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September 16, 2009
Fuse Selection Guide - Part IV

Here is the fourth installation of the OptiFuse Fuse Selection Guide.  The information is a bit "meaty" but hopefully you’ll take a moment to read through it...who knows...maybe I actually wrote the material in a way that can easily digested...

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?

What is the maximum allowable I²t?

A few blogs ago I discuss the idea of I²t for all devices providing short-circuit protection.  Click here to review that blog in detail.

Here’s a short review...it takes a certain amount of "reaction time" for any over current protective device to work.  During the time it takes for the fuse to open, there is energy flowing through the fuse. That energy is measured in I²t.  There two parts to the fuse’s "reaction time".  1)  The time it takes to melt the fuse element and 2) the time it takes for the arc inside the fuse to stop conducting.  The combination of both of these times gives us the total clearing time of the fuse.

During this clearing time, there is energy is being "let-thru" the fuse therefore the downstream components are then subject to this extreme energy as it passes through the fuse (if only for a few milliseconds). 

In order to specify the proper fuse or circuit breaker in a circuit, the engineer must know the withstand capabilities of the downstream circuit components and select a fuse whose let-thru energy is below that of those components.

Are there "in-rush" currents available?

Once again, there is a very good review blog regarding in-rush currents and time-delay in a previous blog.  Click here to review that blog

Depending on the circuit, there are times when a large amount of current is required when a piece of equipment is turned on.  The types of components that can cause this type of in-rush include motors, fans, and capacitors.

When in-a rush current does occurs, it generally only lasts for 2-5 seconds after turning on the piece of equipment (such as your computer power supply or old CRT type TV) but can be as high as 6-10 times that of the normal current (for example a typical TV might draw 3A but the in-rush could be as high as 30A).

During this in-rush, the fuse should not open.  The specified fuse in this case should be a time-delay fuse allowing the piece of equipment to start up properly without a nuisance opening of the fuse.  To learn more about time-delay fuses please click here.

Well we’re getting through the list a few sections at a time...hopefully you find it informative and not too technical or boring.  If you can think of any ways to liven up this very mundane information I’d love to hear from you...

Look for part V next Wednesday as we continue this technical series. 

Thank you once again for your continued support of OptiFuse as we try to inspire imagination...

Jim Kalb


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