A centre tap is a connection made from the centre of the secondary winding of a transformer. It is labeled as Earth, Common or 0V. Grounding secondary winding of a transformer is usually done by connecting the earth to the neutral lead. There exists another system in which earth is connected to the centre tap of the transformer instead of the neutral. This is known as Centre Tap Earth (CTE). CTE systems are very common in electrical distribution systems and site safety operations.

Some common CTE operating voltage
110V CTE 55V-0V-55V
220V CTE 110V-0V-110V
230V CTE 115V-0V-115V
240V CTE 120V-0V-120V

Before selecting a transformer for a system, it is important to understand its earthing system whether it is neutral earthing or centre tap earthing. By having a wrong transformer for the application may cause damage to the connecting devices.

For the following diagrams, 110V CTE will be used.

Mains Supply System

In this system, the transformer has 2 output leads at the secondary coil, neutral and live. Neutral lead may be connected to the supply earth.
Line-to-earth shock potential is a full 110V.

Centre Tap Earth System

In a 110V CTE system, instead of the neutral lead, the centre tap is connected to the supply earth. This system has no neutral, therefore caution must be taken to ensure that no live lead is earthed. Short-circuit may occur and may lead to system power failure. Also, the centre tap of the transformer should be left unconnected when is it not in use in an unearth system.

CTE system is commonly used for site safety portable tools. The centre tap splits the winding into 2 sides with each having only 55V with respect to earth. In case of an electric shock, this limits the magnitude of the shock potential to 55V, half of 110V. Although 55V can still be a fatal shock under certain circumstances, it is better than a full 100V. CTE system for portable tools are usually installed with residual-current device (RCD) for safety reasons.