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August 21, 2009
UL vs EIC Fuses

Some of our customers have often wondered why fuse makers have two different part number for seemingly the same fuses. 

For example OptiFuse FSC / FSD, Bussmann GMA / GDB, Littelfuse 235 / 217 are all fast-acting, glass, 5 x 20mm fuses.  There appears to be two part numbers for each type of fuse.

The reason for this anomaly is that the first part numbers (on the left side of the slash) are designed to meet UL standards and the second part number is designed to meet the European IEC standards.

But wait you say...aren’t both of these agencies involved with maintaining safety?... why the differences?... blame it on "geo-political differences" I suppose...

Mostly the differences in the standards are in the opening times...let me show you an example...

The UL standards say that a fast-acting fuse must operate under the following conditions (UL Std 248-14):
  • At 110% of the fuse’s rated current, the fuse must stay closed for a minimum of 4 hours
  • At 135% the fuse must open within 1 hour
  • At 200% the fuse must open within 5 seconds
The corresponding IEC standards say that a fast-acting fuse must operate under these following conditions:
  • At 150% of the fuse’s rated current, the fuse must remain closed for 1 hour (32mA-6.3A) and 30 minutes (8A-15A)
  • At 210% the fuse must open within 30 minutes maximum
  • At 275% the fuse must open in a minimum of .05 seconds and a maximum of 2 seconds
  • at 400% the fuse must open in a minimum of .01 seconds and a maximum of .3 seconds (125mA - 6.3A)
  • At 1000% the fuse must open within .02 seconds maximum
I underlined two of the above standard conditions showing that it’s impossible to be open (UL standard at 135%) and be closed (IEC standard at 150%) at the same time.

Because of these two different standards, fuse manufacturers are forced to design and construct two very different fuses... depending on whether or not the equipment will be used in Europe or North America (UL and CSA use basically the same standard).

The fuses that have these different designations include:  3.6 x 10mm, 5 x 20mm and microfuses in both fast-acting and time-delay versions.

This topic is something that design engineers should be definitely be made aware of...especially if their product is intended to be used in Europe (this is a perfect way for a salesperson to add value to their product line and differentiate themselves from their competition). 

By the way... there has been some talks recently about unifying the standards... but nothing is complete as of today... I’ll keep you abreast as developments occur...

Thank you once again for your continued support of OptiFuse and our pursuit of unifying the world.

Jim Kalb

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