Date: Sat, 8 Jan 2011 16:21:22 -0800
From: "James R Carr" <n7fcf@hctc.com>
Subject: Re: [Amps] Fuses
Down in prosser, WA about 30 years ago they had a problem with a potato
processing plant and a mechanic came running into the panel room and before
anyone could stop him threw the main 480v 3 phase breaker. The plant was
under full load with motors up the cazoo. When the back emf hit it blew the
breaker out of the panel. The guy was lucky he wasn't hurt.
I retired in 2000 and there is a whole raft of devices that have come on
line since.
Jim
N7FCF
### This is why you want monster size movs across the load itself. Here is a
case, where
the up-stream .... 'whole house breaker' won't do much good..... it's on
the line side of the
breaker, instead of the load side. If you want to see the effects of back
emf.... try running
a 10 kva pole pig flat out... key down cxr....then reach over, and shut the
main contactor OFF !
[ typ 240 in - 4800 v out, then FWB = +6700 vdc no load.... ham supply for a
YC-156, etc]
Now you can see why you want the voltage rating on any 208/240/360/480/600 vac
breaker on the
high side of the actual normal V.
## After a rude awakening trying the above stunt, it's also why I use
parallel, bolt down MOV's [ 277 v rated]
directly across the 240 vac primary of the plate xfmr. I also use similar
parallel 130 v rated movs [ bolt down type]
between each hot leg of the xfmr..and neutral. I also use more big movs,
directly across the contacts of the contactor
I use for the main on/off. Then any mag-hydraulic breaker/ fuses upstream
won't get blasted with the mother of all
back emf. It's also why I use contactor's instead of relay's.
Contactor's are just 2 x sets of contacts in series... with a huge
solenoid spring. This breaks the arc into 2 x arcs. I use contactor's rated
for way in excess of normal average current
being drawn on the 240v primary. [ both to minimize V-drop..and also to handle
the typ 3-5 x normal current... caused by the
high C filter].
## I'm wading through Buss's eng notes. Apparently they have a new style
fuse out, [dual ele] that will clear a hard fault super
fast, yet allow for 500% overload for 4 x seconds... for typ motor start up.
They are sized at 130% of the motor's normal run current.
They have new ones out.. for non motor start.. that will clear faster than
their old line of fuses..[ again, a hard fault]. They mention 'current
limiting
fuses'... [even in their old literature].. which is a bit of a mis-nomer. I
see no mention of 1/2 cycle clear times [ 8.3 msec] ...yet. I'm still
looking.
Buss's idea of .....'current limiting' is really reduced..... 'let through
max peak current' compared to their older series of fuses. You still end up
with a high fault current.... but a lot lower than their old style fuses.. plus
they will clear faster ! This is all being driven by recent changes in
arc suppression requirements by most gov'ts.
Jim VE7RF
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