The need for greater reliability in power distribution systems, mainly in connection with the spread of the use of information systems imposes the reconsideration of current criteria for the employment of devices to protect MV networks against overvoltage. Zinc oxide surge arresters, already proven in HV networks, are being employed in a more extensive way even on MV networks by virtue of their better characteristics compared with other types of devices such as spark gaps or non-linear resistor gapped surge arresters. Still, zinc oxide arresters are the object of further innovation in order to cope with some drawbacks experienced in service and to improve their performance by design optimization. At present, an interesting solution is represented by the synthetically housed surge arresters. Within ENEL (Italian Electricity Board), this interest involved a revision of the manufacturing and testing specifications for this kind of surge arrester. This report presents the main planning/design criteria behind the decision of ENEL to install zinc oxide surge arresters with synthetic housing.