A heater includes a conduit and three insulated electrical conductors located in the conduit. At least one of the three insulated conductors includes an electrical conductor at least partially surrounded by an insulation layer and an electrically conductive sheath at least partially surrounding the insulation...http://www.google.com.hk/patents/US20100089584?utm_source=gb-gplus-share專利 US20100089584 - DOUBLE INSULATED HEATERS FOR TREATING SUBSURFACE FORMATIONS
