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Old 21st March 2005, 03:48 PM   #1
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Default How fast are 'Fast Fuse' and how to identify?

Hi,

I have got some 5A fuse and my new project needs 5A 'Fast Fuse'. How to tell if mine are 'fast' and how fast is 'fast'? I cannot find any marks on the fuse expect the '5A'

Tks for input
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Old 21st March 2005, 03:54 PM   #2
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It will say F5A is it's a fast one or T5A if slow. If unmarked assume slow.
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Old 21st March 2005, 04:31 PM   #3
AndrewT is offline AndrewT  Scotland
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Hi,
wired fuses come in 3 speeds. fast, standard & slow (or time delay), the markings would be F5A, 5A, T5A.
All these are fairly slow compared to an MCB and extremely slow compared to BJT output device. Fuses rely on downstream robustness to survive the short term overload before the fuse opens. Transformers have a large amount of thermal capacity and hence lag to help them meet this overload.
The real purpose of fuses is to stop the equipment or house wiring catching fire.
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Old 21st March 2005, 08:46 PM   #4
K-amps is offline K-amps  United States
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If the fuse has a squiggly wire (not coiled or straight) it's probably fast
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Old 21st March 2005, 08:56 PM   #5
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to test: Place the fuse across the + and - terminals of a car battery. If it blows in under 1ns it is a fast fuse. Be sure to test all fuses in this manner before installing them.


Disclaimer: I'm just kidding of course
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Old 21st March 2005, 09:27 PM   #6
johnnyx is offline johnnyx  United Kingdom
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If a fast fuse is specified, then it's best to forget the ones you have, and get some F5A ones. Look in a parts catalogue.
BTW there are ultrafast fuses designed to protect semiconductors, marked FF5A. Some industrial ones are designed to protect thyristors etc. in inverters for motor control. I'm not that sure how effective they are though, I only get to see the ones that have failed.
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Old 21st March 2005, 10:01 PM   #7
K-amps is offline K-amps  United States
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Quote:
Originally posted by johnnyx
If a fast fuse is specified, then it's best to forget the ones you have, and get some F5A ones. Look in a parts catalogue.
BTW there are ultrafast fuses designed to protect semiconductors, marked FF5A. Some industrial ones are designed to protect thyristors etc. in inverters for motor control. I'm not that sure how effective they are though, I only get to see the ones that have failed.

Hey Johnny X, are you saying with the FF fuses, BJT's no longer need to protect the fuse?
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Old 22nd March 2005, 09:00 PM   #8
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No, I think Murphy's Law (electronic component sub-set) is pretty rigid at this point. "A fast acting fuse will always be protected by the semi conductor blowing first")
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