This Aerospace Recommended Practice (ARP) provides information to guide the selection of electromechanical relays in both electrical and electronic circuits and their 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 cover contact sets from NO to NC (and vice versa); add and replace auxiliary contact sets; change ore replace coils; use coils on systems up to 600 V 50 to 60 Hz; and expect relays to last for many operations. Aerospace relays are designed to be nonrepairable with fixed contact configuration, relatively short life, 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. All of these relays are sealed. Adjustment is not possible. A user can make no changes after relays leave the producing facility. These relays are high-performance, low volume and low mass types with short life, when compared to industrial types, and have little margin to tolerate an application or selection error. Relays approved for aerospace by the military are summarized in MIL-STD-1346. This standard provides a current listing of military standard relays. The user then evaluate these relays to determine with relays are most suitable for the application at hand and select the military relay part number from the detail specification sheet. Each of the manufacturers employ a staff of applications engineers who should be consulted if there is uncertainty in matching a relay to a specific application.