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Which of These 11 Fuse Selection Factors Have You Overlooked?

For the circuit designer, few tasks can be quite as complex as choosing an appropriate fuse for an application. Simply deciphering the information in the various manufacturers' datasheets and comparing them with the application's requirements can be extremely time-consuming.

Fuses have been a staple of circuit protection for more than a century. Littelfuse offers a brief history on its website:

The first fuses were simple, open-wire devices, followed in the 1890's by Edison's enclosure of thin wire in a lamp base to make the first plug fuse. By 1904, Underwriters Laboratories had established size and rating specifications to meet safety standards. The renewable type fuses and automotive fuses appeared in 1914, and in 1927 Littelfuse started making very low amperage fuses for the budding electronics industry.

The fuses used in electrical/electronic circuits today are current-sensitive devices designed to serve as the weak link in the circuit. They protect discrete components or complete circuits by melting reliably under current overload conditions.

There are 11 critical factors involved in fuse selection:

  1. Normal operating current: A fuse's current rating is typically derated 25% for operation at 25ºC to avoid nuisance blowing. A fuse with a current rating of 10 A is not usually recommended for operation at more than 7.5 A in a 25ºC ambient environment.
  2. Application voltage (AC or DC): The fuse's voltage rating must be equal to or greater than the available circuit voltage.
  3. Ambient temperature: The higher the ambient temperature, the hotter the fuse will operate, and the shorter it will last. Conversely, operating at a lower temperature will prolong fuse life. A fuse also runs hotter as the normal operating current approaches or exceeds its rating.
  4. Overload current condition: This is the current level for which protection is required. Fault conditions may be specified, either in terms of current or in terms of both current and maximum time the fault can be tolerated before damage occurs. Time-current curves should be consulted to try to match the fuse characteristic to the circuit needs. Keep in mind that the curves are based on average data.
  5. Maximum fault current: The fuses's interrupting rating must meet or exceed the circuit's maximum fault current.
  6. Pulses (surge currents, inrush currents, startup currents, and circuit transients): Electrical pulse conditions can vary considerably from one application to another. Different fuse constructions may not react the same to a given pulse condition. Electrical pulses produce thermal cycling and possible mechanical fatigue that could affect the fuse's life. Initial or startup pulses are normal for some applications in which fuses incorporate a thermal delay design to survive normal startup pulses and still provide protection against prolonged overloads. The startup pulse should be defined and compared to the fuse's time-current curve and I2 t rating.
  7. Physical size limitations: Refer to the manufacturer's data sheet for information on a fuse's length, diameter, and height.
  8. Agency approvals required: Refer to the manufacturer's data sheet for information on a specific device's agency approvals, such as UL, CSA, VDE, METI, or MITI. Military requirements need special consideration.
  9. Fuse features: Refer to the manufacturer's data sheet for information on mounting type/form factor, ease of removal, axial leads, visual indication, etc.
  10. Fuseholder features and rerating: Refer to the manufacturer's data sheet for information on clips, mounting block, panel mount, PC board mount, RFI shielded, etc.
  11. Pre-production application testing and verification: Verify the selection by requesting samples for testing in the actual circuit. Before evaluating the samples, make sure the fuse is mounted with good electrical connections, using adequately sized wires or traces. The testing should include life tests under normal conditions and overload tests under fault conditions to ensure the fuse will operate properly in the circuit.
  12. To learn more about choosing fuses appropriate to the application, download a free Fuseology selection guide from Littelfuse Inc.

19 comments on “Which of These 11 Fuse Selection Factors Have You Overlooked?

  1. eafpres
    April 2, 2014

    Hi Bharat–thank for the thoughtful guide.  I must reminisce about my home as a teenager.  We lived in a house built in the early 1900s which had a fuse box.  No circuit breakers, but those lamp-base screw in fuses.  The wiring which was protected by these fuses was nothing short of dangerous in the 1980s.  Some of it ran in the attic where glass insulators were attached to the rafters and the wires ran from one to the other.

    For homes such as this, you were not required to change anything as long as you never did any renovation or modification to the main structure which would require a building permit.  If you did so require a permit, you were then required to upgrade the entire wiring scheme to modern standards, including replacing fuses with circuit breakers.

    The fuse box remained well after I left college and in fact was there in the mid 1990's until the house was sold and completely renovated.

  2. Steve Taranovich
    April 2, 2014

    Thanks for sharing your experience Blaine. I brought Littelfuse on because I think that safety is an after thought in designer's minds nowadays and it is a critical area in electronics that needs attention. Bharat will bring us important tutorials that will enhance designers' awareness of circuit protection (Safe electronics so-to-speak) and how to implement it properly.

     

    I too have fond memories of growing up in Brooklyn, NY with fuses in the basement of our 6-family apartment house. I remember when my mom and dad were months behind on the electric bill and Con Ed came to shut off the electricity. The service man came to shut off the power until my dad could scrounge up the money to pay the bill.

    That service man asked me to show him where the fuse boxes were in the baement. When I got down there with him, he told me that he is “officialy” shutting off our power (by removing the glass, screw-in fuses). He showed me how he inserted a penny into the empty fuse socket and said that this would keep the electricity on until we paid the bill (What compassion!)

    He also showed me how to safely remove that penny once the bill was paid with a pair of insulated pliers before the Con Ed service man came back to turn the electricity back on.

    A time long past

  3. eafpres
    April 2, 2014

    @Steve–what a great story.  Now of course, the meters become “smart” and a computer might make the decision to turn off your power.

  4. Steve Taranovich
    April 3, 2014

    Hi Blaine,

    Ha–computers can sometimes be far colder than humans!

  5. Davidled
    April 3, 2014

    In most fuse box, there are several fuses in the serial and parallel pattern. If electric power is overloading in a specific fuse line, then we go to the basement then figure out which fuse should be reset. Instead of manually resetting all fuses in box, if some type interface device such as led may indicate warning sign before fuse is gone. That will be a great tool for householder.

  6. bshenoy
    April 6, 2014

    DaeJ – there are indicating fuses available for some larger fuse products but those fuses indicate after the fuse has blown open. The concept of a fuse indicating that it is close to trip or opening is asked about once in while from our customers. This functionality has some valid benefits for the circuit designer. One being knowing that fuse element is weakening and needs to be replaced before a nuisance trip occurs. So, this is a fuse reliability issue and would be great value for these customers who are worried about nuisance tripping issue. Of course, it would be benefical if the fuse was installed in replaceable configuration such as in a fuse-holder.

  7. RedDerek
    April 9, 2014

    The solution could be a small circuit that monitors the fuse voltage drop versus current. If the fuse is weak, would not the voltage drop increase? Then it would be down to figuring out what voltage drop is acceptable.

  8. Sachin
    April 10, 2014

    It is funny (in an expensive way) just how a small mistake or oversight when choosing the right fuse for a simple design could render everything inefficient or even totally useless. Unfortunately as professional designers we very often simply do not have the time to go through multiple different manuscripts and technical documents trying to compare fuses so mostly we simply take the word of the sellers or manufacturers and leave it at that. So thank you for this simplified checklist.

  9. bshenoy
    April 11, 2014

    RedDerek – I am checking with our fuse designers if it is true that fuse element resistance increases consistently when it weakens or is close to nuisance opening. There are phenomenon such as metal migration that occur as well but those happeing usually do increase the resistance. So, let me check on this.

  10. bshenoy
    April 11, 2014

    SachinEE – We actually have a lot of tools we developed to help customers select the correct fuse. Suggest taking a look at these which go into more details.

    http://www.littelfuse.com/~/media/Electronics_Technical/Application_Notes/Fuses/Littelfuse_Fuse_Characteristics_Terms_and_Consideration_Factors_Application_Note.pdf

    http://www2.speed2design.com/fuse_selection_guide

     

  11. Sachin
    April 12, 2014

    I think many people have and will still get many problems related with frequent replacement of fuse after it blows off. The best thing that fuse developers should actually do is to look for away that might make their customers identify the fuse us close to trip in order to replace it earlier. What am trying to say is that a fuse should indicate a warning before it trips.

  12. bshenoy
    April 17, 2014

    RedDerek – response from our R&D engineer.

    The problem with fuses is that monitoring voltage drop will not give you the entire picture and therefore cannot reliably detect early failure potential.  Voltage drop alone does nothing as it will vary with current load, so monitoring current and voltage drop has some merit in terms of monitoring the differential as compared to a “standard” that could be created for a particular design.  At the end of the day though there are failure modes that voltage drop will not accurately predict.  For instance, current cycling can cause work hardening of the element material (grain growth) and create brittle fracture failures that occur unpredictably.  During this type of scenario there will be no appreciable change in voltage drop before the fracture failure occurs.

  13. Sachin
    April 29, 2014

    When dealing with electricity, the most important factor to be looked into is safety. And I would like to urge to continue enlighten us on matters related to electricity safety. Since not everybody has electricity knowledge, but they use electricity, I think it is perfect for designers to upgrade on the way the fuse boxes are designed. They should include warning alerts which will enable an individual to detect electricity malfunction in prior.

  14. SunitaT
    April 29, 2014

    The eleven factors involved in the fuse collection are basically what will make it easier to use. The fact that the normal operating current is 7.5A makes it easier to operate also not forgetting the voltage characteristics which determine the type of fuse box that will be purchased by the end user. The pre production application testing and its verification is what will improve its functioning and even boost its authenticity status. The only downside to it will be that it's expensive.

  15. bshenoy
    April 29, 2014

    SunitaT0- you are correct. The 11 steps which includes items like temperature de-rating, lifetime re-rating, in-rush pulse withstand calculations, will also ensure that the correct fuse current rating and i2t are selected so that fuse does not nuisance trip or fail during it's lifetime. These steps basically add design-margin over an above the operating current which bumps up selected fuse current rating. The downside for too much design margin is that you will get decreased protection level ; ie the fuse will trip slower when you need it for protection.

  16. amrutah
    April 29, 2014

    Bshenoy: Thanks for the information shared.  De-rating due to usage (I guess its what you meant by lifetime re-rating) is important.  I believe the de-rating due to on-shelf storage is also important since there are issues like metal corrosion, oxidation and so on…

  17. yalanand
    October 31, 2014

    I believe the de-rating due to on-shelf storage is also important since there are issues like metal corrosion, oxidation and so on.

    @amrutah, that is a good point. Do you think de-rating due to on-shelf storage is significant compared to other de-rating factors ? How can we avoid derating due to on-shelf storage.

  18. yalanand
    October 31, 2014

    @Bharat, thanks for the post. I visited the LittelFule website and found it very useful especiall the  Fuse Selection for Electronics Applications section which lists the factors to consider in fuse selection.

  19. amrutah
    October 31, 2014

    @yalanand,

        The on-shelf de-rating is a very slow process compared to the derating due to temperature or over-voltage. 

    For temperature or over-voltage derating for the device lifetime can be computed by accelrated simulators.  I wanted to know if there is anything on how to quantify the on-shelf derating.

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