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The use of conducting armour media between the groundwall insulation and slot iron in large high-voltage rotating machines is required for suppression of in-slot corona during operation. To ensure continued efficient operation of high-voltage machines, the importance of maintaining an adequate degree of conductance along the length of the armour and a sufficient connection between the armour and grading materials cannot be overstated. This paper presents a discussion of the effect of operating environment on the condition of conducting slot armour, and an overview of several types of commercially available materials. An experiment to measure resistance measurements before and after vacuum pressure impregnation (VPI) for six different materials is described. Resistance is evaluated as a function of distance from the ground plane. Resistances for two of these materials were re-evaluated over a temperature range approximating that experienced by the material when in operation. Voltage endurance test results are presented for a series of bars made using two of the materials in the resistance study, including observations about deterioration under high-voltage test conditions.