• To protect components and equipment from costly damage caused by over-currents.
• To isolate sub-systems from the main system once a fault has occurred.

Fuses have many unique performance characteristics, such as:

• Optimum Component Protection

Fuses reduce short circuit (fault) currents that flow to a low value by "current limitation". There is no need for complex short circuit calculations and no concerns about costly future upgrades due to system expansion with increased fault currents. Their compact size offers low cost over-current protection for the highest short circuit levels.

• Safety

Fuses do not produce gas, flames, arcs or other materials when clearing any value of over-current up to the highest level of short circuit. In addition, the speed of operation on high short circuit currents limits significantly the flash hazard at the fault location.

• Reliability

No moving parts to wear out or become contaminated by dust, oil or corrosion and no nuisance tripping. If a fault occurs, the fuse immediately operates in its predetermined manner or co-ordinates with other circuit components. The cause of the fault is then ascertained, corrected and a new fuse fitted. Fuse replacement ensures protection is restored to its original state of integrity.

Fuses shall be selected according to the following criteria.

• Maximum operational voltage – shall be equal or greater than the system operating voltage.
• Rated breaking capacity – shall be equal to or greater than the maximum fault current expected in the circuit downstream the fuse.
• Rated current – shall be equal to or greater than the continuous operating current of the circuit protected by the fuse.
• Breaking range – partial range or full range breaking capacity according to the type of protection required.
• Utilisation category – select according to the nature of equipment to be protected.

Fuses are in most cases marked with all the essential information required to select a replacement fuse or to enable you to select fuses for most common applications. Standard markings are as follows:

• Fuse standard, size and reference. This defines the operating performance and mechanical design of the fuse.
• Voltage rating. Fuses may be operated up to the maximum voltage stated in the rating. Voltage ratings may be colour coded to reduce the risk of incorrect installation.
• Current rating. This represents the value of the current that the fuse can carry continuously without deterioration under specified conditions.
• Rated breaking capacity. This is the maximum value of prospective current that a fuse is capable of breaking under standard conditions.
• Breaking range and utilization category are indicated by letter codes. The first (lower case) letter indicates the breaking range, i.e. the range of prospective currents the fuse is able to break. The second (capital) letter indicates the utilization category, i.e. time-current characteristics for typical applications.
• Fuse manufacturer trademark.
• Manufacturer’s product reference or part number.

IEC 60269 gives guidance how to protect, but does not advice what current rating should be taken. For this recommendation lists of the fuse manufacturers are helpful.

For thermal derating of fuses at temperatures above 40ºC the general rule is that the current rating is decreased by 0.5% for each 1ºC above 40ºC. Where the fuse is mounted in an enclosure use the internal enclosure temperature for calculations.

For installations situated at altitudes of over 2000 metres a general rule is that the current rating of the fuse is decreased by 0.5% for every 100m above 2000m.

Current limiting fuses contain a granular filler, usually high purity quartz sand of a defined grain size and packaging density. The specific grain size distribution provides room to expand for the vapours and gases produced by the arc and offers a large surface for efficient cooling. The filler does melt under the influence of high arc temperatures, absorbing an enormous amount of energy and extinguishing the arc well before current zero. Fused quartz forms a non-conductive fulgurite body that prevents re-striking of the arc.

An overload current is an excessive current relative to the normal operating current, but one which is confined to the normal conductive paths of the circuit. Overloads are often between one and six times the normal current level. They are usually caused by harmless temporary surges in current that occur when motors are started up or transformers are energised. Such overloads (transients) are very brief in duration and any rise in temperature is trivial and has no harmful effects on the circuit components.

Continuous overloads can result from defective motors, overloaded equipment or too many loads on the one circuit. Such sustained overloads are destructive and must be cut off by protective devices (fuses) before they damage the circuit.

An over-current is a situation where a circuit experiences a current that is higher than the normal operating current. This is either an overload or a short circuit (fault) current.

Short circuit currents are also commonly known as ‘fault currents’ and occur when a circuit is given an opportunity to flow through a shorter than normal path. Whereas overload currents occur at rather modest levels, the short circuit or fault current can be many hundreds of times larger than the normal operating current. If not cut off quickly, the resulting damage and destruction can be very serious. Engineers need to know the potential fault current when designing circuits. This is the value of the highest possible current in the event of a short circuit.

The prospective short circuit current is the value of the current that would flow if there was no protection in the circuit in the event of a short circuit. The lower the power factor of the installation, the higher the peak value of this destructive current. This is also commonly referred to as the potential fault current.

The potential fault current is the value of the current that would flow if there was no protection in the circuit in the event of a short circuit. The lower the power factor of the installation, the higher the peak value of this destructive current. This is also commonly referred to as the prospective short circuit current.

Yes, very important! Each fuse is marked with a nominal current rating. The current rating of the fuse must be higher than the circuit’s normal operating current. As to how much greater, it depends on a number of factors, including what type of opening characteristic you desire.

Yes, very important! The voltage rating of the selected fuse must be greater than or equal to the circuit voltage. Since fuses have such low resistance, the voltage rating becomes critical only when the fuse is trying to open. The fuse must be able to open quickly, extinguish the arc after the fuse element has melted and prevent the system open circuit voltage from re-striking across the open fuse element.

Fuse links are voltage sensitive devices and it is important to note that the satisfactory operation of a fuse link under fault conditions is dependent on the system voltage. Therefore, they must not be installed in circuits with a voltage above their voltage rating. They can however be used satisfactorily in circuits at lower voltage levels. For example, you can use a 500V rated fuse in any circuit that has up to 500V.

During normal conditions, the fuse must carry the load current of the circuit without blowing and opening the circuit. However, when an over-current occurs, the fuse must interrupt the over-current and withstand the voltage across the fuse after arcing. It is important that designers take account of temporary conditions such as surges & spikes during fuse selection. To properly select a fuse, the following items must be considered:

• Available short circuit current
• Voltage rating (AC or DC)
• Full load current
• Characteristics of components to be protected
• Any in-rush characteristics of the circuit (eg. Motor start-up)
• The available space
• The ambient conditions
• Any standards requirements
• Suitability to the proposed fuse holder (eg. Power loss)

All fuses are designed to open a circuit to protect other valuable components from over-currents. However, they are not all designed to do it in the same way! There are many different types of fuse performance options and selecting the correct performance for your application is vital. For this purpose, the IEC created ‘Utilisation Categories’ which describe the various operating classes of fuses. For example, fuses that are designed to protect sensitive semiconductor devices are designed to open very quickly and let through a minimum amount of energy during an over-current yet motor protection fuses are designed to withstand large in-rush currents in multiple start-up situations. Some fuses only protect against short circuits when others also protect against overloads. There are many differences. Choosing the wrong fuse could potentially cause serious damage by under-protecting or could cause excessive down time due to nuisance-blowing.