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September 30, 2009
Fuse Selection Guide - Part VI


Almost there...here is the second-to-last installment of the OptiFuse Fuse Selection Guide.  We will finish the guide next Wednesday after which time the entire guide can be downloaded in PDF format from 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?

Does the fuse need to be "field-replaceable"?

Fuses are intended to open when a problem occurs whether it be a short-circuit or over-load.  A decision by the engineer needs to be made as to whether or not the fuse should be field replaceable.

The primary reason for making the fuse replaceable is simply convenience for the end-user in getting their equipment back up and running.

The reasons for choosing not to have the fuse field replaceable are two-fold:
  1. There can be a significantly greater cost to the manufacturer to include a fuseholder as opposed to directly soldering the fuse into or onto the PCB
  2. The manufacturer may not want the end-customer to access the interior of the equipment to replace the fuse for liability issues.  This is especially true when a short-circuit was the cause of the problem in the first place.
  3. The manufacturer may have some "planned obsolesce" of their parts and may want to replace the entire circuit board rather than have someone replace just the fuse

Is resettability an issue?

There are one-time use fuses and resettable type fuses available to the engineer.

Both types of fuses provide short-circuit and over-load protection.  Resettable fuses are limited to circuit applications below 9 amps (at 12V) and even less current at higher voltages.

Circuit breakers can also provide resettability and can range from 1A to 6000A.

One-time fuses are just as their name implies.  Once they are called upon to act, the interior link melts and the fuse must be replaced.  Just because the fuse is replaced, there can still be a short-circuit or over-load still present in the circuit which can cause the newly replaced fuse to open as well.  Care should be taken to correct whatever problem that may have occurred when the fuse opened in the first place...before replacing the open fuse with a new fuse.

What safety agency approvals are needed?

There is an entire alphabet soup of world-wide safety agencies out there.  UL, CSA, IEC, CCC, PSE, VDE, Nemko, Semko and TUV are some of the most popular agencies.

The agency approvals needed by manufacturers depends solely on what type of equipment they are making and where in the world they hope to sell their equipment. 

Fuses typically are available with several approvals (for example UL and CSA).  Even within a single agency, there can be multiple type of approvals such as UL listing versus UL recognized.  To review the differences between listed and recognized products in detail, please click here.

Certain equipment does not require any agency approvals such as many automotive or low-voltage applications.

The biggest issues as it relates to safety agency approvals for fuses is that there are several different test methodology and standards depending on the agency involved.  This might mean two different fuse characteristics for what is apparently the same fuse and/or application.

Please contact OptiFuse with any specific questions and latest updates as to the agency approvals of our products.

Thank you once again for your continued support of OptiFuse where we know that education is a process not an event...

Jim Kalb


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