Solid state relays (SSR) or semiconductor relays are semiconductor devices that can be used in place of mechanical relays to switch electricity to a load in many applications. They are purely electronic devices, normally composed of a low-current control side and a high-current load side (switching side). Many solid state relays (SSR) feature electrical isolation in the thousands of volts between the control side and the load side. Typically, this is achieved through optical isolation using an optoelectronic device (a photocoupler).