There are some important parameters to consider on how to select a fuse. These are the Current Rating, Ampere Square Seconds, Breaking Capacity and Voltage Rating to name some. 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. It is because, 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. Nominal Current Rating
This is the rated current carrying capacity of a fuse. This is normally measured at nominal conditions and ambient temperature of 25ºC. The actual circuit current must not exceed the nominal current rating of the fuse. 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 above technique, be sure 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
The nominal current rating above is only valid for steady state condition. Circuit involving large capacitances will most likely have a very high current during start up (the time the capacitor is charging). I2t is the rating use by manufacturers for time related responses. In some definition, this is called as the melting current. This fuse rating is mostly use 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,
Fuse manufacturers are relating this rating to melting, arcing or total clearing. For more detailed explanation, I suggest to take a look on the fuse datasheet.
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?
Fuse manufacturers are also specifying pulse cycle withstand capability. This is often use together with the I2t. For more information, see the fuse datasheet. So, for this example, since nothing is specified, we will answer this directly as below.
3. Voltage Rating
This rating is often underestimated and misunderstood by some. Fuse is in series to the circuit. It has a very small resistance. So, why voltage rating is relevant?
When the actual open circuit voltage is higher than the fuse voltage rating, arcing, explosion or at worst fire will happen.
This rating is more on safety reason not only for the circuit but for the environment as well. 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. Current De-rating due to Temperature Variation
The current carrying capacity of a fuse is dependent to the operating temperature. Once the operating temperature is high, the current capability will decrease. The fuse will melt earlier as it is design at nominal conditions. Fuse manufacturers provide a graph in their datasheets which contain 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 starts to decrease. For instance, a slow blow fuse current carrying capacity will decrease to around 82% at 65ºC ambient temperature.
5. Breaking or Interrupting Capacity
This is the maximum current the fuse can safely interrupt. During short circuit, the fuse is suddenly expose to very high current. Multiple times to its normal rating. It is important that there is no damage to the fuse body or any sign of rapture. There should be also no incident of arcing or explosion. Breaking capacity will ensure these.
6. 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. 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 bigger than fast blow. This is popular in applications which have large capacitors such as switching mode power supply.