Re: High Voltage but Low Current fuses...

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Terry Fritz" <teslalist-at-qwest-dot-net>

Hi,

At 07:17 PM 7/17/2003 +0000, you wrote:
>Heh. I bought some 50ma fuses awhile ago (Yes 50ma)
>the wire inside is SOOO thin, you have to get the light to reflect off it 
>just so befor you can even see it.
>(And I have very good eyes)
>Course, its only a mini fuse, but im sure they would make high voltage 
>versions

The problem is with high voltage you can draw really long arcs.  Like 6 
inches off a MOT and stuff.  So "real " high voltage fuses are made to 
break high voltages at really high (5000 amps!) sustained currents.  Thus, 
they are very big and long as they provide a very long quenched distance to 
break the arc.  "Mid-voltage" fuses may be say 15kV at 5 amps but they are 
like 17 inches long!

For our need.  We may see a sudden very high current pulse that the MOVs 
can handle.  Then the standard 1/4 x 1-1/4 low current style fuse will have 
a pretty good chance of at least providing a nice arc area to dump 
power.  Once the arc goes out, the voltage standoff is very high.  For the 
purpose of our differential probe.  Two MOVs and two say 0.25 amp pigtail 
fuses provide fair protection against "accidents".  Far far better than 
nothing.

If they could take a primary cap discharge or a MOT, and give the scope a 
reasonable chance of avoiding damage that would be great!  Without 
anything, scope destruction is assured.  We are just "trying" to do 
something just in case.  Of course, it could not hold a direct connection 
to a power line or a "real" lightning strike, but for our need it should do 
well.  There is even a very good chance the probe itself would only need 
new fuses for most mishaps.

Of course, operation over 1000 volts will void the warranty ;o))  But with 
just a few more parts we can add a great deal of protection in case someone 
does goof.

Cheers,

         Terry