For an equipotential surface,
Change in potential difference, ∆V = 0
Work done = q×∆V = q×0 = 0
Work done = Fs cosA = 0
Where, F = Force due to charges,
s = distance between charged bodies, and
A = angled between direction of F and s
Since, the equipotential surface is formed due to cancellation of equal and opposite electric fields, F and s cannot be 0.
So, Work done = 0 if and only if cos A = 0 → A = 90°
Hence, the equipotential surfaces are always at right angles to lines of electric force.