The properties required of a sheath material to be used on a metal free cable for application on long span overhead power lines have been discussed. The dominant factor affecting material choice is its electrical performance. An improperly designed sheath will lead to premature cable failure. The results of calculations of currents and voltages induced on cables in a severe UK situation have been presented, and dry-band arcing has been positively identified as the prime cause of degradation. A range of tests which have been used for materials evaluation has been described. Only tests involving significant lengths of cable (0.25 m to 10 m) have been found to be reliable. The definitive test has yet to be found, and direct material comparison must use a range of wet/dry cycles to model the range of conditions met in service. A semi-insulating sheath is proposed as a possible alternative to arc-resistant materials. Such a sheath might prevent arcing both by reducing the maximum voltage induced on the cable and by preventing enhancement of local electrical field gradients across dry bands.
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