[Elecraft] K3: Power cable question

[David Cutter]

Glad you raised that point, Brian.  In my Sunday school classes I ask the 
question: "what does the fuse protect?"  It's  a revelation to all when I 
give the right answer.

On our 240V distribution system, a melted cable can lead to a severe shock 
hazard and I'm guessing that in the 110V centre-tapped system since the 
shock hazard is so much less, the next major hazard is fire, so, I've often 
thought that we are protecting against 2 hazards, each with a different 
emphasis in each case.

In a 14V dc distribution system there is no real shock hazard (SELV), so the 
remaining hazard has to be fire, so, a slow fuse is not a problem, since 
heating effect in the wiring is so much slower.

Anyone care to comment/add?

David

>
> 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