An electrothermal heater mat (3) is provided for an ice protection system for an aircraft (1) or the like. The heater mat (3) is a laminated heater mat and comprises dielectric layers (50-58), a heater element (501) and a conductive ground plane (71-74) for detecting a fault with the heater element (501). The dielectric layers (50-58) are made of thermoplastic material, and the ground plane (71-74) is formed by spraying metal such as copper onto the thermoplastic material of one of the dielectric layers.