GxxRT-style steel tube resistors are build with steel tubes made from stainless steel that are fitted in a frame. The steel tubes are bended in a U-shape, connected at one end in a connection box. This can be an open box or an IP65/IP66 box. To prevent humidity penetrate the magnesium oxide inside the tubes and degrade the insulation value, the endings of the elements are covered with a layer of epoxy resin. Often a heating wire is embedded inside the epoxy to further protect the elements from humidity.