In recent years, the amount of electrical and electronic equipment installed in motor vehicles has increased greatly, resulting in more complex 12 V systems and load requirements ranging from 1000 W to in excess of 2000 W in highly appointed vehicles. For circuit protection requirements, a modern middle-of-the-range model can have about 30 fuses in it, and there may be 75 or more in a luxury car.
For many years, simple cylindrical glass fuses were used in the UK and USA and exposed fuse element, semi-cylindrical fuses (commonly called ‘torpedo’ fuses) were used on the continent of Europe. The current ratings of these fuselinks were not universally standardised and the UK previously rated its fuselinks at the current
at which the fuse would operate, the true continuous rating being about half that. In some instances fuselinks were marked with a dual rating; as an example, a marking of 10/20 A on a fuselink indicated that it could carry 10 A continuously and that it would operate at 20 A.
The prevailing nominal voltage in motor vehicles is 12 V and larger vehicles such as trucks, buses, emergency equipment or specialised platforms (e.g., military split systems of 12 and 24 V). As these applications evolved, it became necessary to use fuses that could accommodate either voltage range. A common value of 32 V emerged, as this would handle the highest permissible transient voltage in a system operating nominally at 24 V. For short-circuit ratings, 1000 A became an accepted standard for 32 V fuselinks and more recently, some fuselink designs have capacities up to 2000 A, partly in response to application requirements in systems outfitted with high-wattage components and supplies.
The trend towards greater complexity and electrically operated accessories in vehicular systems coupled with governmental regulations concerning energy con-servation and reductions in toxic emissions has driven the transportation industry’s research and development towards new solutions. Electrically, this has meant new approaches to supply and overall vehicular electrical architecture design. At present, there are several solutions that have gained some prominence: 42 V electrical systems (or 42 V/12 V split system, a mild-hybrid), high-voltage drive system for electric propulsion motors (such as 300 V) with either 42 V or 42 V/12 V subsystems for conventional components and accessories (a hybrid), and the latest thrust to develop-ment of fuel cell supplied systems with a high-voltage AC or DC controller for totally electric propulsion and step-down conversion for all other necessary components.
The hybrid-type systems rely on a reduced form of internal combustion engine for supplemental propulsion and system electrical charging while the fuel cell approach uses hydrogen as fuel processed through an exchange catalyst that produces electric current with the ‘waste’ product being water. All of these newer approaches change the complexity of over-current protection requirements and what sort of over-current device will be required to accomplish protection.
As electrical system voltages and current capacities have evolved, so have the approaches towards fusing. The first major evolution in fuse design occurred in the late 1970s. The cylindrical glass fuses and ‘torpedo’ fuses, while satisfactory in performance, were not well suited to amperage ratings exceeding 30 A, were not conducive towards reductions in size requirements for mating fuse holders, and were not easily adapted to high-volume vehicle manufacturers desiring to automate the assembly processes of electrical centres in vehicles. In response to this, a blade-type fuse format was developed and introduced. Three basic sizes have emerged in succession as illustrated in Figure 6.10, which have become the dominant type of fuse used in the transportation industry. These fuses, which are available in standard ratings from 1 to 100 A with 32 V maximum, are capable of withstanding the inrush currents that occur in vehicular electrical systems. The two smaller sized blade-type fuses are typically considered as fast acting. The large size blade-type fuse is generally termed as ‘time lag’ in operation, which means that overloads under 150 per cent of rating take a longer time to operate the fuse than the fast acting styles. This makes the
Figure 6.10 Blade-type automotive fuses
Figure 6.11 Bolt-in automotive fuses
fuses better suited for higher inrush conditions or for short-circuit cable protection.
The blade-type fuses utilise plastic housings that are moulded in various colours to allow for a system of visual recognition tying colour to amperage rating, which establishes standardisation for users.
With larger amperage requirements in some components and the need for main fusing, other styles of fuses have emerged, such as bolt-in fuses. These fuses, as the term implies, are installed into holders with threaded mounting posts that are used to affix wire terminals and fuse terminals together (see Figure 6.11). Bolt-in style fuses can cover current ranges from as low as 30 A to as high as 500 A, also with 32 V maximum.
The emerging voltage increases for advanced vehicular electrical systems have resulted in the availability of some compatible fuses. For 42 V system applications,
several versions of blade-type fuses and bolt-in fuses have been developed with 58 V ratings, which reflects the anticipated maximum transient voltage in a 42 V nominal system. As with earlier 32 V devices, proper mechanical design and clearances have provided short-circuit performance in either the 1000 or 2000 A levels as unfilled fuselinks. The higher arc potential can be successfully managed in the melting element sections of the fuses. The higher operating system voltages do, however, make it necessary to incorporate certain safety features to prevent users from disengaging or engaging fuses or other pluggable devices in live circuits due to the destructive nature of arc energy on mating terminals.
While conventional fuselink design has application in both older and newer sys-tems, other means of fusing (over-current protection) are being explored. Examples would be: positive temperature coefficient (PTC) devices, see Section 1.3.3, which are solid state in nature and can ‘switch’ between conducting and resistive states, surface mount devices (SMD) that could be fuses or solid-state devices directly installed on circuit boards used in vehicular systems, electronically controlled power distribution systems that monitor circuit loads and switch on/off or re-route loads, pyro-fuses that incorporate conventional fuselink design but have firing circuits that force a fuse operation in a catastrophic event such as a vehicular crash to cut all power from the supply, and other electronically activated devices such as a Hall-effect sensor in combination with a field effect transistor (FET) that would shut down a circuit when current overload biases the sensor output to the FET.
Remarkably, glass fuses and ‘torpedo’ fuses are still in use today though pre-dominantly as either service parts or in aftermarket accessory modification products.
The blade-type fuse is still widely used at the vehicular manufacturer level due to its wide availability and economy. Alternative protection devices will continue to emerge in response to newer overall technology improvements and economies of scale. Finally, legislative efforts on toxic materials and vehicle end-of-life disposal requirements will affect fuse design and use.