RE: slightly off topic

Wow... glad this thread has turned into rant from 659827354 people.
Slow Blow Voltage Limiting.  I have the fuse at my house, I'll put up a
spec sheet and post it when I get home tonight.  Anyway.  We replaced it
with a 20KA Current Limiting Fuse, like you speak of.  Works great now.
The next one is figuring out why the server room is drawing 93 amp on 1
leg and 104 on the other at peak.  We left a multi meter on each leg
doing MIN/MAX Sunday night.  The average was about 50-60.  My guess is
with all the work being done in that electrical room yesterday, someone
failed us over to the UPS' and then they had to charge up a bit.  Thank
you all for your insight, maybe we should kill this thread now.  Either
way I will post a spec sheet when I am home.

-----Original Message-----
From: [email protected] [mailto:[email protected]]
On Behalf Of Mark Radabaugh
Sent: Monday, March 03, 2008 5:09 PM
To: [email protected]
Subject: Re: slightly off topic

Blake Pfankuch wrote:
> They do, that's how we came to the conclusion they were slow blow
> voltage regulating 100 amp fuses.
>
>   
Hum... In many many years of industrial power and controls work I can't 
say I have ever seen such a beast.  

Slow Blow Current Limiting fuses are extremely common.   Due to the 
extremely low inductance of the electrical distribution system fault 
currents can be very large and damaging.  Current Limiting fuses are 
designed to limit the fault currents to 200,000 or 300,000 amps and to 
open in less then 1/4 cycle.   In low impedance circuits it is common to

have both a breaker and a fuse.   Breakers are simply unable to 
effectively interrupt the fault currents caused by catastrophic 
faults.   Current limiting fuses work by vaporizing the material inside 
the fuse which creates enough pressure to stop the arc.

What is the manufacturer's marking on the fuse?   The part number alone 
should be enough to find the manufacturers data sheet.

I have seen plenty of truly odd voltage readings on 3 phase circuits 
when a single leg of a 3 phase circuit is open.   Depending on the 
configuration (delta/wye) and where the ground is the readings can be 
all over the map.   Very high impedance digital meters (very common 
these days) can also give odd voltage readings on open circuits.   The 
voltage and current on the surrounding conductors can induce hundreds of

volts onto an open wire (though it has very little current available) 
and the meter will show voltage.  Check the same wire with an older 
analog meter or wiggy 
(http://www.us.schneider-electric.com/us/products/machine_safety.nsf/uni
d/58870E11543C976885256D500050927B/$file/wiggy.htm) 
and you won't find anything.

Mark
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