Electrical power sensors are used to measure characteristics of electric power such as voltage, current, power, and potential degradation of insulation due to age. For AC circuits, an electric power sensor can also be used to measure power quality. Some electrical power sensors are incorporated into transformers or other components of the power distribution grid. Others are added to existing systems. Typically, these electrical power sensors measure one or more physical characteristics and convert them to electrical signals. Basic electric power sensor categories include electrical capacitance sensors, electrical frequency sensors, electrical resistance sensors, and electrical conductance sensors.