High voltage relays are used to transfer power for high voltage and high frequency applications. They are heavily insulated and consist of strong materials to increase contact life. High voltage relays use several isolation methods to achieve high dielectric strength. Some devices include a vacuum to produce the greatest possible isolation between contact electrodes. Others contain specialized or proprietary gas mixtures at several atmospheres of pressure. High voltage relays that contain fluids such as oil are commonly available. Other isolation methods use reed relays or air as the dielectric. Hermetically sealed devices feature an airtight seal that prevents contamination. Intrinsically safe (IS) relays are incapable of releasing sufficient electrical or thermal energy under normal or abnormal conditions to cause ignition of a specific hazardous mixture in its most ignited concentration.
High voltage relays are available with a variety of pole and throw configurations. Specifications include throw type, number of poles, normal state, operation time, and release time. There are two throw types for high voltage relays: single throw (ST) and double-throw (DT). ST relays have a pair of contacts that are open in one position and closed in the other. DT relays have a third contact for making or breaking the connection of two conductors in a single branch circuit. Both ST and DT relays are available with 1, 2, 3, or 4 poles. Single-pole (SP) devices control one line with each throw. By contrast, double-pole (DP) devices control two lines. Typically, the first line is hot and the second line is neutral. Triple-pole (TP) devices control three pairs of contacts and four-pole devices (4P) control four pairs of contacts. Depending on the device, contacts can be normally open (NO) or normally closed state (NC). Operation time or pick-up time is the time required from actuation to the full current-carrying state. Release time or drop-out time is the time required from deactivation to the open state.
Selecting high voltage relays requires an analysis of performance ratings, coil ratings, and mounting styles. Performance ratings include maximum AC voltage, maximum DC voltage, dielectric strength, continuous carry current. Dielectric strength or isolation voltage is the maximum potential gradient that a material can withstand without rupture. Continuous carry current is the maximum current that contacts can carry for a sustained period of time. Coil ratings include AC coil voltage, DC coil voltage, and coil resistance. Mounting methods include both panel mounts and flange mounts. Some high voltage relays mount on printed circuit boards (PCBs). Others are equipped with a fastener that attaches to a DIN rail.
High voltage relays are used in medical equipment such as heart defibrillators, kidney stone lithotripters, and magnetic resonance imaging. They are also used in aerospace and military equipment such as high frequency antenna couplers, multi-mode radar, laser rangefinders, space and satellite applications, and lightning protection. Commercial applications for high voltage relays include electric vehicles and rapid transit, undersea cable branching systems, scientific and test equipment, silicon wafer processing equipment, and oil well applications.
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