Agreed, The first instant in time you have the full output of Bonnyville Dam
on the fault. But then the impedance of the system kicks in and limits it.
But still depending on the size of the utility transformer, it's eficiency,
the length and size of the service conductors, you can still have several
thousand amps at the main panel and down the line. About 20 25 years ago
they had to increase the fault clearing ability all the breakers in the
panels from 5000 amps to 20000 amps due to the increased use of low loss
transformers by the utilities. basically the breaker can clear it without
exploding. It probably will be terminally damaged though. I have seen buss
bars twisted into pretsels by a dead short and a breaker that didn't clear
quick enough.
Jim
N7FCF
----- Original Message -----
From: "Jim Thomson" <jim.thom@telus.net>
To: <amps@contesting.com>
Sent: Saturday, January 08, 2011 2:18 AM
Subject: [Amps] Fuses
> 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
> _______________________________________________
> Amps mailing list
> Amps@contesting.com
> http://lists.contesting.com/mailman/listinfo/amps
_______________________________________________
Amps mailing list
Amps@contesting.com
http://lists.contesting.com/mailman/listinfo/amps
|