S c is the compressive strength.
This criteria is often used when designing with brittle materials such as concrete or some cast irons. Applies only in the elastic range. States yielding occurs when the largest of the 3 principal strains becomes equal to the strain corresponding to the yield strength. If then the three shear stresses would all be zero and there could be no yielding - regardless of the hydrostatic stress. The magnitude of the hydrostatic stress has no effect on the size of the Mohr circle, but move it along the normal stress axis.