Also, fuses rated 250 V AC are typically rated only 32 V DC. That's
because, with AC, the current goes to zero twice per cycle, so the arc
can go out. Not so with DC. I suspect the standard 28 V DC once used in
many aircraft was chosen because that's about the highest voltage which
can be switched with a mechanical relay or switch. (Something about the
energy required to strip electrons from the metal contacts.) Solid-state
switches, of course, don't have that problem, hence the popularity of
higher voltage DC systems now.
Long ago, I put a 3AG fuse in series with the finals' cathodes of a
T-4XB. The first step in replacing that fuse when it blew was to shake
all the glass out of the radio.
73,
Scott K9MA
On 7/15/2018 05:22, Roy Koeppe wrote:
One reason 'all fuses' aren't just rated at 250v to be done with it is
that a shorter element section in a lower voltage fuse can have a
lower resistance, due to the shorter arc gap allowed. Thus lower
voltage drop for a low voltage circuit, etc.
73, Roy K6XK
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