[Amps] high voltage fuses

[Carl]

The HV fuses Im referencing and have here came from military and industrial 
equipment. Perhaps those engineers  didnt know as much as you (-;


----- Original Message ----- 
From: "Jim Thomson" <jim.thom at telus.net>
To: <amps at contesting.com>
Sent: Thursday, October 10, 2013 7:21 AM
Subject: Re: [Amps] high voltage fuses


> Date: Mon, 7 Oct 2013 12:23:06 -0400
> From: "Carl" <km1h at jeremy.mv.com>
> To: "Jim Thomson" <jim.thom at telus.net>, <amps at contesting.com>
> Subject: Re: [Amps] high voltage fuses
>
>
> Instantaneous peak current can be a lot more than 5-10% more than key down
> and long term stressing of an underated fuse will eventually fail.
>
> A tube arc/short instantaneous current will take out a 2-2.5X rated fuse 
> as
> fast as a smaller one.
>
> All the commercial TX Ive ever seen have the fuse in the same area as the
> tube and that includesmilitary ones Ive worked on.
>
> Dentron used a pair of 1 Ohm 1/2W carbon composition resistors in parallel
> at the base of the RFC as the fuse, seems to do the job for some failures
> but not all.
>
> I suppose the old style 220V cartridge fuse could be rebuilt for HV but I
> would never suggest any bare wire with a simple covering at much over 
> 2000V.
> I suggest those who believe it is OK should see the results of a plasma 
> arc,
> nothing stops it until power is cut.
>
> Carl
>
> ##  Instantaneous peak current isnt gonna be more than max dc plate 
> current... not at
> least in normal key down RTTY-FM operation.
>
> ##  Installing HV fuses rated at 2-2.5X is plane nuts imo.   1.05- 1.10 is 
> ample.
> 50—60% more is the highest I would ever go if stuck for a fuse that will 
> safely do the
> job.
>
> ##  Fuses rated at 1X- 1.2X    will open up faster than  a 2-2.5X  rated 
> fuse.   Now this
> is when used in conjunction with a 50 ohm glitch R.    The 50 ohm glitch R 
> will limit the
> peak fault current to a specific value based on B+ of course. 3kv / 50 
> ohms =  60A.
> I had all this tested  thoroughly  in the lab.  The fusing wire opens off 
> cuz of the
> heat created...it’s a heat activated device.   All as we want is  a HV 
> fuse to open off
> asap when a fault condition arises.  Its not like we are trying to open 
> off the HV fuse
> cause the plate current on your 8877 has risen to 1200 ma cuz of bad swr 
> due to an
> ice storm.
>
> ##  Case in point.  It takes 1 msec  to open off a BUSS-HVU-2 amp rated 
> fuse..when 10A
> of current is flowing.  With a 3A rated fuse, and the same 10A, it now 
> takes  a full 20 seconds
> to open off the fuse.   The 3A rated fuse of course has a  50%  higher 
> rating.   Use a fuse rated
> for 2-2.5X of normal CCS current...and you are asking for trbl.
>
> ##  In the case of commercial gear, a glitch R is probably not used...and 
> they are relying on the Z
> of a HV choke instead..used with a low C  filter.. hence potentially much 
> higher peak fault currents.
> Most commercial  Broadcast TX gear I have seen is typ over 
> fused..including pole pigs out on the
> street, with 9-11 A slo-blows.  Ditto with large vault xfmrs, etc.
>
> ##  IE:  FM broadcast TX using a GG triode......and located  up a snow 
> covered hill, way outside of town
> in winter.  They don’t want the slightest chance of a HV fuse blowing, 
> unless it’s a 100%  hard fault.
> They are trading off protection vs reliability and being off the air.  OFF 
> air means loss of $$...esp when
> its gonna take time to get somebody outa bed to 4 wheel drive up a hill to 
> change a fuse.  A ham amp
> is close by....like within 5-30 feet away.   I have several amps, so if 
> one blew a HV fuse, who cares, switch
> on a spare amp..then fix the fuse later on.   Telco’s over fuse everything 
> too..like main buss, and bay feeders.
> The equipment blows up 1st.
>
> ##  I wouldn’t be using a 2 to 2.5 A  rated fuse for an 8877 anytime soon.
>
> Jim  VE7RF


The HV fuses Im referencing and have here came from military and industrial 
equipment. Perhaps those engineers  didnt know as much as you (-;