Phase to phase and phase to earth clearance?

A clearance is defined as shortest distance through the air between two conductive elements.

Practically all generators and many motors and high-power motors have 3 power conductors: the 3 phase wires.
Many also have a 4th "ground" or "earth" wire.

It is important to keep the physical distance (typically measured in inches or mm) between any one phase conductors and any other phase conductor, and between any one phase conductor and anything else (such as the earth conductor) big enough so that
(a) the normal voltage between the conductors should not cause a spark to arc between them, and
(b) the common surges of voltage caused by distant lightning strikes should not cause a spark to arc between them.

Many systems deliberately put the ground conductor relatively close to each phase conductor through a glow-discharge tube or other lightning arrester, such that
(c) the surge of voltage on a phase conductor, caused by nearer lightning strikes, causes an arc to the safety ground through the arrestoer and dissipates most of the energy relatively harmlessly.

See the related links for some information on how big those clearances should be.

phase to phase implies: voltage measuring between two lines at different voltages
while latter implies:" b/n one line to ground at zero potential"