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9/8/98 AC 43.13-1B (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 ap- plications typically have a metal face and pot- ting 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. d. 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 mo- tion. Inadvertent operation of switches can be prevented by mounting suitable guards over the switches. 11-48. RELAYS. A relay is an electrically controlled device that opens and closes electri- cal contacts to effect the operation of other de- vices in the same or in another electrical cir- cuit. The relay converts electrical energy into mechanical energy through various means, and through mechanical linkages, actuates electri- cal 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 con- trols. 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. Installa- tion procedures vary by the type of connection and should be followed to ensure proper op- eration of the relay. c. Repair. Relays are complicated elec- tromechanical assemblies and most are not re- pairable. 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 derating factor to use for other types of current loads. (Ref. MIL-PRF-39016, MIL-PRF-5757, MIL-PRF-6016, MIL-PRF-835836.) (2) Operating a relay at less than nomi- nal coil voltage may compromise its perform- ance 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 hard- ware, especially at electrical connections. The use of properly calibrated torque wrenches and following the manufacturer’s installation pro- cedures is strongly recommended. This is es- pecially important with hermetically sealed relays, since the glass-to-metal seal (used for Par 11-53 Page 11-19PDF Image | AFS-640
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