Date: Mon, 7 Oct 2013 12:23:06 -0400
From: "Carl" <km1h@jeremy.mv.com>
To: "Jim Thomson" <jim.thom@telus.net>, <amps@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
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