[Amps] Fuses

[Jim Thomson]

Date: Sat, 8 Jan 2011 16:21:22 -0800
From: "James R Carr" <n7fcf at 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