earthing is provided to pull down the floating neutral to ground and provide a path for zero-sequence currents. Not all neutrals are grounded, and not all grounded neutrals are hard grounded (meaning, sometimes we do put some impedance while grounding, to limit the zero sequence currents).
In delta, the zero sequence currents just flow through the windings. There is no path for them to sink into earth.Only neutral point is earthed to provide a path to unbalance current of 3 phase system. Delta connected circuits do not have any neutral point. So there is no question of earthing.
But in the case with Star connected circuit , where one can find neutral point, which is earthed as per requirement.
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