The paper discusses the electrical characteristics and performance in service of porcelain insulators for outdoor high-voltage apparatus. It is emphasized that if large power systems are to be operated with a minimum of breakdowns a carefully planned scheme of field testing, supplemented by laboratory investigation, is essential.Part 1 describes the laboratory apparatus and the field testing equipment used by the Central Electricity Board. For laboratory investigations, two outdoor high-voltage testing sets are installed, having maximum voltages of 85 kV and 250 kV. A test tower and rack are associated with each equipment, and continuous pollution tests can be made simultaneously on 60 insulators. Counters are installed on all the insulators to record the leakage-current “surges.”Field tests are made with the lines in service, with an apparatus consisting of a capacitance potential-divider and electrostatic voltmeter. The potential distribution on the insulator is measured, and faulty units are detected by the abnormally low potential-drop across them. The surge counter is also applied to equipment in service in order to indicate when insulator cleaning is necessary.Part 2 gives some of the results of investigations made with the apparatus and technique described in Part 1. Information is given regarding the performance of line insulators, post insulators and bushing porcelains. The behaviour of insulators in fog, and similar weather conditions, is discussed, and examples are given of the highly irregular potential distributions and leakage-current surge records obtained under such conditions. Comparative results are given for “normal” and “anti-fog” insulators. The mechanism of insulator flashover in fog is described in detail. The requirements of a good anti-fog insulator are discussed, and for an insulator for use in highly polluted and humid atmospheres a leakage path of 2 in. per kV of applied voltage is recommended. A new design of insulator unit is described in which the potential distribution is stabilized by resistors.Information, based on service experience, is given on the deterioration of insulators. With high-quality modern insulators, the rate of deterioration should not exceed 0.1% per year, but it is nevertheless essential to make field tests, as a single faulty unit may lead to the complete failure of the insulator string. Confirmatory tests made in the laboratory show that in most cases (approximately 90%) the field diagnosis of faulty insulators is correct. Deterioration takes the form of cracks in the porcelain head, and the mechanism of insulator failure due to cracking is described.The cause and characteristics of radio interference due to power lines are discussed, and data are given of the interfering field strength due to high-voltage lines operating in various weather conditions. Under the worst condition—fog—the interference increases by 40 db. above the fine-weather value. The attenuation of the interfering field is very rapid at right angles to the line, and a decrease of 25 db. occurs in 50 yd.; at this distance from the line, local broadcasting stations may be received virtually free from interference even under the worst weather conditions.