The on-load tap changer (OLTC) is used to change the tapping connection of the transformer winding while the transformer is energized. The tap changer can be designed as a single unit for single and three-phase applications with one common neutral point. Depending on the three-phase rating, it might require three separate units, each having its own insulated phases. Tap changers can be located either inside the transformer main tank or outside in its own compartment. Switching from one position to another has to be performed through an impedance to avoid a short circuit between two steps of the regulating winding. The transition impedance can be either a resistor or a reactor. Most of the tap changers are of the resistor type.