The Fuse - a strong link (Part 1)

Everyone knows something about the fuse - many people still have re-wirable fuses in their homes. Even so, it may still surprise many to know that the first recorded use of a fuse to protect electrical equipment was way back in 1864. However, there is more to modern fuses than meets the eye and this method of circuit protection is ideal for most modern requirements. Today's fuses present many benefits for both installers and end-users.

Fuses protect electrical devices, components and circuits from over-currents and short-circuits. This is achieved through the extremely rapid melting of internal element(s) through which the fault current flows. The fuse is rated so that current flow is interrupted when it reaches a predictable magnitude for a fixed time period. Fuses limit overloads to a single circuit and minimise the danger of fire in the equipment and the circuit leading to it. Fuses are also relatively low-cost items and are highly compact. The space saving alone can save a lot of money. Let’s take the fuse’s many advantages in turn.

Current limitation:

Current limitation is probably the strongest feature of the fuse. Cartridge fuses limit the short circuit fault current and energy let-through. Magnetic and thermal stresses in the fault circuit are therefore dramatically restricted, so reducing damage to sensitive circuit components at the point of fault itself. Even higher current limitation is possible to protect very sensitive circuit components.

Current-limiting circuit breakers now offer some current limitation, but such products do not compare with the current limiting properties of fuses, although a possible combination is the circuit breaker backed up by a properly co-ordinated fuse.

Reliability:

High reliability is a major advantage of using the fuse for overcurrent protection. The reliability of the fuse derives from its simplicity. There are no moving parts, so that after operation, fault rectification and replacement, the circuit is restored to its original state of integrity. Importantly, the fuse replacement time is very small compared with the fault rectification. Prior to any operation, the fuse will be in its passive state, carrying normal load currents, for perhaps several decades. If a fault does occur, the fuse immediately operates in its predetermined manner. Other protective devices contain mechanical parts (which could deteriorate or fail). Moreover, contact erosion can take place during operation. At high fault currents many of these devices are only capable of a limited number of operations. Moreover, fuse characteristics are fixed and tamper-proof. Finally, nuisance operation because of ambient temperature changes does not occur with fuses, but it can with other devices.

Other benefits:

• The current limiting features of fuses lead to a number of safety and performance benefits. One such is a high breaking capacity. Modern cartridge fuses can clear the highest breaking capacities and the most onerous fault conditions - for example, 300kA at 600V AC. Even the humble 13A plug top fuse has a breaking capacity of 6kA with a power factor below 0.4.
• Another benefit is safety. When a short circuit fault occurs, arcing occurs at the seat of the fault. The fuse’s strong current limiting dramatically reduces the associated flash hazard. Not only this, when the fuse operates, it also emits no gases nor other products, so giving safe, quiet operation. Circuit breakers, on the other hand, need to be positioned with care to ensure against possible flashover.
• Good power quality is an increasingly important factor, particularly with supplies to modern electronic data handling and processing equipment. The fuse will operate in a few milliseconds, causing minimum disruption in local healthy circuits.
• Economies are always being sought. The fuse’s excellent current and energy limitation properties allow it to be very compact. This, in turn, allows associated switchgear to be smaller. Moreover, while the complete circuit breaker has to be replaced after clearing two or three short circuit faults, only the comparatively inexpensive fuse has to be replaced after operation.
• Standardisation is yet another benefit of using fuses. Progress continues to be made in the National, BS, European, EN and International IEC standardisation of fuse performance, characteristics and dimensions. One often-overlooked advantage is the long-term availability of replacement fuses. They can be in service for many years, but standards activities ensure the availability of safe, updated replacements. Other protective devices are often not interchangeable and older designs may be obsolete. A recent Standards activity is the preparation of basic application guides, such as the draft Application Guide for LV fuses. This will become IEC 61818.

Applications for fuses:

1) Motor circuit protection

The fuse allows well co-ordinated protection with other circuit components. An excellent application example is motor circuit protection. Here, the fuse has to withstand the motor starting current and may need a higher rating than the motor’s full load current. Fuse manufacturers provide data for suitable gG or gM fuses *. The overload unit provides the low overcurrent protection and the fuse provides the short circuit protection.

Co-ordination recommendations are made by the manufacturers of motor starters in accordance with IEC 60947-4-1 to give the desirable Type 2 co-ordination using fuses. Verification tests are undertaken with standardised gG or gM fuse.

The fuse’s excellent current limiting characteristics prevent contactor and associated overload relay thermal damage. Mechanical damage is also prevented because of the fuse’s low peak let-through (cut-off current). If the contactor / relay were not protected by the fuse, the electromagnetic forces associated with the fault current would be more likely to cause damage and contact welding. Starter manufacturers recommend suitable gG or gM fuses to provide Type 2 co-ordination.

2) Overcurrent discrimination

Fuses can also provide overcurrent discrimination **. An integrated system protected by fuses excels in this application by giving minimum system disruption. The standardisation of gG fuse characteristics ensures that discrimination between fuses can be achieved on a 1.6:1 ratio of current ratings for most practical situations. The 1.6:1 ratio represents two steps in the R10 series of ratings, i.e. a 100A (downstream) fuse will discriminate with a 160A (upstream) fuse.

3) Cable protection

Fuses provide excellent cable protection. Standardised gG fuses can clear any overcurrents that cause the fuse elements to melt. They do this before a temperature rise detrimental to the cable insulation, joints, terminations or surroundings can occur ***. A cable overload test is included in the fuse standard for verification of protection for low overcurrents. For higher overcurrents the fuse characteristic will be inherently faster than the cable withstand.

References and explanations:

* Motor circuit protection - gG and gM are internationally standardised characteristics for general purpose or motor circuits respectively. More details can be found in guide IEC 61459:- ‘Guidance Notes: Co-ordination Between Fuses and Contactors/Motor Starters’.

** Overcurrent discrimination - This is defined as: ‘co-ordination of the relevant characteristics of two or more overcurrent protective devices such that, on the occurrence of overcurrents within stated limits, the device intended to operate within these limits does so, while the other(s) do(es) not’.

*** Cable protection - Simply the rated current, 1n, of the fuse is selected to be equal to or greater than the operational current, 1B, of the circuit and equal to, or smaller, than the continuous current carrying capacity of the conductor in accordance with the lEE Wiring Regulations.

This article will be continued next week.

Cooper Bussmann
Burton-on-the-Wolds, Loughborough, Leicestershire LE12 5TH
Tel: 01509 882737
Fax: 01509 882794
Web: www.bussmann.com