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August 31, 2009
Fuse Selection Guide - Part II

Last week I briefly wrote about all of the different criteria needed to properly select an over-current protective devices.  Today’s blog is the second in this series...

After creating a general list of criteria in order to select the proper devices

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 normal operating current of the circuit?

In order to select the right amperage fuse you first need to know the full-load steady-state current of the circuit at an ambient temperature of 25º C (68º F).  Once the current value is determined, then a fuse should be selected to be 135% of this value taken to the next standard value.

For example, if the normal steady-state current is calculated to be 10 amps, then a 15A fuse rating should be selected [10 amps x 135% = 13.5 amps, the next larger standard size is 15A].

It is important to note that if the fuse is intended to be used in an environment with possibly very high ambient temperatures, then the fuse would need to be sized significantly higher.

What is the operating voltage?

Several weeks ago I talked in great detail about the voltage rating of overcurrent protective devices.  To re-read the article, please click here.

The basic rule of thumb (without having to go back a re-read the technical bulletin) is that a fuse must always be selected so that the voltage rating of the fuse is higher than the circuit voltage. 

For example, if the circuit voltage is 24 volts, then the fuse voltage rating must be higher than 24 volts (yes...it can be 250 V...just so long as it’s higher than the circuit voltage).

Is the circuit AC or DC?

Once again, a fairly detailed article is available in our archive covering this subject.  To re-read the blog please click here.

To summarize the article...there are both AC (alternating current) and DC (direct current) available.  AC power is what you will typically find in your home from the utility, and DC current generally is delivered via a battery (as in automotive systems) or solar power systems.

With AC power, the current and voltage oscillate back and forth.  This oscillation helps the fuse to clear quickly.  DC power on the other hand doesn’t oscillate so the fuse must find other means to clear itself when opening.

Because of these differences, some fuses are designed specifically to used in DC applications (such as automotive fuses).  Some AC rated fuses may be used in DC applications, however there may be a voltage de-rating in these cases. 

More to come...Additional parts of this continuing series will be available each Monday for the next several weeks.

Thanks to the feedback from the many readers, the blog will be send only three times per week using the following schedule:

Monday - Technical Brief
Wednesday - Product Information
Friday - News and Commentary

Thank you once again for your continued support of OptiFuse where your opinion matters

Jim Kalb


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