Mechanical relays are very delicate instruments which must often operate in environments polluted with dirt and corrosive chemicals. They are also often subjected to strong and continuous vibration. Never-the-less, their reliability must be extremely high if they are to protect equipment. Therefore, routine testing and maintenance is necessarily frequent.
Manufacturers now offer most relays with solid-state components which eliminate many of the maintenance problems of the mechanical types. Solid state devices are of two basic types: Analog devices operate according to the equations of natural laws of pressure, temperature, flow, etc. Digital devices operate by comparing numbers. The digital devices permit degrees of precision impossible with mechanisms having moving parts. Furthermore, digital devices can consider a large number of conditions. Digital computers programmed to monitor the operating conditions of a substation or plant can also be considered as relays.
The analog devices use transistors, diodes and SCR devices as parts of an electronics package. These can perform specified sensing and control tasks in the operations of a power system. The newer circuit breakers with solid state trip elements are examples of electronic devices.
