In order to choose a suitable fuse for some special occasions, subscribers should have a good mastery of the following features and some concepts of application.
Environmental Temperature: It means the air temperature directly around the fuse. Do not interblend with the room temperature. On many practical occasions, the fuse temperature is rather high as the fuse is closed such as the fixed rack installed in the electrical panel or the fuse is fixed on other heating components, such as the neighborhood of the impedance and the transformer.
Interruption Capability：Please see the blowout rated value.
Current Rated Value: Nominal amperage noted on the fuse. The value is the current that the fuse can carry and the value is set by the manufacturer according to a series of already-controlled testing conditions. (Please see the decrease of rated value.)
The content code of the fuse products contains serial sign and ampere parameter. Please see those contents about conducting correct choosing guide in the Fuse Choosing Guide and Time Current Curve List.
Decrease of Rated Value：When at the environmental temperature of 25℃, the working current of fuse that we present should not be over 75% of the nominal current value because that nominal current value is set according to a series of already-controlled testing conditions. These testing conditions are one part of the Fuse Used for Assisting Overcurrent Protection 198G Standard of Underwriters Laboratories, whose main aim is to set the common testing standards required for the serial control of the components which are used for guard against fire. The commonly seen alteration factors of these testing standards contain fully-closed fuse holder, high-touch resistance, air flowage, instantaneous peak value and changes in the aspect of connecting cable (diameter and length). Fuse Is actually temperature-sensitive component. Although the controlled testing conditions change a little, the expected service life of the fuse would be largely affected, especially when the load is its nominal value. The nominal value is commonly expressed as 100% rated value.
Circuit design engineer should clearly know that the aim of setting these controlled testing conditions is to make the fuse manufactures be able to keep the performance standard of the products unified so he must know various change conditions while using the fuse. In order to compensate for these changes, the load added to the fuse often should not be over 75% of the nominal rated value listed by the manufacturers while the circuit design engineer is design the equipment with protection equipment which is both safe and has long service life. Meanwhile, he must also provide enough overload and shortcut protection.
Fuse Is actually temperature-sensitive component. Each parameter of the fuse is set at the environmental temperature of 25℃. The fuse temperature produced when the fuse passes current increases or decreases along with the change of the environmental temperature.
The Environmental Temperature in the section of Fuse Choosing shows that the environmental temperature has effect on the nominal rated current value of a certain fuse. The material adopted for designing the majority of traditional Slo-Blo fuses has comparatively low melting temperature thus it is quite sensitive to the changes of the environmental temperature.
Dimension: The dimension has millimeter as its unit if without other regulations. The fuse dimension scope of this product content ranges from the minimum dimension of 0603 circuit wafer (1.60mm length*0.79mm width*0.46mm height) to the maximum of 5AG. 5AG is usually called mini-type fuse (10.3mm diameter*38.1mm length). During the past years many kinds of new products were developed out, which boosted the continuous evolution of fuse dimension to satisfy the requirements of various circuit protection. The earliest fuse is very simple easy-disconnection component. Later in the 19th century, there occurred the first plug-in fuse with the fine lead closed in the reading light holder by Edison. In 1904, the Underwriters Laboratories set the various items specifications of the dimension and rated value to suit the safety standards. In 1914, there occurred reusable fuse and automobile fuse. In 1927, some companies began to manufacture fuse wire of very low ampere for the budding electronic industry.
The fuse dimension in the blank below begins with the early glass fuse which was used for automobiles, among them there are the sign of AG and as A represents automobile and G represents glass, namely Automobile Glass. Figures used before AG is set according to the chronological order. Because fuses of every new specification and dimension are made by different manufacturers, for example, 3AG is the third fuse dimension that is put into the market. Although dimension and structure of other non-glass fuses are set by the function requirement, the fuses still preserve the length or diameter of the glass fuse. Their signs are changed to AB to replace AG, which means that the fuse tube is made of bakelite, glass fiber, ceramics or other similar non-glass material. The fuse with the largest dimension in the blank below is 5AG, namely mini-type. This name is adopted by American electric industry and the State Electric Code scope. American State Electric Code Scope always views fuse of 14.3mm*50.8mm as the smallest standard fuse which is in use.
Fuse Performance：Performance in fuse design means the degree that the fuse conducts speedy reaction to various current loads. Fuse performance is generally separated into 3 types, namely super quick blowout, quick blowout and slow blowout. The features of the slow-blowout fuses are that this sort of fuses is designed with additional thermal inertia to undertake the overcurrent increase during the normal start-up.
Fuse Structure：The internal structure will change along with the different changes of the rated ampere value. Fuse pictures in this product content give the model structure of the fuses that have special rated ampere value in the fuse series.
Fuse Holder：On many practical occasions, fuses are fixed in fuse holders. The fuse together with the assisting fuse holder is not used only as a shape to directly connect or cut the power supply.
Blowout Rated Value：Also called disconnection capability or shortcut rated capacity. Blowout rated value is the biggest allowable current the fuse can carry under the rated voltage. When there is shortcut, the fuse will pass instantaneous overcurrent many times, which is bigger than the normal working current. Safe operation requires the fuse to keep complete state (without detonation or rupture) and to eliminate shortcut.
Along with different designs of the fuse, the blowout rated value ranges from fuse of 250V, 5*20mm in the metric system and 35-ampere AC to fuse of 600V and 200，000-ampere AC in the series of KLK. Subscribers can get data of other series of fuses from the manufacturers.
The blowout rated value of fuses classified according to the UL Standard 198G must be 10，000 ampere. There are also some exceptions. Please see the section of Standard. Fuses of these exceptional cases have safety guidelines which are largely over the guidelines of those possible shortcut current on many practical occasions.
Interference Open Circuit：Interference open circuit is often caused by imperfect circuit analysis. Among all of the factors listed in the Fuse Choosing Guide, we should pay special attention to the first, the third and the sixth item, namely the normal working current, the environmental temperature and the overcurrent increase. For example, a commonly-seen cause of conventional power supply interference open circuit is that we are not able to fully consider the nominal fuse I2t rated value of the fuse. We couldn’t choose fuses only according to the normal working current and the environmental temperature. Under the application condition, the nominal fuse I2t rated value of the fuse must also meet the requirements that various inrush currents put forward to the fuse, which are caused by the power supply smoothing filter’s inputting capacity to the electric device. There are steps of transferring various wave shapes into wave shapes required by I2t circuit in the Fuse Choosing Guide. For those safe and long-service life fuse protections, a good designing method is to keep the I2t of the inrush current wave shape not higher than 20% of the nominal fuse I2t rated value of the fuse while choosing fuses. Please refer to the Overcurrent Increase of the Fuse Choosing Guide.
Impedance：Fuse impedance isn’t important in the total impedance of the circuit. Because impedance of the fuse, whose amperage is lower than 1 is only several ohms, we should take this point into consideration while adopting fuses in low-voltage circuit. Subscribers can gain the practical impedance value of the fuses from the manufacturers. Most fuses are made of positive temperature coefficient. Therefore, we will often mention cold impedance and hot impedance (voltage drop under rated current). The practical working impedance is between the two. Cold impedance can be obtained through testing current, which is not higher than 10% of the nominal rated value of the fuse. The cold impedance value given in this publication is both nominal and model. If this parameter is a limit value as far as the design analysis is concerned, subscribers should consult with the manufacturers. Hot impedance is gained when the current value passing the fuse is equal to the nominal rated current. We can send subscribers with fuse impedance data on getting requirements. And we can also provide fuses according to the impedance control tolerance regulated by the subscribers and we will collect additional cost fee for this.
Shortcut Capacity： Please refer to Blowout Rated Value.
Soldering Notes：Because most fuses have soldered joints we should pay special attention while planning to fix these fuses through soldering method. Too much quantity of heat during the soldering process will make the solder inside the fuse reflux and accordingly change its rated value. Fuse is a thermal-sensitive component which is similar to semi-conductor so we recommend that subscribers use decalescence facilities while soldering.
Sampling Notes：As it needs destructive inspection when confirming whether a certain specification is qualified or not, sampling inspection method should be conducted to each passel of finished products according to the principle of statistics.
Time-Current Features Curve List：As a picture indication of blowout features, Time-Current Features Curve List is commonly middle curve. The reason why we give this curve list is to make it as an assisting method of design but not as a part of fuse specifications. Time-Current Features Curve List is very helpful while choosing fuses because fuses of the same rated current value may well have rather different features curve. The fuse specification often contains the service life requirement when the working current is 100% or 110% of the rated value or contains the longest open circuit time when overcurrent passes (often 135% or 200% of the rated value). Destructive inspection is needed when confirming whether a certain specification is qualified or not. Destructive inspection should be conducted to each passel of finished products according to the principle of statistics. Time-Current Features Curve List gives the average data required by the design. However, to any passel of special products, this average may have some difference. So we should inspect some samples to identify their performance immediately after choosing a fuse.
Underwriters Laboratories：Please refer to Underwriter Laboratory Classification. The fuse must satisfy each item of requirements set by the underwriter laboratory standard, namely the regulations of No198G of Fuse Assisting Overcurrent Protection. Some 32-voltage fuses in this product content are classified according to the UL Standard 275. On the other hand, Component Precis Acquisition Recognition of Underwriters Laboratories means that this item of product gains the application Appraisal document approved by the Underwriters Laboratories component précis.
Voltage Rated Value： Voltage rated value labeled on the fuse that the fuse can certainly safely interrupt the rated shortcut circuit current when the voltage is equal to or smaller than the rated voltage of the circuit. The voltage rated value series is contained in the regulation of American N.E.C and is also a requirement of the Underwriters Laboratories used for guarding against fire dangers. For most small-dimension fuses and minitype fuses, the standard voltage rated value adopted by fuse manufacturers are 32V, 125V, 250V and 600V.
In the electronic equipment which has rather low output power supply or whose shortcut current value is lower than 10 times of the fuse current rated value, the commonly-used method is to regulate that fuses with its voltage rated value being 125V or 250V can be used for secondary circuit protection of 500V or circuit of even higher voltage.
Just as what has been mentioned above (Please refer to Rated Value Decrease.), the fuse is sensitive to current changes but not to voltage changes. The fuse will keep its original state under any voltage between zero and its maximum rated value. Circuit voltage and active power become problem only after the fuse has melted and caused electric arc. Safe melt of the circuit is relative to circuit voltage and active power, which has been discussed in Blowout Value.
Generally speaking, fuses can be used without any damage to its melting features under any voltage which is smaller than its rated voltage. If the maximum power electrical level, which occurs in the fuse under the condition of complete shortcut circuit, can only low-energy and non-destructive electric arc, the fuse can be used under various voltages which are higher than its already-tested rated voltage value.
Nominal Melting Performance I2t Derivation：Conduct laboratory test to every fuse to confirm the energy needed to melt the part. This energy is called Nominal Melting Performance I2t. The testing method is like this, inflict a current increase to the fuse and measure the time of occurring melting. If there is no melting phenomenon within about 0.008 seconds or even shorter time, we should increase the intensity of impulse current. Repeat this testing procedure until the melting of the fuse part is limited to about 0.008. The aim of conducting this testing procedure is to ensure that the procreant heat energy has not enough time to dissipate by heat conducting through the fuse part. That is to say, all of the I2t is used for melting. Once the testing result of the current （I）and the time （t） have been confirmed, it is very easy to calculate melting heat energy I2t. When the melting process finishes, there occurs electric arc first and then the fuse is disconnected. The nominal I2t value given in this publication belongs to elimination, namely the section of the melting state of disconnection.