[Milsurplus] Fuses to protect transformers

[Richard Brunner]

That has to be a typo, it's probably 45 volts dc.  Slow-blow is no surprise. Fuses are inherently not fast devices, they will carry rated current forever, will operate "eventually" at 135% of rating, in about two minutes at 200% of rating, and fairly fast at 10 or more times rating. At ten times it may take 5 cycles to clear, and with a good short circuit it's fairly fast.  To interrupt, the fuse element must melt and/or vaporize, which takes time. Circuit breakers are usually faster.  There are now current-limiting fuses which will interrupt before the current wave reaches its peak, but you have to hit it hard to current-limit, at least 20,000 amps.

Richard, AA1P


>ac interruption is easier than dc because there is the current zeroThat makes a great deal of sense but leads me to ask if you, or someone, knows why the Littelfuse series 808 is rated for 450VDC but only 250VAC? Seems backwards, but that's what's on the datasheet. So far, I haven't found a 450VAC or higher pigtail fuse in the neighborhood of 1A that isn't slowblow.

  Wayne
  WB4OGM


  -----Original Message-----
  From: Richard Brunner <brunneraa1p at comcast.net>
  To: milsurplus <milsurplus at mailman.qth.net>; hwhall <hwhall at compuserve.com>
  Sent: Thu, Mar 13, 2014 3:06 pm
  Subject: Re: [Milsurplus] Fuses to protect transformers


Firstly, those are ac rated fuses and will not work anywhere as well on dc. 
ac interruption is easier than dc because there is the current zero.  The 
voltage rating on a fuse is the highest voltage it will reliably interrupt. 
At higher voltage and dc the fuse will melt and arc, dropping the voltage, 
which may be enough to save the equipment and transformer, but a better 
solution is using hv rated dc fuses, if you can find them.

Second thought: Since it will be feeding rectifiers, the current will be dc 
pulses with a current pause, so there is some opportunity for current 
interruption.  However, since it is way over the ac fuse voltage rating it 
is hit-or-miss whether it will interrupt, and probably not.

Richard, AA1P

> For this 40s vintage Link Trainer electronics I'm working on, I'm thinking 
> that it'd be a good idea to fuse not only the primary but the secondary HV 
> winding, since these transformers are not too easy to find anymore. I'm 
> thinking of pigtail fuses inserted in the wiring under the chassis, to 
> maintain appearances. But I'm not sure about what fuse selection would be 
> adequate protection.
>
> Would a 1A fast blow fuse be OK for the secondary that probably delivers 
> around 100ma DC? Or to put it another way, how much over the normal 
> current output should I select a fast blow fuse? The filters are dual 
> section choke input with 15 & 25 mfd caps. I haven't reached the point of 
> firing up the whole thing, so I have to guess for now at the current.
>
> The common fuses I have available are marked 250V and the HV is 450V. Can 
> they be relied on to open and not arc?
>
> Any advice? Thanks!!
>
> Wayne
> WB4OGM