Fused thinking

Any belief that only circuit-breakers provide the most reliable form of circuit protection and will in time supersede fuses is dramatically misplaced. It is true that circuit-breaker technology has benefits in the home, but high rupturing capacity (hrc) fuses have much higher breaking capacities than equivalent miniature circuit-breakers (mcbs) and operate much faster. This is why they are more widely used in industrial applications and in particular for protecting electronic circuits.

Although the operating principle of fuses has remained basically unchanged since the second half of the 19th century, fuses have evolved enormously. The most recent developments have improved the performance of fuses, widened the scope of their applications and made them easier and safer for installers and users.

After operation, which involves the over-current melting of the elements, the fuse must be replaced. This is a small price to pay for high performance and total reliability. Fuses are relatively low cost items and coupled with the small space they take up, they are very economic. Further, they can be recycled and the component metals reused.

Today, users have the choice between quick-acting and time-delay fuses. Quick-acting types are typically used in circuits with little or no inrush currents or where high over-currents or short-circuit currents must be quickly interrupted. Resistive loads such as heater elements are examples where quick-acting fuses are used. They are also now being widely used to protect high-tech electronic equipment.

Time-lag components are used where high starting inrush currents occur and decay gradually, examples include high, loaded-motor, switch-on currents (inductive load) or a battery charger (capacitive load). With motor circuits there are two types of over-current: motor overload and short-circuit. The former is guarded against by overload relays; the latter is best served by fuse protection.

In industrial applications, short-circuit over-currents can be as high as 50 kA. Such over-currents demand very fast operation of the protection device to avoid severe damage to the installation and possible fire. High-speed operation makes fuses ideal for this purpose.

New technology

Among recent developments are nh fuses with dual indication of their status. This is a device that overcomes the long-standing past difficulty of finding which fuse in a large installation has operated when an over-current or short-circuit has occurred.

The NH fuse link is the tried and tested low voltage fuse type for general industrial applications. It comes in a wide range of sizes and ratings. Many such fuses have been fitted with an indicator to show which fuse has blown, but when fitted to typical fusegear it is not always possible to see the indicator.

Dual indicating devices enable a blown fuse to be identified from the top, bottom or sides depending where it is located, ie in a base, fuse switch-disconnect or a vertical fuse rail. Fuse links with dual indication can significantly reduce downtime costs and remove the need to stock a variety of NH fuse links with different types of indicators.

Another recent innovation is the highly compact, advanced current protection fuse holder that is specifically designed to fit into 45 mm-wide motor starter systems for 32 A IEC motor circuits. It is IP20 finger safe, meeting global standards for minimising contact with current carrying parts. An integral din rail spring allows for easy installation without tools. Its major benefit is that when used in motor starter applications, the wiring time in group installations can be reduced by up to 80%.

One of the latest fuses developed is the CUBEFuse from Cooper Bussmann. This is available in 30, 60 and 100 A versions. It has several benefits: all its components – fuse, fuse base and wire ports – are IP20 finger safe. Also its footprint is very small – this is important as it frees up panel space, allowing more components to be installed if needed. More available space also makes for easier access.

The dovetail base design of this fuse allows 30, 60 and 100 A fuse bases to be ganged together, permitting them to be snapped onto a 35 mm DIN rail or directly panel-mounted.

A step change in fuse technology came about only 15 years ago, with the introduction of chip fuses. These are made by depositing fusible elements onto a ceramic substrate. Using this procedure fuses as small as 3 x 1·5 mm can be made. These are suitable for automated direct mounting on to printed circuit boards for the protection of electronic equipment. It should be remembered that mobile phones don’t have a circuit breaker; they have a chip fuse. This is an example of modern fuse technology realising new applications.

Current-limiting semiconductor fuses are particularly suitable for providing short-circuit and overload protection for diodes, thyristors, insulated gate bipolar transistors (IGBTs) and other power semiconductors. This is because they have very fast trip characteristics and are highly compact. They have exceptionally low I2t, low power losses, arc voltages and peak let-through.

The pace of change in this field is rapid and new products are being constantly developed to ensure that future protection is provided. Some examples include the new IGBT flat pack fuse and the size five high-speed fuse for high power rectifiers up to 9000 A.

For maximum thermal conductivity the low mass fusible elements in these fuses must be made of silver and the quartz powder in which the elements are embedded must be high quality. This is to ensure that in the event of an over-current the heat created is removed rapidly. The better the heat conduction, the faster the fuse will operate under high currents.

The fuse continues to make the transition from being a reliable, low-cost technology, to a thoroughly modern method of circuit protection. With all the original benefits of high speed operation and high breaking capacity, there is the added advantages of today’s enhanced performance capabilities, increased safety, still more compact size and greatly increased applications.