A heating system for a subsurface formation includes an electrical conductor and an insulation layer at least partially surrounding the electrical conductor. A jacket that includes ferromagnetic material at least partially surrounds the insulation layer. The outside surface of the jacket is configured to be at little or no electric potential while the electrical conductor is conducting electricity and while the jacket is at temperatures below the Curie temperature of the ferromagnetic material.
 The Government has certain rights in the invention pursuant to Agreement Nos. SD 10634 and NFE 062050824 between Sandia National Laboratories (operating under Agreement DE-AC04-94AL85000Sa for the U.S. Department of Energy) and Shell Exploration and Production Company.