according to the concept of equipotential surfaces work done in moving a charge from one point to other over equipotential surface is zero. so a test charge must continuously keep moving over the equipotential surface. but this is not what happens . wh
The component of Electric Field parallel to an equipotential surface is zero, as the potential does not change in this direction. Thus there is no net force on the test charge to keep to moving over the equipotential surface. It'll require a constant radial force for a test charge to be in motion over an equipotential surface as it is circular and not a plane.
An anology to understand the same may be taken as the work done by a coolie in moving laguage from one place to another is zero as the force acting is perpendicular to the direction of displacement. Similarly on an equipotential surface electric field is perpendicular to the surface.
Hope this helps.