Tap changers are devices used to maintain a constant voltage in a transformer or to maintain it within the prescribed limits.

An off-load tap changer, (OLTC),  or De-Energized Tap Changer (DETC),  allows the output voltage to be adjusted by changing the taps in the transformer using a control device. As the name implies, these types of tap changers require that no load is connected to the transformer or that the transformer is completely de-energized before any switching maneuver.