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...

ties, more definite information than that ascertain-recorded by several successive hourly tests. About able by the usual indicating and recording instruments 30 per cent of the failures have been detected on will be available in the future. these tests. At monthly intervals a complete check is made of MAINTENANCE all tubes by means of a portable test set. At the same time all 132 kv circuit breakers are tripped by The carrier equipment is checked hourly by the the relays to insure that all electrical and mechanical substation operators and readings of carrier signal devices are in proper working condition. strength noted on ammeters connected in each reAt penrodic intervals a complete calibration test ceiver relay circuit. The test is quite simple and is made of all line protective relays. As the time can be completed in a very short interval, since it settings are very short and the selectivity between requires only the pressing of a button to transmit the vanrous relays is in the order of a few cycles, special signal, and the reading of an ammeter.high speed timing devices are used for this purpose. A code signal is used to identify the line being At Pleasant Valley the automatic oscillograph is tested, and by means of a prearranged schedule, the used for checking the timing of the relays after they initiating station transmits a warning signal and re-have been individually calibrated. The record ceives a "ready" signal by the receiving substation. shows the performance of the combined relay inSignals of approximately 2 seconds' duration are then stallation, including the time margins between starttransmitted by the initiating station and answered ing of the carrier signal and individual relay tripping by the receiving station, thus permitting a check of and resetting operations. signal strength of each circuit at each substation.At other substations an electronic timer is used Poor condition of any of the tubes is usually in-for checking the calibration of the individual relay dicated by a gradual decrease in signal strength as units.

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