Studies in Lightning Protection on 4,000-Volt Circuits-III

The advancement in knowledge of lightning protection, as developed by the investigations in Chicago since the 1920 paper on the same subject, has been largely. due to an intensive study in the past five years made with the active cooperation and assistance of the manufacturers of lightning arresters. The development in recent years of methods of laboratory tests with the lightning generator and the cathode ray oscillograph render unnecessary a continuance of these statistical research investigations. The examination of 474 transformers burned out by lightning indicates the desirability of line transformers with non-aging insulation which will be lightning-proof against induced discharges when protected by the modern types of arresters. A comparison of the troubles on the distribution system due to lightning in Chicago with similar results in other cities indicates that considerable advantage is obtained by the use of the four-wire three-phase system with grounded neutral. Induced discharges of 400 kv. crest value have been noted. Direct strokes account for only 5 per cent of transformer troubles. No evidence could be found of shielding effect against induced discharges due to adjacent high trees, buildings, or other structures. Traveling waves are not a factor in the transformer troubles on the distribution system in Chicago. Modern types of lightning arresters excel the best type of 20 years ago in reduced annual charges, but not in efficiency as a protective device. Extra expense to reduce resistance of lightning arrester grounds below 50 ohms is not warranted.