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Semiconductor fuses in enclosed holders?

Semiconductor fuses in enclosed holders?


Be very careful when using semiconductor fuses in enclosed fuse holders!

This issue is becoming more prevalent and unfortunately we are aware of a few recent cases where the mismatch of semiconductor fuses and fuse holders has resulted in a fire and expensive damage to a board. It's actually easy to prevent such events from happening, but first you need to know why they happen...

There are many performance categories of fuses. The most common category (by volume) is the Class gG fuse used for general applications, cable and line protection. Another category is the semiconductor fuse, also known as ultra rapid, high speed and solid state fuse. As its name suggests, it is designed to protect semiconductor devices from over-currents. The most common semiconductor fuses are the Class gR and the Class aR.

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In order to achieve their very fast performance characteristic, semiconductor fuses are engineered with a high purity silver element and high grade silica filling inside the body housing. They have been designed to efficiently move heat from the fuse core to the outside of the fuse. Therefore, in operation, they can run very hot and emit a lot of heat to their surrounding environment. This heat is called 'power loss' and is measured in Watts.

All fuse holders have a designated power loss tolerance, meaning the amount of heat (also measured in Watts) that a fuse holder can withstand before losing its physical integrity. Ideally, a semiconductor fuse should be mounted in an application which provides adequate ventilation or cooling. If a fuse holder totally encloses a fuse, it needs to be able to tolerate the heat that the fuse is emanating and this is where you have to be very careful.

In other words, the Power Loss tolerance of the fuse holder has to be higher than the Power Loss value of the fuse. If this rule is not adhered to, there is a very real possibility that the fuse holder will melt, buckle and even catch fire during operation!

Power Loss values can be easily obtained from the respective fuse and fuse holder's technical data sheet, commonly available from the manufacturer or the product supplier.

Sometimes people get caught out because they are using an existing fuse holder that was compatible with a semiconductor fuse of a certain current rating, but when a fuse of the same series is used of a higher current rating, the Power Loss values of the new fuse are higher than the tolerance of the fuse holder.

For example, if we consider a SIBA 10x38mm DIN Rail Mount Single Pole Fuse Holder Part Number 51 063 04, it has a Maximum Power Loss Tolerance of 4W. If you needed a semiconductor protection solution for an application, you may assume that you could use any 10x38mm semiconductor fuse because if it fits in the holder, then it should be right...Yes?

The answer is a resounding 'No'. You need to check the Power Loss values of the fuse before you make your selection. In this case, let's say for example that you were considering the SIBA 600V gR Part Number 60 033 05 series. Inspecting the data sheet reveals that to keep within Power Loss tolerances, you can only use fuses rated up to 12A. Even though 60 033 05 fuses are available up to 30A, you can't use a fuse rated above 12A.

In such cases you would need to move to a larger physical size fuse and holder, say a 14x51mm or a 22x58mm. Please contact Fuseco for guidance in such situations. 

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It is important to note that this caution only relates to the use of semiconductor fuses. All other fuse classes have Power Loss values well under the Power Loss tolerance of their corresponding fuse holders.  

So in conclusion, when applying semiconductor fuses, select open fuse holders and environments that provide adequate ventilation, airflow or cooling. If that is not possible and you need to use an enclosed holder, then check the respective Power Loss values of the fuse and the holder and ensure that you select a fuse that has a power loss value that is lower than the Power Loss tolerance of the fuse holder. 


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