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.

What are semiconductor fuses?

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. 


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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Freight details

Fuseco can help! We provide a 24 hour 7 day a week service for customers who are experiencing a breakdown situation or need a fuse urgently after-hours or during the weekend.

To use this service, please call our after-hours number 0411 104 363 and one of our customer service team members will be available to assist you. Please note that an after-hours fee applies to this service.

Fuses shall be selected according to the following criteria. 

  • Maximum operational voltage – shall be equal or greater than the system operating voltage. 
  • Rated breaking capacity – shall be equal to or greater than the maximum fault current expected in the circuit downstream the fuse. 
  • Rated current – shall be equal to or greater than the continuous operating current of the circuit protected by the fuse. 
  • Breaking range – partial range or full range breaking capacity according to the type of protection required. 
  • Utilisation category – select according to the nature of equipment to be protected.

Fuses are in most cases marked with all the essential information required to select a replacement fuse or to enable you to select fuses for most common applications. Additional information, such as performance characteristics can be supplied by the fuse manufacturer. Standard markings are as follows:

  • Fuse standard, size and reference. This defines the operating performance and mechanical design of the fuse.
  • Voltage rating. Fuses may be operated up to the maximum voltage stated in the rating. Voltage ratings may be colour coded to reduce the risk of incorrect installation.
  • Current rating. This represents the value of the current that the fuse can carry continuously with out deterioration under specified conditions.
  • Rated breaking capacity. This is the maximum value of prospective current that a fuse is capable of breaking under standard conditions. In other words, the maximum amount of current that a fuse can tolerate without losing its physical integrity (physically breaking apart). Industrial fuses according to IEC 60269-2 have a minimum breaking capacity of A.C. 50kA and D.C 25kA respectively.
  • Breaking range and utilization category are indicated by letter codes. The first (lower case) letter indicates the breaking range, i.e. the range of prospective currents the fuse is able to break. The second (capital) letter indicates the utilization category, i.e. time-current characteristics for typical applications.
  • Fuse manufacturer or trademark.
  • Manufacturer’s product reference or part number, required for additional information on the fuse.

Additional markings may include:

  • Rated frequency if other than 50 or 60 Hz. 
  • Date of manufacture, useful to trace back product design changes.
  • Certification mark, asserting product quality by an independent certification body. 
  • Conformity mark, stating conformity to applicable national directives. 
  • Recycling mark, identifying responsible recycling organization. 

In most cases, people contact Fuseco when they need to replace an existing fuse that has blown. In such cases, it is paramount that either the identical fuse or an exact equivalent is provided. Care must be taken to make sure that the fuse provided is of the same utilisation category and has the same electrical performance of the original. If you need assistance with this, I welcome you to contact us and we would be very happy to assist you. Also, we provide many types of cross-reference charts on this website, wall charts and various tools that can assist you to identify equivalent fuse types.

Fuses are rated by current, voltage, breaking capacity (interrupt capability) and whether they are designed to operate on AC or DC circuits. The current rating of a fuse is the current that it can carry for indefinite time periods without opening. The voltage rating relates to the ability of the fuse to function and extinguish internal arcs when it opens. The breaking capacity is how large a short-circuit or fault current the fuse can interrupt or stop safely without allowing a continuing arc and without damaging the fuse body or fuse holder.

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It’s been a pleasure working with Fuseco on this project. Initially, we thought that the power quality issues on this job were insurmountable. Fuseco conducted a power quality site audit and consulted with us to find a suitable solution that would work reliably in this challenging environment. The reactors and harmonic filters specified & supplied by Fuseco have been the perfect solution.

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I would like to acknowledge the exceptional service provided by one of your employees over the Xmas break. On Christmas day we lost a 22kV underground feed to a very important part of our business and were desperate for some replacement HV fuses. I found your emergency contact details on the web a and immediately called. The person who answered was very helpful and arranged an emergency transport to our site. The next day, we were back up and running! We are a remote operation 1800kms from the nearest city and average service is the norm.

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I have dealt with Fuseco for the last 6 years for all our fuse requirements and find they provide the highest quality service and on-going support to our business for our day to day operations and for emergency/ heatwave as they arise. During a heatwave event in January 2014, SA Power Networks had critical fuse demands. Fuseco were extremely responsive in expediting stock requirements and organising special air freights to meet our urgent demands. Their customer service is excellent and key KPI’s measured against the contract are always above target.

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