver contacts. In gen- eral, silver contacts should not be used to con- trol 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 con- tact materials for low-energy loads, but is not applicable to hermetically sealed switches. (1) Typical logic load devices have a voltage of 0.5 volts to 28 volts and a continu- ous 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 electro- mechanical switches of certified quality.) TABLE 11-5. Selection of contact material. TABLE 11-4. Switch derating factors. Nominal System Voltage 28 VDC 28 VDC 28 VDC 28 VDC 12 VDC 12 VDC 12 VDC 12 VDC Type of Load Derating Factor Lamp 8 Inductive (relay-solenoid) 4 Resistive (Heater) 2 Motor 3 Lamp 5 Inductive (relay-solenoid) 2 Resistive (Heater) 1 Motor 2 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 de- pendent 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. 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. Par 11-53 Page 11-17PDF Image | AFS-640
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