Fuse - Basic Properties, Terminology and Theory
What is a Fuse?
Fuse is an overcurrent device with a fusible link that operates and opens the circuit in an overcurrent condition. The modern fuse can take many forms and use many different techniques to interrupt overcurrents. When potentially damaging overcurrents occur, the link will melt very quickly, protecting conductors and circuit components.
Fuses serve two main purposes:- Protect components/equipment from damage caused by overcurrents
- Isolate sub-systems from the main system once a fault has occurred
There are five basic types of fuses: Time-delay fuses (time lag), Dual element time-delay fuses, Non- time-delay fuses, Very fast acting fuses and Limiters.
Time-delay & Dual element
Time-delay fuses in small dimension packages must meet the following requirements; up to 3 ampere fuses must have opening times of 5 seconds or greater at 200% and 3.1 - 30 ampere fuses must have opening times of 12 seconds or greater.
A major type of time-delay fuse is the dual-element fuse. This fuse consists of a short circuit strip and a soldered joint spring connection. During overload conditions, the soldered joint becomes hot enough to melt. The spring shears the junction loose. Under short circuit conditions, the short circuit element operates to open the circuit. All dual-element fuses are considered to be time-delay, but not all time delay fuses are dual element.
Time-delay fuses are ideal for circuits with a transient surge or power-on in-rush. These circuits include: Motors, transformers, incandescent lamps and capacitive loads. This in-rush may be many times the circuit's normal full load amperes. The in-rush can be 20 times the normal current level. When using fast-acting, single-element type fuses, it is necessary to size the fuse at 150% to 300% of the circuit's full load current. With this sizing rule, in-rush may cause nuisance openings. Time delay fuses allow close sizing of the fuse without nuisance opening. Size the time-delay fuses from 125% to 150% of the circuit's full load amperes.
Non-time-delay
Non-time-delay fuses have no intentional built-in time-delay and are used in circuits without transient in-rush currents.
Very fast-acting
Very fast-acting fuses often have silver links. Because of the fuses' current limiting ability, these fuses are frequently used to protect semi-conductor circuits.
Limiters
Limiters are overcurrent protective devices that are used for short circuit protection only. In fact, some limiters will not open under overload conditions and are designed to open short circuits only.
maximum available short circuit current
A fuse must be able to open the circuit under a short circuit without losing case integrity. The interrupting rating of a protective device is the maximum available current, at the rated voltage, that the device can safely open without the possibility of rupturing.
A quick method of calculating the maximum available short circuit current that can be delivered from a transformer is to take the transformer full load amps, multiply by 100 and divide by the percent impedance of the transformer. Additional factors that affect the short circuit include the size of the conductor and the distance from the transformer to the fault.
IEC Fuse Standards
International Electrotechnical Commission (IEC) fuse standards are very different from Underwriters Laboratories (UL). IEC writes the standards followed by many European and Asian countries. Because the electrical characteristics of these fuses are so different, UL and IEC rated fuses are not interchangeable. When designing products to go overseas, it is important to consider that world standards may require a different fuse than those used in North America.
Some these countries conduct their own testing such as the German testing agency VDE. However, most accept the testing of Svenska Elektriska Material Konllanstalter or Semko, the Swedish testing agency. Those products tested by Semko that pass the IEC code are marked with a circle around an S.
Testing by European agencies revolves around voltage drop, time-current characteristics, breaking capacity and endurance tests. The biggest difference between North American and European standards are the time current characteristics as shown.
Fuses are available in maximum voltages from 32 AC through 700 AC/DC, and 0.001 (1/100) through 800 current.