Fuses are among the best known of electrical devices because most of us have quite large numbers of them in our homes and, unless we are extremely fortunate, we are made aware of their presence from time to time when one must be replaced because it has blown or, to use the official term, operated. They are basically simple and relatively cheap devices, although their behaviour is somewhat more complex than may be generally realised. The underlying principle associated with fuses is that a relatively short piece of conducting material, with a cross-sectional area insufficient to carry currents quite as high as those which may be permitted to flow in the protected circuit, is sacrificed, when necessary, to prevent healthy parts of the circuit being damaged and to limit the damage to faulty sections or items to the lowest possible level. As an example, a fuse element a few centimetres long with a particular cross-sectional area could be used to protect an electrical machine winding containing a considerable length of conductor, maybe kilometres, of a cross-sectional area slightly greater than that of the fuse element. In this case the volume of conducting material to be sacrificed in the event of a fault would only be a tiny fraction of that being protected and the cost of the protection would clearly be acceptable.

To determine the arcing-period duration, it is necessary to predict the current variation from the end of the pre-arcing period and to thus determine when the current will fall to zero. To do this, the relationship between the current through the fuselink and the voltage across it during arcing must be known or be calculable. The following are also presented: basic conditions during the arcing period; arc model; positive column; power supplied to an anode; power lost by an anode; power supplied to a cathode; power lost by a cathode; power balance; and complete mathematical model.

In this book chapter, descriptions of the constructions of high-voltage (HV) fuses produced and used in the UK are given first and then the practices in other countries are stated, particular attention being given to significant differences in design. The chapter covers the following subjects: non-current-limiting fuselinks including expulsion fuses and liquid fuses; current-limiting fuselinks including constructions of back-up or partial-range fuselinks, current-interrupting abilities and categories of partial-range fuselinks, and full-range fuselinks; Continental European practice; and North American practice including current-limiting fuses and non-current-limiting fuses such as expulsion fuses, weak links, boric acid fuses, and automatic sectionalising link (ASL).

This book chapter covers typical applications of fuses and deals with the following topics: (1) general aims and considerations which apply in all applications including time/current relationships, I²t values, virtual time, published time/current characteristics, cut-off characteristics, operating frequency, and application of fuses to DC circuits; (2) discrimination and co-ordination of protective devices in a circuit; (3) protection of cables; (4) protection of AC induction motors; (5) protection of power transformers; (6) protection of voltage transformers; (7) protection of capacitors; (8) protection of semiconductor devices and equipment incorporating semiconductor devices including rectifiers, DC thyristor drives, inverters, power transistors, and circuits with high surge current flow; (9) protection against electric shock; (10) arc flash; and (11) power quality.

The standards set for the construction of prototype fuses in both materials and manufacturing processes become the standards which must be maintained when bulk production commences. This is essential to ensure that the production fuses will have the performance characteristics indicated by the type tests and that they will conform with the appropriate specifications. This involves the preparation of detailed specifications and procedures for all stages of manufacture from the purchase of materials and components, through production processes to final inspection and testing. In parallel with this, inspection routines and procedures are set up to monitor current standards and achievements continually in order to provide assurance that standards are being met and that a continuous feedback of corrective action is applied to the ear lier stages in the production process to maintain the most economical course towards achieving the set objectives. The procedures adopted by various manufacturers obviously vary in detail but comply with the requirements of ISO 9001: 2000. Typical quality assurance/quality control practices employed are outlined in the following sections.