Fuses are designed to operate when over-currents, large and small, occur within electrical equipment; they thus interrupt the flow of current, preventing damage. They are needed for various power electric systems, for stationary and automotive applications, as well as power grid components like PV systems and distribution lines. Different types of equipment and voltages require special fuses, and their behaviour must be understood to guarantee correct choice and safe operation.
For the 3rd edition in 2004, Wright and Newbery's classic had been revised to include pre-arcing and arcing behaviour, retrofitting of fuses, chip and automotive fuses, and insulated gate bipolar transistor (IGBT) protection. Edition 4 includes updates on standards and new applications. Data, standards descriptions and many illustrations have been revised and updated. Chapters cover pre-arcing and arcing behaviour of cartridge fuselinks, constructions and types of low-and high-voltage fuses as well as miniature, plug and other small fuses, various applications, standards, manufacture, quality assurance and inspection.
This reference is indispensable for researchers involved with power electric equipment, grids and motors, and experts engaged in fuse development, design and production.
Inspec keywords: recycling; ISO standards; arcs (electric); electric fuses; asbestos
Other keywords: copper; asbestos; electric fuses; circuit breakers; dust; ISO standards; arcs; quartz; standards; recycling
Subjects: Engineering mechanics; General electrical engineering topics; Recycling; Switchgear; Industrial processes; General topics in manufacturing and production engineering; Engineering materials
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, may be 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.
Fuses incorporate one or more current-carrying elements, depending on their current ratings, and melting of these, followed by arcing, occurs when excessive over-currents flow through them. They can be designed to interrupt safely the very highest fault currents that may be encountered in service, and, because of the rapidity of their operation in these circumstances, they limit the energy dissipated during fault periods. This enables the fuses to be of relatively small overall dimensions and may also lead to economies in the cost and size of the protected equipment.
Because of the above advantageous features, fuses have been and are used in a wide variety of applications, and it appears that the demand for them will continue at a high level in the future. They were undoubtedly incorporated in the earliest electric circuits in which the source power and value of the equipment were significant.
All fuses incorporate one or more elements which melt and then vaporise when excessive currents flow through them for sufficient time, and thereafter the resulting arc or arcs must be extinguished to achieve satisfactory interruption. The means of arc extinction vary with different types of fuses. This chapter will deal with the pre-arcing behaviour of low-voltage high-breaking-capacity fuses and then variations from this process will be dealt with in later chapters when other types of fuses are being described.
The previous chapter dealt with fuselink behaviour during the pre-arcing period which ends when breaks are formed in the element because of parts of it running away in liquid form or vaporising.
It was stated in Section 2.1.1 that there must be a very short period during which voltages build up across the breaks and these lead to ionisation and the formation of arcs. The arcs then persist until the current reaches zero, at which time, arc extinction occurs, and clearly it is desirable that interruption should be maintained and that the arcs should not restrike. This process must always occur and it is important that satisfactory clearance is achieved at all current levels. At currents in a limited range above the minimum fusing value, however, the duration of the period in which arcing takes place is very short relative to the total clearance time and the effects of arcing on such factors as the energy let-through to the circuit being protected are usually insignificant and therefore not of interest. In these circumstances, it is possible to neglect the arcing period and predict the behaviour and operating times by only considering the pre-arcing performance as outlined in Chapter 2.
This is not possible, however, when the behaviour at very high currents is being considered because the period for which arcing persists may well be comparable with or even greater than the pre-arcing period and the energy let-through to the protected circuit during the arcing period represents a considerable fraction of the total.
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 as can be seen from the following section.
It was stated in Chapter 1 that fuses are classified into three groups, namely low-voltage, high-voltage and miniature. This chapter will deal with the construction of fuses in the first category and the other categories will be described in Chapters 5 and 6. For clarity in each of the chapters, British practice will be described first and then the practices in other parts of the world will follow.
This chapter will deal in a general way with the constructions of low-voltage fuses produced for industrial and domestic applications and also those suitable for the protection of semiconductor devices.
By definition high-voltage (HV) fuses are for use in AC systems operating at frequencies of 50 and 60 Hz with rated voltages exceeding 1,000 V. They are sometimes called 'medium-voltage fuses' because most are used in systems classed, in some countries, as 'medium voltage' (generally greater than 1,000 V and less than about 100,000 V), but IEC and IEEE fuse test standards refer to them as 'high-voltage' fuses.
These fuses fall into non-current-limiting and current-limiting classes, the latter being used exclusively in some countries, although both are used in other countries, including the UK.
Each fuselink should ideally be able to interrupt satisfactorily all currents from that needed to melt its element or elements up to its rated maximum breaking current. Those current-limiting fuselinks which can meet this condition are categorised as 'full range'. As explained in Section 5.2, the majority of current-limiting fuselinks produced for use in HV circuits are not, however, designed to provide the above performance, but they are nevertheless suitable for a wide range of applications. Such fuselinks may be categorised as 'partial range' or Back-Up fuses.
Following the pattern of the previous chapter, descriptions of the constructions of 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.
Although miniature, domestic plug and other small fuses are physically similar, they are grouped into several different application categories and must comply with different specification standards.
Miniature fuses are defined as being for the protection of electric appliances, electronic equipment and component parts thereof, normally intended for use indoors. A British Standard, BS 646, for such fuses, was introduced in 1935, but it was not until after World War II that they became of technical and economic importance because of the rapid development of the electronics industry.
Plug fuselinks are classified as domestic fuses, which are defined as being for use in domestic and similar buildings, for example dwelling houses, blocks of flats and office buildings. Within this class are the semi-enclosed and cartridge fuselinks used in equipment such as supply-authority fuses and consumer units. The constructions of these fuses were described in Chapter 4 because of their similarity to other low-voltage fuses.
This chapter will deal with the smallest types of fuselinks that are produced, namely the miniature and domestic plug fuses referred to above and also automotive fuses. The total world market for these components is estimated at being well over 10 billion per annum and the continuing increase in the number of small electrical appliances being marketed makes it essential that the designs and methods of manufacturing these fuses should enable them to be produced in large quantities at low cost with high quality and reliability. The various types of fuses differ in some important respects and therefore they are considered separately in the following sections of this chapter.
Fuses are used for so many different applications that it is impossible to consider all of them and therefore only some of the more common are discussed in this chapter. There are, however, some general aims and considerations which apply in all applications and these are dealt with initially.
A fuselink which is to protect a particular piece of equipment or circuit should ideally satisfy a number of criteria. This is illustrated by considering a simple example based on the circuit shown in Figure 7.1.
First, the minimum fusing current of the fuse should be slightly below the current which the cables and item of equipment are able to carry continuously.
The item of equipment will usually be able to carry overload currents for limited periods, and the fuse should operate at these current levels in times slightly shorter than the corresponding equipment time ratings. Clearly, the cables should also be able to cope with this duty without suffering damage.
Higher currents may flow as a result of faults within the item of equipment and, in these circumstances, the primary requirement is that consequential damage to the remainder of the circuit should be prevented. The extreme case will occur in the event of a short circuit between the line and neutral terminals of the equipment. Interruption of the fault current (clearance) must then be effected before damage is caused to the cables.
A further possibility is a short circuit between the conductors of the connecting cable. The most severe situation would arise if the fault was at the input end, i.e. between points A and B, and, in these circumstances, the fuse would have to interrupt the circuit before the source and supply cables could suffer damage.
This chapter covers typical applications and additional information pertaining to low-voltage, high-voltage and miniature fuses is given in Section 8.1.8 - Application guides in standards.
It was mentioned in Chapter 1 that the Electric Lighting Acts, introduced by the British Parliament in the 1880s, gave the UK Board of Trade the responsibility of introducing regulations to secure the safety of the public and ensure a proper and sufficient supply of electrical energy. This led to the production of several documents which specified regulations, and eventually, in 1919, British Standard 88 was introduced for fuses with rated currents and voltages up to 100 A and 250 V, respectively. Since that time, this standard has been extended and updated and other British Standards have been introduced. Other major countries in which fuses are manufactured have also produced their own national specifications and standards and understandably their requirements have differed from those in the UK. This has been inevitable because of factors such as the different units of length which are used in the various countries, i.e. centimetres and inches, and the different operating voltages which have been adopted.
All national standards are continually being reviewed and revised to meet the changing requirements and also to give wider international acceptance. There is also a desire to have internationally agreed standards which will be used throughout the world and these are being produced by the International Electrotechnical Commission (IEC) and the format and contents of many national standards have been aligned with those of the IEC. Most developing countries, in fact, use IEC standards and thus do not have the complicated infrastructure of national standards.
Within IEC, fuses are the responsibility of technical committee 32 (TC32), with sub-committees SC32A, SC32B and SC32C being responsible for standards for high-voltage, low-voltage and miniature fuses, respectively.
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 earlier 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. Typical quality assurance/quality control practices employed are outlined in the following sections.
Manufacturers will have third party validation of their quality systems and it is also likely that the manufacturers will also have procedures to meet ISO 14001, environmental management system.
As the arc chambers of fuses are enclosed, all the materials the fuses were originally been made from remain preserved even after operation. Because of the silver and copper contents, operated fuses have always been professionally disassembled to remove the metal parts, especially the end contacts, for the purpose of scrap recovery and utilisation. Such an approach, however, is disadvantageous in several aspects: Metal utilisation in not done in systematic and country-wide way; The metal in the fulgurite is in most cases left unconsidered; Opening of the fuse body requires excessive time and is not unproblematic, as with the quartz and potential fine quartz dust, and in the event of fuses from early manufacture - asbestos fibre, maybe released.