Date: Sat, 8 Jan 2011 08:34:30 +0000
From: Ian White GM3SEK <gm3sek@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
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