[Amps] Fuses

[James R Carr]

If you can get a copy of ferms Fast finder from your local library, "its a 
cheat sheet for the NEC" in the back he go into available fault current and 
it's calculation. In exterme cases you have to install a sevice main with 
current limiting fuse ahead of the breaker to clear something like 50 75K 
available fault current before it can destroy the breaker. The breaker does 
the job 99% of the time and the fuse just gathers dust. But when the 
catostrophic event occures the fuse blows and saves the system.
I remember the adjustable trip motor controler breakers, but they must not 
have invented hydraulic timed ones yet.
Panels have to be co-ordinated also for AIC and trip out first lest the main 
send a fault current downstream to a panel incapable of clearing it.
Jim
N7FCF
----- Original Message ----- 
From: "Jim Thomson" <jim.thom at telus.net>
To: <amps at contesting.com>
Sent: Saturday, January 08, 2011 4:25 AM
Subject: [Amps] Fuses


> Date: Fri, 7 Jan 2011 22:11:50 -0800
> From: "James R Carr" <n7fcf at hctc.com>
> Subject: Re: [Amps] Fuses
>
> Who makes it and what's the model number? I'd like to see the specs on it
> especially the UL test results. When I retired in 2000 the only way they
> were able to get a breaker to clear a fault in less than three full cycles
> was to put 18" of wire between it and the buss bars. And even then it 
> didn't
> get down to 1/2 cycle. Fuses were tested as a bolted fault across the buss
> bars.
>
> ##  per all the eng notes  from airpax, and a host of other breaker 
> manufacturers,
> a good quality Thermal breaker should trip in 35 msecs. [this was with a 
> gross overload,
> like several hundred percent, IE; dead short].  There is also a breaker 
> known as a
> 'thermal-magnetic'  and also the 'hydraulic magnetic'.   The 
> hydraulic-magnetic is the
> fastest of the bunch.  The hy-mag breakers have loads of options, to 
> handle everything from
> medium in-rush currents, to heavy motor start inrush.  You can also get em 
> with optional
> cam followers  to tweak the trip characteristics even more.  Other options 
> included
> various arc chutes, aux contacts, lighter duty alarm contacts, etc.   You 
> can also get them
> with no breaker at all, and configure them as an on-off switch.  You can 
> also obtain them with external
> sense leads, and  manually adjust the exact trip point, both thermally, 
> and over trip, and even various
> timing schemes for max % of overload.
>
> There is 2 x ways to open off the mag-hydraulic breaker.  one was with a 
> slight overload, like 125%...
> in which case the  hydraulic portion of it functions. With gross 
> overloads, like several hundred percent,
> the magnetic portion would take over. Internally, there is a fluid 
> reservoir. Per the eng notes, to change the
> basic trip curves, ['instant trip' , normal, more lag, even more lag, then 
> motor start, etc... they changed the
> viscosity of the oil used inside the reservoir.   The 'instant trip' 
> versions had no oil in the reservoir at all, were the
> most sensitive to any kind of overload, and are only recommended  for use 
> in delicate circuits.  These instant
> trip versions are way faster than the typ  35 msecs you get from a plane 
> jane thermal house breaker. Thermal
> breakers are typ 35-116 msecs... depending on % of overload.
>
> Their are several caveats/eng notes  when using ....'instant trip' 
> ...mag-hydraulic breaker's, esp when used in conjunction with
> any kind of... 'pulse train'  type  supplies.    IE: switching supplies. 
> The original  pulse train, is of course, any ham type
> High C , B+  supply.    By pulse train, they mean super high current 
> pulses..every  8.3 msecs, like you get  from any FWB
> and a high C filter.   The  mag hydraulic  breaker makers say the 
> individual pulse  every  8.3 msecs  can't be any longer than
> 10 msecs  max in duration..or the duration of the pulse will be long 
> enough to fool the magnetic portion of the mag-hydraulic
> breaker...and cause it to trip, cause of a combo of high peak current and 
> pulse duration.  Using Duncan's PSUD, plus various
> experiments  with scopes and current probes, the AVERAGE current  during 
> each pulse is still
> pretty low. The peak current, of course is sky high, depending on Z of 
> primary/sec of plate xfmr..and all wiring back to the
> main 200A panel, drop wire, all switch's, contactor's  etc.
>
> Interesting enough, in my quest to lower the Z  from plate supply back to 
> main panel... by using bigger ga wire, and thus
> lowering the peak V  drop,  results in  mixed results.  With small ga 
> wire, and higher Z, the peak current every 8.3 msec
> are slightly lower.. and peak V drop is still bad.  With bigger ga wire, 
> the Z is lower, and now the peak current pulses are a lot higher.
> Trbl is, the higher peak current pulses  times the lower Z wire, still 
> results in a big V drop on peaks.  The pulse width will vary too,
> as though it's trying to compensate. With a lower Z source, the width of 
> each pulse gets a little wider.   Overall results are still better
> with bigger ga wire, esp with bigger VA xfmr's  and  bigger C  filters. 
> Peak current's can still be anywhere  from 2-5 times average, it's
> all over the map, depending on how many uf used, ga and length of wire, 
> how many va the xfmr is, etc.  I didn't want to take a chance,
> so sized my mag-hydraulic breaker up by one notch, since it was a one shot 
> purchase, and a custom order..and no refunds.  With fixed
> breakers like these, you don't want em on the hairy edge.  With the 
> breakers that incorporate an external, adjustable trip level,  no big
> deal.  As I mentioned b4,  with a pair of HV fuses, the type of breaker in 
> the 240 line is sorta a moot point.  The thought here was..initially
> if a breaker is used, lets use the fastest one made.
>
>
> BTW, we had a bad fire in on of our bigger telco's  just a few yrs back, 
> in the huge panel that contained all the motor control stuff
> for the HVAC in the building. [ 6 x separate units].  The entire panel 
> burned up. The cause  was one of those adjustable  external trip
> sensors.  It malfunctioned, and would not send the control signal to the 
> big 3 phase  contactor it controlled.  Something had shorted,and
> the wiring just sat there and burned up.  A real mess, the fire spread, 
> and burnt everything..including the 1st 20'  of all wires that exited the
> panel, via 17 x conduits on top of the same panel.  Insides of the 
> conduits just charred black.  IMO, these adjustable trip devices  should 
> still have
> to use supplemental breakers, which these did.  Trbl was, the supplemental 
> breakers/fuses  I believe, were further downstream.  The larger upstream
> breakers  for the burnt up panel, were located in another panel, across 
> the room.  With a 4000 lb, 450 kva vault xfmr,  you can burn stuff to a 
> crisp..fast.
> It's almost like having a 15A breaker fail shut in your main panel, then 
> trying to rely on the 200A breaker to do the job, it won't.
>
> Later... Jim   VE7RF
>
>
>
>
> ----- Original Message ----- 
> From: "Paul Kraemer" <elespe at lisco.com>
> To: "AMPS" <amps at contesting.com>
> Sent: Friday, January 07, 2011 6:50 PM
> Subject: Re: [Amps] Fuses
>
>
>> It is a terminology thing as in magnetic / hydraulic trip
>> Also, we routinely use supplemental IEC protection devices that have an
>> adjustable thermal trip plus a near instantaneous overcurrent trip
>> Actual tests in our shop have demonstrated to us that they will in fact
>> trip
>> faster than a fast semiconductor fuse will blow
>> That is why we frequently install them ahead of motor drives to protect
>> the
>> expensive semiconductor protection fuses.
>> Paul K0UYA
>> ----- Original Message ----- 
>> From: "James R Carr" <n7fcf at hctc.com>
>> To: "Jim Thomson" <jim.thom at telus.net>; <amps at contesting.com>
>> Sent: Friday, January 07, 2011 8:01 PM
>> Subject: Re: [Amps] Fuses
>>
>>
>>> Where pray tell do you get a hydraulic breaker? I have installed several
>>> thousand over the years but have yet to see one filled with oil.
>>> As for current limiting devices, the fastest breaker will hold in for
>>> three
>>> to seven full cycles. To be current limiting, a fuse has to clear in 
>>> less
>>> than 1/2 cycle.
>>> Jim
>>> N7FCF
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