[Elecraft] Fuses (WAS: Power cable question)

[Ron D'Eau Claire]

Granted, a fuse often isn't as fast a fuse as a small-signal transistor, but
fuses do a good job saving power transformers, rectifiers, voltage
regulators, and many other expensive and hard-to-replace parts in equipment.


I'm no fuse designer, but I am aware that at least some fuse manufacturers
recommend not exceeding 75% of the fuse rating in service and many won't
guarantee their fuses to carry the rated current for long. Littlefuse, for
example, only guarantees their fuses to last 4 hours running at 100% of
their rated current. They might last thousands of hours at their rated
current, or not. 

One thing I suspect causes failures in some systems I've worked on (radars,
nav gear, commercial HF/VHF radios, etc.) is the turn-on surge current,
which can be several times the normal current as capacitors charge, blowers
come up to speed, etc. While the total surge may be within the fuse ratings
for current vs. time, the repeated heavy "hits" don't do them any good. 

I've often suspected that is one significant cause of the fuse failures I've
seen where the measured currents are quite normal once the fuse is replaced.


Fuses may be shock-sensitive, sometimes surprisingly so. I've dropped a
brand new just-checked fuse, picked it up and found the element cracked or
broken although the glass tube was not damaged. (Murphy dictates that the
most shock-sensitive fuse is the last one you have within an hour's travel).
These are brand-name, normally excellent quality fuses too. I treat the
replacement fuses in my kit just like I'd care for an incandescent bulb or
vacuum tube. 

Ron AC7AC

-----Original Message-----


On Jun 16, 2008, at 9:59 PM, <d.cutter at ntlworld.com> <d.cutter at ntlworld.com 
 > wrote:

> A regular fuse should blow at 2.6 x the rating within 30s, in other
> words very slowly.  It's only a short circuit that will normally  
> blow a new fuse.  Have not seen any data on old fuses which might  
> get metal fatigue.  There are faster fuses, but the fastest are rf  
> transistors - on three legs anyway...

Most people don't know that the purpose of a fuse or circuit breaker  
is to protect the power distribution wiring, not the electronic  
component itself. Wire will carry a surge without too much temperature  
rise. The idea is that the fuse will blow or the breaker open before  
there is any chance of damage to the wire.

This means that a fuse or breaker cannot protect your active devices.  
If you want that level of protection you need something like a power  
supply with fold-back current limiting.

Brian Lloyd