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In this example we will revisit the basic problem of determining the potential between a grounded plate and a circle/sphere on an electrostatic potential. In  we had approximated the potential with a homogeneous charge on the ring and grounded the plate by putting a mirror charge behind the plate.
In more complex situations the position and shape of a mirror charge is not trivial. Therefore we use a numerical PDE solver.
As we have seen on tha page "FiPy: Solving PDEs with Python"  boundary conditions can only be defined on exterior faces of a mesh. Even in the simple case of a grounded plate and a circle on a defined potential one has to generate a mesh that fullfills the boundary conditions in gmsh .
Before running the simulation one has to adapt the script from  to import a mesh instead of generating a regular one -- as you can see here. Then one can successfully define a boundary condition on the inner exterior face of the mesh and run the simulation.