There are some important parameters to consider on how to select a fuse suitable for your operation. These are the Current Rating, Ampere Square Seconds, Breaking Capacity and Voltage Rating. These will be discussed below so keep on reading. Read also the fuse basic operation and fundamental for further information.
A useful design practice in selecting a fuse which is also recommended by fuse suppliers is to consider the 75% factor. A 75% factor means that the continuous circuit current should only be 75% of the fuse continuous current rating. The purpose of this is to compensate the environment temperature effect since at high ambient temperature a fuse breaking point will decrease. For instance, a 10 ampere total circuit current should use a 13 amperes fuse. However, the designer should ensure that the circuit can withstand the 13 amperes of current in a short duration until the fuse blow up.
Now we will go back to the parameters to take note on how to select a fuse.
Parameters to Consider on How to Select a Fuse
1. Current Rating
This is the rated current carrying capacity of a fuse which is normally measured at nominal conditions and ambient temperature of 25ºC. This rating must not be fully utilized in the circuit. A good rule of thumb is to set the circuit typical current to 75% of this rating only. Mathematically,
A circuit has a nominal current of 10A. What should be the typical current rating of a fuse to be used?
In adopting this technique, be sure to ensure that the circuit components are able to withstand the excess current before the fuse act its role. In other words, the components in series with the fuse should have a current rating higher than the melting point of the fuse. With this, every time there is sudden increase of current, only the fuse will blow.
2. I2t Rating
For circuit involving large capacitances will most likely have a very high current during start up (the time the capacitor is charging). The current rating above is only valid for steady state condition and cannot cover this phenomenon. Thus I2t is introduced by manufacturers. In some definition, this is called as the melting current. In short, this fuse rating is designed for transient conditions. The product of the square of the circuit current and the time should be less than the I2t of the device to avoid damage. Mathematically,
A particular fuse has an I2t of 100A2seconds. What should be the maximum allowable inrush current of the circuit if the permitted transient period is 1 second?
3. Voltage Rating
This rating is often underestimated and misunderstood by some circuit designers. Fuse is connected in series to the circuit and has a very small resistance so why voltage rating is relevant? In the event of fuse melting or simply when a fuse open up, if the open circuit voltage is higher than the capability of the device, explosion may occur and might trigger fire. This rating is more on safety reason not only for the circuit but as a whole environment. Say if an open circuit voltage (when fuse blows) is 120V, therefore the rating of the fuse should be greater than this value. Mathematically,
4. Temperature Variation
The current carrying capacity of a fuse will be greatly affected by the operating temperature. Once the operating temperature is high, the current capability will decrease and the fuse will melt earlier as it is designed at typical or nominal conditions. Fuse manufacturers provided a graph in their datasheets which containing the current versus operating temperature. To site an example, see below.
Above figure is taken from the datasheet of a certain fuse manufacturer. As you can see at 25ºC ambient temperature the current carrying capacity of a three types of fuse is at 100%. However beyond this temperature the current carrying capacity started to decrease. For instance, a slow blow fuse current carrying capacity will decrease to around 82% at 65ºC ambient temperature.
5. Other Information
Fuses nowadays come in several packages so select the most appropriate for your design. Fuses are also characterized as fast blow or slow blow one. Fast blow is fast acting type wherein the I2t rating is small. This is suitable for sensitive and or critical circuits. On the other hand, a slow blow fuse is a time lagging type wherein the I2t rating is relatively higher than fast blow. This is popular in applications which have large capacitors such as switching mode power supply.