In Star Connection meeting point of 3 phases is taken as earthing or ground,but what of Delta circuits?

There are a few common techniques for grounding a delta system. These are:

1. Leave it ungrounded, so the only ground connection is through parasitic capacitance of the system. It is common to apply some form of voltage-based ground fault monitor on this system, since a phase-to-ground fault causes vary little current flow.

2. Connect one phase to ground, then use two conductors to distribute the other two phases. This is referred to as a 'corner grounded' system. It is still a three-phase system.

3. Use a transformer with midpoint tap. Ground the midpoint tap. In the U.S., this is seen as 240V delta, with 2x 120V from the transformer winding with the midpoint tap.

4. Apply a separate grounding transformer, such as a zigzag or wye-delta, with or without additional impedance in the neutral to limit ground fault current.