Penetration of an applied quasistatic electric field inside an open, conducting, spherical shield is analyzed. The shield is either empty or can be loaded with a concentric spherical conductor, and its opening is assumed to be circular. The electric potential and field multipole expansions are determined and, in the case of a hollow cavity, closed analytical expressions are provided. When the cavity is loaded, convergence of the multipole expansions is improved by a preconditioning technique which isolates the field singularity at the sharp edge of the aperture, and makes its analytical treatment possible. Numerical results for the electric field complement the theoretical analysis.