Substations can be classified into two categories according to voltage level: high- voltage and medium- or low-voltage. High-voltage includes 500kV, 330kV and some 220kV substations, while medium- or low-voltage includes 22kV terminal substations, 110kV, and 35kV substations.  High-voltage (transmission) substations are large outdoor sites. Low-voltage (distribution) substations are indoor systems located in urban areas to handle high load density. In high end voltages greater than 110kV, simultaneous sampling requires time to stamp readings and a software program to analyze the data. Customers are interested in capturing large amplitude variations on the current and voltage channels, the dynamic range. In midrange voltage stations between 110 and 3.5 kV, it is necessary to time stamp readings and software programs are also required to analyze the data in simultaneous sampling. For low end voltages less than 3.5kV, non-simultaneous sampling 14 bits or less can be used.