Fuse is a very simple electrical part and yet needs to be properly selected. Its role in the circuit is very important actually. Selecting a right fuse starts with a good understanding on fuse electrical parameters. There are some important fuse electrical parameters to consider. These are the Current Rating, Ampere Square Seconds, Breaking Capacity and Voltage Rating.
Fuse Electrical Parameters Explanation
1. Nominal Current Rating
We will start with the current rating. This is an obvious specification of a fuse. This is defined as the rated current carrying capacity of a fuse which is normally measured at nominal operating conditions and ambient temperature of mostly 25ºC. This rating will change with higher temperature so be careful to avoid issues on your design. 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 particular circuit has a nominal current of 10A. Determine the typical current rating of the fuse to be used.
In the above rule of thumb, it is obvious that the current that will flow to the circuit devices is higher than the nominal current in the event of abnormality. Be sure that the circuit components are able to withstand the excess current before the fuse act its role. In other words, you need to ensure that the components in series with the fuse have a current rating higher than the melting point of the fuse. So, every time there is sudden increase in current, only the fuse will blow.
2. I2t Rating (Melting point)
The second fuse electrical parameter to talk with is the I2t rating. Circuits involving large capacitance will have likely a very high current during start up (the time the capacitor is charging). This is called inrush current in other terminology. The current rating discussed above is only valid for steady state condition and cannot cover this phenomenon. Thus I2t is introduced by manufacturers. In some people or books/writeups, this is called as the melting current. In short, this fuse rating is designed for transient conditions. To avoid abnormal tripping of the fuse every time there is high inrush current, the I2t of the fuse must be higher than the I2t of the circuit. Mathematically,
If 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
The third fuse electrical parameter to consider is the voltage rating. This rating is often underestimated and misunderstood by many circuit designers. Their argument is that fuse is connected in series to the circuit and has a very small resistance so why voltage rating is relevant?…In the event of a fuse melting or simply when a fuse open up, when the open circuit voltage is higher than the voltage capability of the fuse, explosion may occur and might trigger fire. This rating is more on safety reason not only for the circuit right functionality. 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 last fuse electrical parameter to tackle with is the temperature variation. In electronics circuit design, temperature is one of the big enemies to withstand. The current carrying capacity of a fuse will be greatly affected by the variation of the operating temperature. When the operating temperature is high, the current capability will decrease and the fuse will open earlier as it is designed at nominal conditions. All fuse manufacturers provided a graph in their datasheets which contains the current versus operating temperature. If there is no data like this in the fuse you are using, request one from the manufacturer. To site an example, see below.
Above illustration is taken from the datasheet of a certain fuse. As you notice, at 25ºC ambient temperature the current carrying capacity of a three types of fuse is in the fullest, at 100%. However beyond this temperature you can observe that the current carrying capacity starts to decrease. For example, a slow blow fuse current carrying capacity will decrease to approximately 82% at 65ºC operating ambient temperature.
To summarize, the purpose of the fuse is to protect the circuit in the event of high current. However, the fuse should not also over reactive with the short duration currents that it will open in unspecified condition. By considering all these fuse electrical parameters you can be sure that the fuse will operate as expected.