The SSRL series of solid state relays are used to control large resistance heaters in conjunction with temperature controllers. Solid state relays are SPST, normally open switching devices with no moving parts, capable of millions of cycles of operation. By applying a control signal, an SSR switches ON the ac load current, just as the moving contacts do on a mechanical contactor. Three-phase loads can be controlled using 2 or 3 SSR's. Use 3 SSR's for Y or star 3-phase loads using a neutral line. Two SSR's will control “delta” loads with no neutral line. Three solid state relays are also used when there is no neutral load to provide redundancy and extra assurance of control.