The CP SB1 is a transformer switch box designated for automatically measuring the ratio and winding resistance and testing the tap changer of three phase power transformers. It is an accessory to the CPC 100. Automatic control of the On Load Tap Changer is included. Testing of power transformers over all taps and all phases is fully automated. Therefor, no rewiring is required. The CP SB1 is controlled from the CPC 100 via its serial interface. The results are recorded in the CPC 100 with the ratio and tap changer test cards, can be analyzed with the computer tool set (CPC 100 Excel File Loader).