This SAE Aerospace Recommended Practice (ARP) provides information to guide in the selection of electromechanical relays to be used in electrical/electronic circuits for application in aerospace@ ground@ and shipboard systems to achieve proper performance. Relays intended for use in aerospace industry and military applications are substantially different from those produced for industrial applications. Most industrial relays are designed so that the user can convert contact sets from NO to NC (and vice versa); add and replace auxiliary contact sets; change or replace coils; use coils on systems up to 600 V 50 to 60 Hz and expect relays to last for many operations. Typically@ aerospace relays are designed to be nonrepairable with fixed contact configurations and generally for systems not exceeding 200 V and 400 Hz. A family of relays is approved by the Military for applications not exceeding 270 VDC. These relays are generally for 28VDC@ 270VDC@ and 115VAC 400hz applications. The applicable system requirements are typically in accordance with MIL-STD-704 and RTCA-DO-160 (Radio Technical Commision for Aeronautics). These relays are high-performance@ low volume and low mass types@ when compared to industrial types@ and have little margin to tolerate an application or selection error.