Synopsis.-Usual methods of connecting arresters for the By interconnecting the ground lead of the arrester and the transprotection of distribution transformers are often inadequate. In former secondary neutral, the voltage across the transformer insulaservice some transformers still flash over although the arrester, in tion is limited to that permitted by the arrester alone. itself, is capable of protecting the transformer with a large margin Tests have been made to determine the conditions of surge reof safety.quired to duplicate flashovers that occur in practise. Under such Surge current flowing to ground through the ground lead of the conditions measurements were made of the voltages appearing on arrester gives rise to inductive drop and resistance drop voltages. a representative secondary circuit when bushing flashovers are These voltages added to that permitted by the arrester can be great permitted, and when flashover is prevented by interconnecting the enough to flash over the transformer bushings to secondary neutral. arrester ground and the transformer secondary neutral. GENERALThe case of low-voltage class distribution trans-ALTHOUGH the low-voltage distribution trans-formers is left free, for the safety of the workman who formers have a very high impulse strength as may touch it while working on the live primary concompared to the normal voltage rating, they ductors. will still flash over and fail in service even when they The secondary neutral is grounded at each customers are protected by arresters which in themselves could service entrance. This ground is made to the water adequately protect the transformer insulation. The service pipe as it is as good a ground as can be obeffect of arrester ground resistance alone is insufficient tained. In addition to the customers ground, there is to account for these failures, as very low grounds still usually a driven ground at a distance of one span from permit failures. the transformer. Lightning surges in the common 2,300-volt, or THE EFFECT OF ARRESTER GROUND CIRCUIT ON THE higher, urban distribution circuits manifest themselves PROTECTION AFFORDED in a somewhat different manner from those on high-Tests show that when a surge is applied to the voltage transmission lines. In the case of a distribution primary of a distribution transformer that has a circuit, the arrester spacing will be usually of such secondary ground, the isolated case will take on a low order that the distance traveled by the voltage wave electrostatic potential above ground so that some will be short, the crest voltage appearing on the line 80 to 95 per cent of the surge voltage wilt appear from will be limited, and the surge will manifest itself as primary lead to case. For simplicity in the consideracurrent through one or more arresters. The current tions that follow it will be assumed that the case takes wave, both as to magnitude and shape, is determined on a potential the same as that of the secondary very largely by the lightning stroke, and very little neutral.by the characteristi...

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