The procedure used for assessing the zone of protection of early streamer emission (ESE) systems is outlined, according to standards which have recently been adopted in the national codes of some countries. Much importance has been placed on the upward leader velocity by previous investigators, when assessing the zone of protection offered by an ESE device. However, by applying a simple model to the attachment process, it is shown that it is the velocity ratio, between the upward and downward leaders, that determines the theoretical zone of protection offered by any air terminal. Various assumed values of the velocity ratio are examined, with reference to existing attachment process models and measurements on natural lightning discharges. The influence of discharge intensity on ESE zone of protection is examined. A question is raised on the ability of the upward connecting discharge from an ESE device to propagate as effectively as that from a Franklin rod, in view of thee fact that a leader launched 'early' will be subjected to a lower electric field. Evidence in the literature suggests that this consideration could prove detrimental to discharge propagation.
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