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The paper is intended to present a perspective of the safeguards that have an economic utility in a supply system to ensure an uninterrupted supply of electricity to consumers. The safeguards are classified broadly under the headings of general, routine, protective, and ultimate. Although the most consistent application of general and routine safeguards can never completely eliminate the risk of faults, the appropriate blending of these with protective safeguards, which isolate faults automatically in their initial stages, should prevent any fault from developing into a breakdown of the major order, such as to involve the functioning of ultimate safeguards.After a general discussion of the application of protective safeguards to supply systems according to whether they include cables or overhead conductors (which, owing to differing conditions, have to be dealt with differently), there follows a survey of protective safeguards available for use with various components of a supply system against short-circuits transient faults, and excess voltages. These include feeder and busbarzone protective systems; methods of reducing unnecessary outages of overhead lines; and rapid operation of circuit breakers. Particulars are given of high-speed small-oilvolume circuit breakers developed on the lines of utilizing in improved ways well-tried principles and components, and of an automatic reclosing 132-kV oil circuit-breaker with an arc duration of one cycle at full rating.It is claimed that, when a fault has developed in a component of a supply system provided with protective safeguards such as automatic instantaneous protection and fast-acting circuit breakers, the result is a switching operation with very little if any damage of the faulty component, and, depending upon the system layout, an outage with either no interruption or only a slight interruption of supply. If, however, a fault develops in an unprotected component of a supply system, the sustained feeding of power into it may lead to severe breakdown and fire, with risk of serious interruption of supply.The authors suggest that, if the supply industry is of the opinion that it is detrimental to have such interruptions of supply as have occurred in the past (for example, because of outages from transient faults on overhead lines or because of outages due to fires resulting from sustained arcing on unprotected apparatus), and that such interruptions are an uneconomic risk for the future, one of the first steps to be taken is a consideration of safeguards adequate to prevent them.

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Two new methods of measurement in alternating-current networks are described. Each method involves the use of a cathode-ray oscillograph, together with a precision-type phaseshifting transformer, a calibrated potential divider, and amplifying equipment. One, a null method, is capable of very high accuracy. The other, although not capable of the same degree of accuracy, has certain definite advantages. It is particularly useful, for instance, in the analysis of networks containing elements with variable parameters, and in the investigation of transient phenomena in networks.The manner of operation of the measuring equipment is illustrated by a series of photographic records.

The risks to life and plant are set out. Means for prevention are discussed in full and summarized in conclusion. In the main an endeavour has been made to collate present practice and to ventilate some demands which are in a nebulous stage, including the duplication of components, the changing over of live circuits, the meaning of the breaking-capacity rating of circuit breakers, the use and effect of reactances, charging resistances and lightning arresters, and the best means of economy in switchgear without loss of safety. In reviewing known automatic protective systems, stress is laid upon the importance of what has been termed, for the purpose of this paper, the "stability ratio." Stability takes precedence over sensitivity. In balanced automatic protective systems stability is jeopardized by influences, some of which are of recent discovery and have been brought to light by new conditions of service. They include the unbalancing effect of high-frequency oscillations on main lines, and interference by induction between the main line and the pilot line. Suitable remedies are available and have been applied. The relation of interference with telephone service and fault occurrences on power supply systems provides a further argument in favour of good bonding, earthing the neutral and the instantaneous isolation of faults. Mechanical consideration is given to the layout of switch-gear and to the terminal construction of plant, and a diagrammatic illustration of the alternator main, field, and neutral connections is shown.

This paper outlines a picture of the progress during the last seven years in the field it deals with, and also sets out the problems of to-day and the lines of possible future practice. The period under review has seen a change of national policy in Great Britain, which practically puts an end to isolated plants for public service, and enhances the scope for economy by the running of the largest plants with interconnections between them and all other plants and loads. New conditions therefore arise, demanding a reconsideration of the problems dealt with by the author in their earlier form in 1925. The construction of the various types of metal-clad switch-gear is diagrammatically represented by illustrations brought upto date and concluding with a forecast of the full application of the principle of complete enclosure and immersion of all conductors and insulators for all voltages as the means of safeguarding life and property, and securing continuity of supply. Certain basic constructional features are dealt with more fully. These are, in general, typical of changes, some brought about by research and practical experience, and others evolved in the quest by designers for something ?different? in a period of intensified competition. An attempt is made to classify the prevalent forms of arcing contacts and busbar enclosures, and to state the effects of such details, particularly of the latter, on the general lay-out of the whole switching equipment. The use of metal-clad switchgear has of itself effected such economy in buildings that outdoor switchgear has not been widely used in Great Britain; but reasons are given in favour of outdoor construction. A review is given of the present position of rating, performance, and selection of circuit breakers, in order to lead up to a better understanding of the interpretation of these terms, and in view of the need for an international agreement on standard conditions for assessing ratings. Results of tests in a British 1500000-kVA testing station are recorded as examples. The need for economy is accepted as a fundamental object for the designer, but it should be effected by the reduction of components rather than by lowering the quality of details. Some ways and means of effecting economy along these lines are therefore proposed. The automatic isolation of faults has a particular bearing on the protection of property and supply; in the latter its performance in procuring stability is considered to be of paramount importance, and in the former its speed of operation is a criterion of equal merit. In the light of these principles the known protective systems are reviewed, and some new developments are mentioned. This study is in the main a history of the quest of the ideal, which, for the sake of economy alone, excludes the pilot cables of the earlier systems, and thus an opening occurs for the use of telephone pairs. Some original research is recorded for the avoidance of interference with the other uses of such telephone conductors arising from accidents on main lines. Further, the resistance of heavy arcs has been experimentally ascertained, with a view to making clear the necessity of using reactance instead of impedance as the true means of measuring distance for the accurate loaction and discrimination of faults in the distance types of proctection. The evolution of automatic and remote-control systems is traced from the earlier power-station control-room to the present-day design of regional control from a central room, and the forecast for the future favours the increasing application of mechanical principles, which in the first place tends to avoid errors, and in the second place takes over the responsibility of action in an emergency.