Fuses ratings

[Karl W4KRL]

Andre,

 

Thanks for pointing this out. It would be nice to have a 1A fuse blow
instantly at 1.1A but physics won’t allow it. That’s what we have electronic
circuit breakers for. It takes pretty much a fixed amount of energy to raise
the temperature of the fuse element from room temp to melting point. For
currents above about five times the fuse rating the energy required is E = I
^2 * R * t where R is the resistance of the fuse element. Since we usually
don’t know the value of R it is common to rate fuses (and other electrical
equipment) with an I^2 *t (I squared t) value. If you look at the 10E curve
for example you can calculate the I^2 value above 50A as 2,500 A^2/s.
Likewise the I2t for the 100E fuse is 250,000 A^2/s exactly 10 squared times
the value of the 10E fuse. The entire chart can be normalized to the ratio
of the overload.  For example, a 5A fuse loaded with 50A (a 10:1 overload)
will blow in 0.25 seconds and a 80A fuse loaded to 800A (10x) will also blow
in 0.25 seconds. The I-squared-t value for this family of fuses is
approximately 2,500 times the rating squared divided by 100.

 

Below 5 times overload other factors affect the blow time such as ambient
temperature and how well heat is conducted away from the fuse by the fuse
holder.

 

In most high power semiconductor applications the fuse rating is chosen
primarily to coordinate with the I^2 t rating of the semiconductors rather
than the current rating. Semiconductor fuses generally use notched silver
links packed in silica. I can’t find any reference for this but when I was
selecting fuses at Westinghouse to protect 3000A thyristors on the Penn
Central Metroliners it was a commonly held belief that silver link fuses had
been discovered in captured German U-Boats where they were used to protect
the propulsion battery circuits.

 

73 Karl W4KRL

 

From: Andre Kesteloot [mailto:andre.kesteloot at verizon.net] 
Sent: Monday, July 23, 2012 10:02 PM
To: Tacos
Subject: Fuses ratings

 

Tacoistas

Last Saturday, at Tacos, part of the technical discussion was focused on
short circuit-protection and electrical fuse ratings.
I remarked that a 1 amp fuse would not normally melt while passing a DC
current of one amp.

Here are the data for a medium delay fuse:
http://www.cooperindustries.com/content/dam/public/bussmann/Electronics/Reso
urces/Data%20Sheets/bus-elx-ds-4395-gmc.pdf
CooperBussman states 
at current rating  -> opening time
100%                            none
135%                            60 minutes max
200%                            120 seconds max

For a slow-blow ceramic fuse, Littlefuse  offers
100%                             4 hours max
135%                            60 minutes max
200%                            120 seconds max


Time vs current characteristics are usually of the type shown in
http://www.sandc.com/edocs_pdfs/EDOC_002721.pdf

73
André N4ICK




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