Electronics > Beginners
500mA fuses blow at 1A, am I testing them incorrectly?
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westfw:
Aren't fuses designed to mostly protect against severe overloads?  What failure mode do you envision where your 400mA-max device would start drawing 600mA, that justifies blowing a fuse?

(Hmm.  Moving parts?  I once blew a fuse in an "automotive 12V adapter" by putting a load on an air pump.   If you use lubrication or overload a motor that should be running at 200mA and it starts running at 400mA instead, that seems like a reasonable time to blow a fuse before "extreme overload.")
bson:
For such small values it doesn't terribly matter.  Just use a fuse that won't eventually fail under normal use.  I'd pay more serious attention to a 10A fuse...
hamster_nz:
The other thing to watch out for is that if you have appreciable resistance (say a long run of thin wire between) the source (say a car battery) and the load (say a 12V/25W LED bulb) that the wire resistance might delay or even prevent the fuse from blowing, even with a dead short across the load.

In such cases you need to make sure that you use chunky enough wire that your fuse can do its job.
james_s:
Since the fuse is sized to protect the wire, this shouldn't matter. If the fuse doesn't blow, the wire should not get hot enough to be a problem.
EEVblog:
As others have said, it's actually quite a complex field if you want to get into it:

http://www.littelfuse.com/~/media/electronics/application_guides/littelfuse_fuseology_application_guide.pdf.pdf
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