Assembly and method of installing an electrode of a predetermined size and configuration in an electric heater envelope having an electrode opening at a preselected position. The assembly and associated method consist of affixing a hollow compression fitting about or within the electrode opening and providing a sleeve of a refractory deformable dielectric material within the fitting through which the electrode passes into the envelope. Dielectric spacing means are positioned between the electrode and electrically conductive portions of the fitting or envelope to prevent electrical conduction therebetween. Compression fastening means are then provided, securable to the fitting, to impart sufficient compressive force to the assembly to deform the sleeve about the electrode, thereby effectively sealing and securing the electrode within the fitting without the electrode grounding against the fitting or the envelope.