This chapter focus on basics and general principles of switchgear. A fundamental question is the type of question that children specialise in asking, and, in order to respond correctly and fully, you have to give the subject more thought than would otherwise be the case. My grandchildren specialise in these sorts of thought provoking questions and, as I am sure that all readers are familiar with the function of gooseberry bushes, I shall confine myself to try to address the fundamental question 'Why do we have switchgear?'. Certain electrical distribution customer's senior engineers used to go out of their way to say that switchgear was a necessary evil. It cost money to buy, install and maintain and that it did not earn any revenue. This is clearly an oversimplification as the end user only buys electrical power, so anything that makes that possible must contribute to that end. These customer's engineers did, however, concede that switchgear was necessary to isolate equipment that became faulty, and they could allow the system to be split into sections to allow quick restoration of power supplies. While electricity distribution systems are relatively passive, for example, the situation in a factory, particularly one using manufacturing processes, or in a generating station can be active, the switchgear takes a critical part in controlling what is taking place. So, switchgear is needed: (a) to isolate faulty equipment; (b) to divide large networks into sections for repair purposes; (c) to reconfigure networks in order to restore power supplies; and (d) to control other equipment.

This chapter presents the calculation of short circuit currents when fault occur in the electrical distribution network. When planning a new installation, or modifying an existing installation by adding in a transformer or making a cable connection to another substation, the effect on the fault level needs to be determined. This is to ensure that the installed plant will still be within its rating and will be able to carry and interrupt the fault current safely. There are a number of specialist companies, and some software packages available, to carry out these calculations but a practising engineer should be able to determine the likely fault level at the feasibility stage.An example of a very simple fault level calculation follows, where there is only one element of impedance. You will notice that in the calculations the network impedance upstream from the transformer is ignored, as this will have a very small value is also discussed in this chapter.

It is a fundamental phenomenon of electricity that a force is exerted between conductors carrying electrical current. Under normal load current conditions, these forces are very small, however, many engineers will not be aware of the enormous forces that are generated when the normal current is replaced by a short-circuit current which can be 40-50 times larger in magnitude. This force generated phenomenon forms the basis of many desirable aspects of electrical engineering, such as the operation of measuring instruments and electrical motors, but in switchgear these forces are potentially dangerous in terms of the stresses induced in both the conductors and their supporting insulators. Two factors influence the magnitude of the electromagnetic force that will be experienced. These are the strength of the magnetic field and the current flowing. The field strength can be derived from Laplace's Law. This states that the field strength created at a point in space due to the passage of electric current through a conductor is inversely proportional to the square of the distance between that point and the conductor, and is directly proportional to all other factors. The force experienced by a conductor in a magnetic field is derived from Biot-Savart's law in that the force is proportional to the flux density and the length of the conductor.

It is obvious that conducting components within distribution switchgear have a high voltage potential difference both interphase and with respect to earth when in service. These components have to be securely mounted and fixed in position by using materials which are very poor conductors of electricity. These are known as insulating materials and form a range of components that are continually stressed throughout the whole life of the equipment. It is certain, therefore, that latent defects within insulating materials, due to inadequate selection, design or manufacture, will manifest them selves during the life of the equipment. It follows, therefore, that great care must be taken when designing insulation systems that are to be incorporated within distribution switchgear. Insulating materials are, by definition, very poor conductors of electricity, which is why they are used to cover conductors, give support to busbars and other conductors, and, in a gaseous form, are used to fill compartments to provide both insulation and a dry, clean environment.

This chapter discussed the primary switchgear which is the first stage in the process of conducting electrical power from the grid to the end user. The importance of the strategic position of a primary substation and its switchgear within the system means that the layout, design and operation must ensure maximum availability and reliability.

Because of the relatively high cost of electrical distribution by buried cable, power supplies to sparsely populated areas are invariably provided by overhead line conductors. The present-day overhead line distribution equipment is the product of innovation and evolution resulting from many years of application in the field of rural electrical distribution. Advances in technology, as they have become avail able, have been adopted to provide better security and continuity of the supply of power to remote centres of consumption, as well as providing the power supplier with information which was not readily available a few years ago.

It is not the purpose, or possible, within the scope of this book to provide a detailed step-by-step guide on the procedures to be followed when conducting development and type testing of switchgear. Indeed, such a guide would run into several volumes and, due to detailed changes in specifications that take place from time to time, it would quickly become obsolete. It would also have to cover all types of switchgear including circuit breakers, switches, fuse switches, fuses, earth switches and disconnectors. However, the intention here is to give an overview of the important stages in testing that have to be completed in order to provide switchgear that is safe and able to operate correctly in the circuits and ambient conditions for which it is intended. By way of example, particular reference will be made to circuit breakers. The opportunity will be taken to highlight certain aspects of each type test in order to show the relative importance and the way in which solutions to encountered problems can, and usually will, have an influence on the performance of other type tests.

The cost of switchgear ownership can be very much greater than simply the initial capital cost of purchasing the switchgear. Some accountants are only interested in the expenditure within a current financial year. However, it could be argued that this first cost will only be a small percentage of the total cost of ownership over the lifetime of the switchgear. The true cost of ownership should include the cost of the substation, as different types of switchgear will occupy different volumes of substation space and, therefore, influence the overall substation cost. In addition, costs will be incurred for the erection, installation and commissioning of the switchgear. Once the switchgear is installed, other costs will be incurred, such as maintenance, labour, materials and outage time. Finally, disposal at the end of useful life must be considered. For example, SF₆ switchgear will incur disposal costs, as only specialist companies have the facilities necessary to safely dispose of the equipment without risk to personnel and without contaminating the atmosphere. Conversely, disposing of oil and vacuum switchgear at the end of their life will generally be self-financing or even yield a small profit. All of these costs combine to give the true lifetime cost of ownership.

This chapter lists sources of information which will assist the reader to search and explore aspects of switchgear in greater depth.