The need of a better understanding of the origin, distribution, and scaling of halo currents flowing through plasma and vacuum vessel during Vertical Displacement Events (VDE) is one of the critical issues for the ITER project. JET can play a key role in this study, provided that new and more detailed information becomes available. This fact motivated the design of a set of additional sensors specifically aimed at improving the diagnostic capability of halo currents at JET. This new system is described in the paper. It consists mostly of Rogowski coils and of a small number of toroidal field pick-up coils. The former will measure directly the current flowing through the tiles of the first wall. The latter will allow the estimation of the total poloidal halo current where the insertion of Rogowski is technically difficult and provide complementary data for a better characterization of the phenomenon. The system is a compromise between the maximization of the spatial resolution in both toroidal and poloidal direction and the actual JET capability of adding new in-vessel probes and new conduits needed to bring the signals outside the vessel. The main system issues are briefly reported, then the modifications of the in-vessel components, necessary to house the probes, will be described together with the technological solutions adopted to improve the measurement quality. Electromechanical analysis is performed on modified tiles to allow the measurement of the current flowing through them. Its results are also presented in the paper.