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October 21, 2009
Ambient Temperature Re-Rating
 
 


In my experience, the most common reason a fuse fails to open or open too quickly is that the fuse was never sized properly in the first place. 

Often the fuse was initially selected based on the steady-state current of the system but there is little taken into account for ambient temperature variations and available in-rush currents.

Fuses (and circuit breakers) are thermal devices that open (or remain closed) based on the amount of heat that they see.  Higher than normal currents generate the heat needed to open a fuse however the temperature of the surrounding air also adds or subtracts from the heat of the current flow.

Below you will find a handy fuse re-rating graph and chart to help select the proper fuse rating.  The resolution of the graph is a bit poor for the blog but can be found on the OptiFuse website starting tomorrow.

I have also included a chart of the re-rating values that the graph was derived from.

 


                     FUSE RE-RATING TABLE

Temp
°C
%
Factor
Temp
°C
%
Factor
-25
14
0.877
23
0
1.000
-20
13
0.885
30
-2
1.020
-15
12
0.893
35
-4
1.042
-10
11
0.901
40
-6
1.064
-5
10
0.909
45
-8
1.087
0
9
0.917
50
-10
1.111
5
8
0.926
55
-13
1.149
10
6
0.943
60
-16
1.190
15
4
0.962
65
-19
1.235
20
2
0.980
70
-22
1.282


Fuse Re-Rating Graph


The formula for selecting the proper fuse therefore is calculated as follows:

Fuse Rating = Iss x  1.25 x Temp. Factor

where Iss = Normal steady-state current @ 23°C

Once the fuse rating has been determined, experimental testing should be completed to ensure that the selected fuse is right for the particular circuit application.  OptiFuse can sample fuses to engineers at no cost to make absolutely sure that the fuse is correctly selected.
 
Thank you once again for your continued support of OptiFuse where as we try to improve the results of your efforts...

Jim Kalb
President
OptiFuse
jimk@optifuse.com

 

IMPORTANT NOTE:  Some of you caught my mistake in Monday’s Blog.  The proper part numbers for the line-voltage resettable fuses are RS120 and RS240 (not R120 and R240 as written in Monday’s blog)...thanks for the catch team!
 

 
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