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September 9, 2009
Fuse Selection Guide - Part III

Here is the third 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 operating ambient temperature?

Ambient temperature is a fancy way of saying the outside air surround the fuse.  Normally, the fuses are tested in "laboratory conditions" by the safety agencies such as UL and CSA.  The lab conditions are almost always set at 25°C or 77°F.  Unfortunately, most real world conditions are not those found in a laboratory. 

A fuses is a heat sensitive device meaning that it takes heat (via the over current) to melt the fuse element inside the fuse.  The more heat...the faster it takes to melt the the fuse element...the less heat...the longer it takes to melt the fuse element.

If a fuse will be subjected to a higher temperature than 25°C, then the fuse amperage will need to be increased as to compensate for the higher temperature (to avoid "nuisance tripping").  Likewise, if the fuse will be used at a lower temperature, then the fuse amperage needs to be lowered (or else it might never open).

The rule of thumb is that for every 20°C higher or lower in temperature, the fuse should be re-rated 10-15%. 

An example of the re-rating for higher ambient temperature:

Normal full load current:  700mA
Normal fuse sizing: 1A (130% of full load current)
Ambient Temperature:  65°C
Re-rating:  1.3A (130% of normal fuse rating)

Conversely, when a fuse is intended to be used in extreme low temperature conditions, then the fuse must have a lower rating than in normal conditions. 

An example of the re-rating for lower ambient temperature:

Normal full load current:  700mA
Normal fuse sizing: 1A (130% of full load current)
Ambient Temperature:  -15°C
Re-rating:  700mA (70% of normal fuse rating)

What is the available short-circuit current?
The available short circuit current is the measured or calculated current that can be delivered to a circuit when a short-circuit is present in the piece of equipment.  These are the dangerous currents that the fuse is intended to open.

The available short circuit calculation can be very complex and generally should be left to qualified engineers.  The calculation is generally based on the following factors:
  • How much short-circuit current is available from the utility?
  • What is the resistance of the wiring from the utility to the piece of equipment where the fuse is installed?
  • What is the internal resistance of the piece of equipment where the fuse is installed
Once all of these factors are known, then the engineer can calculate the available short circuit current to the fuse. 

The fuse must be selected as to have a greater short circuit rating than what is available in the circuit (otherwise the fuse can explode and cause great harm to people and damage to equipment!)

On a side note, I’ve had several interesting e-mails telling me that the technical blogs are too technical while others tell me that they are too watered-down...My goal here is not to create engineers but rather explain some of the key concepts in a relatively "easy to understand" manner...well...at least that’s the goal...

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

Thank you once again for your continued support of OptiFuse where we want try to deliver something for everyone...

Jim Kalb


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