[Amps] Fuses

[Jim Thomson]

Date: Sat, 8 Jan 2011 08:34:30 +0000
From: Ian White GM3SEK <gm3sek at ifwtech.co.uk>
Subject: Re: [Amps] Fuses

James R Carr wrote:
>Where pray tell do you get a hydraulic breaker? I have installed 
>several thousand over the years but have yet to see one filled with 
>oil. As for current limiting devices, the fastest breaker will hold in 
>for three to seven full cycles. To be current limiting, a fuse has to 
>clear in less
>than 1/2 cycle.

A fuse provides NO current limiting until it blows; and a breaker 
provides NO current limiting until it breaks.

A lot of damage can be done in those first few milliseconds, which is 
why the fuse/breaker is only PART of the solution.

We always need a surge limiting resistor in the B+ as well, to provide 
instantaneous current limiting until the fuse/breaker takes over and 
finally breaks the circuit.


-- 

73 from Ian GM3SEK
http://www.ifwtech.co.uk/g3sek

## Agreed. The purpose of the mag-hydraulic  breaker is [A]  handy dandy
way of opening off the 240 v line, without having to kill the dedicated breaker in the
main 200A panel. [B]  eliminate any follow on current. 

##  On my  latest 7700 vdc supply, I use a Buss  HVU series, sand filled  HV fuse..rated at 3A..in series 
with the B+ lead.    The HV fuse gets inserted just prior to the kw rated glitch R, which consists  of  4 x 
parallel globar type AS  energy absorbing type  resistor's, 1.5" diam x 18' long.  [ 4 x 200 ohm in parallel= 50 ohms]
7700/50 ohms = 154A.  max fault current.    Now 154 A of fault current, flowing through a 3 A rated
fuse, will open  VERY fast. [ the fuse is running at 5100%  of it's rating].   I use a 50 ohm glitch R  on the 
smaller supplies as well... with the provisio that a smaller rated HV fuse is used..like 1A, etc. 

##  I also use a 2nd Buss HV fuse, in the sec of the plate xfmr, [one leg only], between  sec and input to
FWB.  In normal operation, with any B+ to chassis fault, the B+  HV fuse alway's blows open 1st..so the
breaker used in the 240 v line  is sorta  a moot point.  That  B+  fuse  concept has been tested  34 x times
now, and nothing ever happens..except a blown HV fuse.   Once, some of the blown  HV fuses  were
'refurbished'  with a single strand, soldered on the outside..and wrapped with 88 tape.   In one case, the
wire ga of the single strand refurbished  job was too big.. and the end result was the parallel pair of  100
ohm, 225 W- WW's  were literally incinerated, burnt to a crisp.   That was with a 165 uf filter cap..and a 253 lb
dahl hypersil xfmr. That particular glitch R  was replaced with 4  parallel  200 ohm, 225 W, wirewounds
[ $12.00 each, from mouser],and the correct size HV fuse.  Zero problems since then, and you can literally
 cro-bar it all day long.  I would not trust any of these glitch R's, even big ones..without a prior series  HV fuse,
not with big uf caps, huge amount's of joules involved, and then trying to rely on a breaker [ or even the
2nd HV  AC fuse]. 

## In one  instance, one of the hv lytics  [ 3900 uf @ 450 vdc] towards the hot end of the string, arced through a 
crack in the plywood, and launched itself.  Since the fault  was before the  B+  fuse.... the  2nd HV fuse blew.
[ the one located between  sec of xfmr and FWB].  The 96 x 6A10's  in the FWB were not damaged.  In 2 x instances the
240 vac breaker popped open.  Once when a screw removed from a 6" fan..and the AL fan swung down like a pendulum
and hit one side of the 240 vac buss. [ and blew a chunk out of the fan casing].  The 2nd time was a B+ fault.  

##  I use a simple  50 ohm-50W  WW  on my L4B's..with the stock drake HV B+ fuse.  [ consist of a .82 ohm, 1 watt carbon R ]. 
The stock .82 Ohm R  always blows cleanly in 1/2.    I have only had  2 x ever blow open  since 1977. 

later... Jim   VE7RF