Constant Boundary Condition

Is there a way to specify a constant boundary condition (in 3d)? I am doing analysis (capacitance, voltage, etc) of a composite electrical component. So we know that inside a conductor, we have no electric field, thus constant voltage. To model this constant voltage, I can set a very high conductance parameter, but then the solver seems to be unstable - So instead, is there a way to set constant voltage condition wherever a conductor is present?

The equation is, simply,
Laplacian of (epsilon * V) = 0