Fuses versus Breakers

On Fri, 11 Mar 2005 09:24:23 +0100, Bert Willing wrote:

There are components which are not fused internally (i.e. T/S indicators,
Colibri etc).

Bimetal breakers are disastreous in this case as they are very slow and will
ultimately less pass about 10 times the nominal value before cutting off.
This winter I was loocking into replacing my fuses by breakers, and I
decided that I didn't want to have this crap in my gliders. The breakers to
use are magnetic ones which are pretty fast - but the price tag is very
different... so finally I kept going with fuses.

Fuses for individual instruments don't protect those instruments, but
do stop the larger battery fuse from blowing if the instrument fails
in a way which draws a high current from the battery.
This only works if the individual instrument fuses have a current
rating smaller than the common battery fuse.
If the common battery fuse blows, you lose ALL the instruments. If a
single instrument fuse blows, you lose that instrument, but you were
going to lose it anyway, as it is the failure of the instrument which
blows that fuse.
Cheers, John G.

Yes and no. Of course one needs a big fuse (mine is 7 Amps) on the battery
to protect the cable running to the instrument panel.
Individual fuses on simple instruments like a T/S do protect the main fuse
to blow off. On more sophisticated intrumentation like a radio or a flight
computer, the protect the instrument in the case you connect the battery
with the instrument switched on. That usually produces a spike which some of
the instruments (the LX160s is a good example) absolutely don't like.
Fuses which protect an instrument need to be fast fuses, fuses protecting
the main fuse or cables are slow fuses.

--
Bert Willing

ASW20 "TW"


"John Giddy" a écrit dans le message de news:
...
On Fri, 11 Mar 2005 09:24:23 +0100, Bert Willing wrote:

There are components which are not fused internally (i.e. T/S indicators,
Colibri etc).

Bimetal breakers are disastreous in this case as they are very slow and
will
ultimately less pass about 10 times the nominal value before cutting off.
This winter I was loocking into replacing my fuses by breakers, and I
decided that I didn't want to have this crap in my gliders. The breakers
to
use are magnetic ones which are pretty fast - but the price tag is very
different... so finally I kept going with fuses.

Fuses for individual instruments don't protect those instruments, but
do stop the larger battery fuse from blowing if the instrument fails
in a way which draws a high current from the battery.
This only works if the individual instrument fuses have a current
rating smaller than the common battery fuse.
If the common battery fuse blows, you lose ALL the instruments. If a
single instrument fuse blows, you lose that instrument, but you were
going to lose it anyway, as it is the failure of the instrument which
blows that fuse.
Cheers, John G.

On Fri, 11 Mar 2005 14:57:37 +0100, Bert Willing wrote:

Yes and no. Of course one needs a big fuse (mine is 7 Amps) on the battery
to protect the cable running to the instrument panel.
Individual fuses on simple instruments like a T/S do protect the main fuse
to blow off. On more sophisticated intrumentation like a radio or a flight
computer, the protect the instrument in the case you connect the battery
with the instrument switched on. That usually produces a spike which some of
the instruments (the LX160s is a good example) absolutely don't like.
Fuses which protect an instrument need to be fast fuses, fuses protecting
the main fuse or cables are slow fuses.

You would need very fast fuse to stop the spike you describe. Maybe a
surge supressor on the line near the sensitive instruments would do a
better job ? e.g. a 5W or so zener diode with a breakdown voltage of
14v. (or 16v if you are one of those who advocate 14v batteries) This
will also blow the battery fuse if you connect the battery in reverse
polarity by mistake, and prevent large negative voltages being applied
to insufficiently protected instruments.
It also saves you having to carry a supply of instrument fuses so you
can replace the fuse of the instrument which was ON when you connected
the battery.
Cheers, John G.

John Giddy wrote:
On Fri, 11 Mar 2005 14:57:37 +0100, Bert Willing wrote:


Yes and no. Of course one needs a big fuse (mine is 7 Amps) on the battery
to protect the cable running to the instrument panel.
Individual fuses on simple instruments like a T/S do protect the main fuse
to blow off. On more sophisticated intrumentation like a radio or a flight
computer, the protect the instrument in the case you connect the battery
with the instrument switched on. That usually produces a spike which some of
the instruments (the LX160s is a good example) absolutely don't like.
Fuses which protect an instrument need to be fast fuses, fuses protecting
the main fuse or cables are slow fuses.


You would need very fast fuse to stop the spike you describe. Maybe a
surge supressor on the line near the sensitive instruments would do a
better job ? e.g. a 5W or so zener diode with a breakdown voltage of
14v. (or 16v if you are one of those who advocate 14v batteries) This
will also blow the battery fuse if you connect the battery in reverse
polarity by mistake, and prevent large negative voltages being applied
to insufficiently protected instruments.
It also saves you having to carry a supply of instrument fuses so you
can replace the fuse of the instrument which was ON when you connected
the battery.
Cheers, John G.

Uh, there is no voltage spike produced from doing that. That's an old
myth that comes from power planes, where the starter motor can induce
big spikes in the system during starting. You don't want sensitive
instruments powered on at that time.

wrote:
writes:


Generally, you size a breaker several times the normal operating
current. So if your vario draws 0.1A a 0.5A breaker would be a good
choice. The voltage drop is proportional to current, so at 0.1A it
will be less than 0.4V. This should be of no consequence to your
vario. A fuse will have a lower voltage drop, but isn't resetable.


No, you should size a fuse acording to the WIRE! So if you wire is
rated for 5A, then fit a 5A fuse.

The fuse rating certainly shouldn't exceed the rating of the wire (or
switches used). It wouldn't be uncommon to have wire rated for 20 amps,
which is more than you want for the fuse rating in this application.


Be sure to check inrush current
and fit a `slo blo' if needed.


Bad idea. Slow blow fuses have much higher resistance than
normal ones, leading to increased voltage drop.



While a fuse is highly reliable, it isn't perfect. And you can be
assured that if it does fail it will be at the worst possible
moment.


The best way I know of to increase fuse reliability is to replace
them every few years. Or it is for 3AG or the like.

Never seen that one before. I've flown for 25 years without replacing
fuses, and never had one fail.

-Dave