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October 7, 2009
Fuse Selection Guide - Part VII

I’m happy to announce that in today’s blog, we will conclude the narrative for the OptiFuse Fuse Selection Guide.  Hopefully by this weekend, the entire Fuse Selection Guide will be posted in a printable PDF format on the OptiFuse website.

Just to review...below are 15 information items that are needed before selecting the proper fuse for an application.  Each week I’ll attempt to explain 2 or 3 of these items in detail.  After completing the series, I’ll post the entire selection guide on the OptiFuse website:

In order to select the proper protective device you need to consider the following factors:

  1. What is the normal operating current of the circuit?
  2. What is the operating voltage?
  3. Is the circuit AC or DC?
  4. What is the operating ambient temperature?
  5. What is the available short-circuit current?
  6. What is the maximum allowable I²t?
  7. Are there in-rush currents available?
  8. Is the protective device being used for short-circuit protection, over-load protection, or both?
  9. What are the physical size limitations?
  10. Is the PCB surface mount or thru-hole?
  11. Does the fuse need to be "field-replaceable"?
  12. Is resettability an issue?
  13. What safety agency approvals are needed?
  14. How will I mount the device?
  15. What are the cost considerations?

How will the fuse be mounted?

One of the most careful consideration that need to be made is the mounting of the fuse in the circuit.  There are several options at hand:
  1. Direct Solder - In this method, the fuse is directly soldered into or onto the printed circuit board (PCB).  The drawback to this design is the lack of field replaceable parts as discussed in great detail in section 11 but cost can be significantly reduced with this mounting method.
  2. Fuse Clips - Fuse clips are relatively inexpensive and allow for field replaceability.  Fuseclips are typically mounted on a PCB so any attempt at replacing the fuse will require opening of the piece of equipment by the end-user.  Additionally, removing a fuse from a PCB without disconnecting the power source could lead to an electrical shock when touching the fuse.  Fuse clips are available for all "tube" fuses as well as mircofuses.  Typically fuseclips are limited to 15A of normal current.  Fuse clips are generally not listed or recognized by any safety agencies.
  3. Panel Mounted Fuseholders - Panel mounted fuseholders allow for easy access for the end-user to replace the fuse in the field.  The panel mount fuseholder is shock-safe meaning that the fuse is removed safely when the cap of the fuseholder is removed preventing the possibility of electrical shocks.  Fuseholders are typically tested and approved by safety agencies such as UL and CSA.  Fuseholders are typically available up to 30A.
  4. Fuse Blocks - Fuse blocks are like fuse clips however they do not need to be mounted on the PCB.  Fuses mounted in fuseblocks are typically only accessible by opening the piece of equipment which could lead to electrical shocks if the equipment is not disconnected from the power source.  Fuseblocks are one of the few methods to mount fuses of large amperage.
  5. Inline Fuse Holders - Inline fuse holders are typically used as a part of a wire harness assembly or where no surface is available to secure another type of fuse mount.  Inline fuse holders are generally available up to 100A in lower voltage applications and up to 30A in higher voltage applications.

What are the cost considerations?

The costs considerations can vary by several degrees depending on the size, performance, and mounting of the fuse.  Generally speaking, the larger a fuse is, the most it will cost (due to higher material costs to build the fuse).

The performance characteristics of a particular fuse is also a large cost consideration.  A low voltage automotive fuse might a fraction of the cost as compared with a 500V super high-speed, ceramic tube fuse both rated at 10A.

Safety agency approvals will also add to the overall cost of the fuse.

One of the largest costs of a fuse is the fuse holder.  A typical panel-mount fuse holder may cost in upwards of 10 times than that of the fuse itself. 

OptiFuse is proud to deliver fuses and fuse holders at a significant cost savings to our customers as compared to Bussmann and Littelfuse.  Our overhead is significantly lower than our competitors therefore we are able to pass these savings on to our customers.

Thank you once again for your continued support of OptiFuse where value isn’t just a word, it’s a way of life...

Jim Kalb

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