Fuses versus Breakers

[ContestID67]

I have some fuses in my glider. One is on the main battery itself.
The other fuses are in the instrument panel and are for individual
devices (radio, computer, vario).

I got it in my head that I should replace the panel fuses with
breakers. There are several advantages, like being able to reset the
breaker in flight and being able to "pull" the breaker.

I have been told that all new breakers must be able to be manually
pulled (tripped) in case of emergency. Older breakers could be reset
but not pulled. Obviously fuses can be pulled also but I have an image
of my fumbling the loose glass fuse in the cockpit which isn't a pretty
picture.

Looking at a particular brand of breaker, the Texas Instrument Klixon
7277 line (http://www.ti.com/snc/products/controls/acb-7277.htm) from
Aircraft Spruce, I found that at low current draws you can loose a volt
or more from your primary.

1/2A 2.00 Vdc drop << my vario
3/4A 1.45 Vdc drop
1A 1.10 Vdc drop << my computer
2A 0.70 Vdc drop
3A 0.33 Vdc drop << my radio
4A 0.30 Vdc drop
5A 0.25 Vdc drop

This is not good. It got me thinking that a glass fuse isn't too bad
after all especially considering a fuse is $1 each and the breaker is
$25 each.

The question is, what is the typical forward drop of a fuse? Also, why
does the breaker have such a high forward drop at low amperages?

Thanks, John

[Eric Greenwell]

ContestID67 wrote:
> I have some fuses in my glider. One is on the main battery itself.
> The other fuses are in the instrument panel and are for individual
> devices (radio, computer, vario).
>
> I got it in my head that I should replace the panel fuses with
> breakers. There are several advantages, like being able to reset the
> breaker in flight and being able to "pull" the breaker.
>
> I have been told that all new breakers must be able to be manually
> pulled (tripped) in case of emergency. Older breakers could be reset
> but not pulled. Obviously fuses can be pulled also but I have an image
> of my fumbling the loose glass fuse in the cockpit which isn't a pretty
> picture.
>
> Looking at a particular brand of breaker, the Texas Instrument Klixon
> 7277 line (http://www.ti.com/snc/products/controls/acb-7277.htm) from
> Aircraft Spruce, I found that at low current draws you can loose a volt
> or more from your primary.
>
> 1/2A 2.00 Vdc drop << my vario
> 3/4A 1.45 Vdc drop
> 1A 1.10 Vdc drop << my computer
> 2A 0.70 Vdc drop
> 3A 0.33 Vdc drop << my radio
> 4A 0.30 Vdc drop
> 5A 0.25 Vdc drop
>
> This is not good. It got me thinking that a glass fuse isn't too bad
> after all especially considering a fuse is $1 each and the breaker is
> $25 each.

In 5000 hours of flying gliders, I've only had to replace a fuse once in
flight (fuse actually went bad), and disconnect only one fuse (gear
warning going off at the wrong time). I think electrical problems where
a breaker would be handy are so rare, I like to keep it simple: I have a
master switch to disconnect the battery if there is smoke, and then just
fuses on the usual things. I carry a fuse of each type in a little
baggie in the pocket so I can replace fuse if I need to.

The engine controller on my motorglider does have a circuit breaker that
is used as a switch for the Ilec controller. It came that way from the
factory.


--
Change "netto" to "net" to email me directly

Eric Greenwell
Washington State
USA

[Tim Newport-Peace]

X-no-archive: yes
In article >, Eric Greenwell
> writes
>ContestID67 wrote:
>> I have some fuses in my glider. One is on the main battery itself.
>> The other fuses are in the instrument panel and are for individual
>> devices (radio, computer, vario).
>>
>> I got it in my head that I should replace the panel fuses with
>> breakers. There are several advantages, like being able to reset the
>> breaker in flight and being able to "pull" the breaker.
>>
>> I have been told that all new breakers must be able to be manually
>> pulled (tripped) in case of emergency. Older breakers could be reset
>> but not pulled. Obviously fuses can be pulled also but I have an image
>> of my fumbling the loose glass fuse in the cockpit which isn't a pretty
>> picture.
>>
>> Looking at a particular brand of breaker, the Texas Instrument Klixon
>> 7277 line (http://www.ti.com/snc/products/controls/acb-7277.htm) from
>> Aircraft Spruce, I found that at low current draws you can loose a volt
>> or more from your primary.
>>
>> 1/2A 2.00 Vdc drop << my vario
>> 3/4A 1.45 Vdc drop
>> 1A 1.10 Vdc drop << my computer
>> 2A 0.70 Vdc drop
>> 3A 0.33 Vdc drop << my radio
>> 4A 0.30 Vdc drop
>> 5A 0.25 Vdc drop
>>
>> This is not good. It got me thinking that a glass fuse isn't too bad
>> after all especially considering a fuse is $1 each and the breaker is
>> $25 each.
>
>In 5000 hours of flying gliders, I've only had to replace a fuse once in
>flight (fuse actually went bad), and disconnect only one fuse (gear
>warning going off at the wrong time). I think electrical problems where
>a breaker would be handy are so rare, I like to keep it simple: I have a
>master switch to disconnect the battery if there is smoke, and then just
>fuses on the usual things. I carry a fuse of each type in a little
>baggie in the pocket so I can replace fuse if I need to.
>
That may be so Eric, but breakers stop the incorrect value fuse being
used if it is the only one available.

One option worth considering is the use of Self Resetting Fuses. These
electronic devices work like breakers but reset themselves when the load
is removed.

Tim Newport-Peace

"Indecision is the Key to Flexibility."

[[email protected]]

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.

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.

Tom

[COLIN LAMB]

Reset-ability is a two edged sword. I was just reading the review of a nice
Russian amphibian aircraft. The circuit breaker panel is in the back of the
aircraft so that the pilot cannot reset them during flight.

From a safety standpoint, when a fuse blows or a circuit breaker pops, it is
telling you that probably there is problem. Although it can be a simple
matter, it can also be telling you there is an electrical failure that could
start a fire in flight, or cause smoke problems.

I am sitting on the fence as I have a few fuses in my panel and a few
circuit breakers. The fuses were there and are not essential to flight.
One is for the vario. And, who needs that? If the radio circuit breaker
goes out, I will get the handheld out and not turn the main radio back on
until on the ground. The nice thing about a glider is that the electrical
stuff is not essential, so fly the glider down and park it, then fix the
problem.

My electrical engineer friend says the fuse is designed to protect the wire
from overcurrent and properly designed equipment should be protected itself.

Just wanted to complicate something that ought to be simple.

Colin

[[email protected]]

ContestID67 wrote:
> I have some fuses in my glider. One is on the main battery itself.
> The other fuses are in the instrument panel and are for individual
> devices (radio, computer, vario).
>
> I got it in my head that I should replace the panel fuses with
> breakers. There are several advantages, like being able to reset the
> breaker in flight and being able to "pull" the breaker.
>
> I have been told that all new breakers must be able to be manually
> pulled (tripped) in case of emergency. Older breakers could be reset
> but not pulled. Obviously fuses can be pulled also but I have an
image
> of my fumbling the loose glass fuse in the cockpit which isn't a
pretty
> picture.
>
> Looking at a particular brand of breaker, the Texas Instrument Klixon
> 7277 line (http://www.ti.com/snc/products/controls/acb-7277.htm) from
> Aircraft Spruce, I found that at low current draws you can loose a
volt
> or more from your primary.
>
> 1/2A 2.00 Vdc drop << my vario
> 3/4A 1.45 Vdc drop
> 1A 1.10 Vdc drop << my computer
> 2A 0.70 Vdc drop
> 3A 0.33 Vdc drop << my radio
> 4A 0.30 Vdc drop
> 5A 0.25 Vdc drop
>
> This is not good. It got me thinking that a glass fuse isn't too bad
> after all especially considering a fuse is $1 each and the breaker is
> $25 each.
>
> The question is, what is the typical forward drop of a fuse? Also,
why
> does the breaker have such a high forward drop at low amperages?
>
> Thanks, John

Warning! Circuit breakers may well not have a suitable response time to
protect some of your valuable devices.
Save a fuse- lose an instrument.
UH

[[email protected]]

I have installed a 1 amp version breaker of this type for my sn-10b. I
checked the voltage as reported by the sn-10 and only saw a .3 volt
drop from measuring directly at the battery.

Regards,

John

[Don Hammer]

>The question is, what is the typical forward drop of a fuse? Also, why
>does the breaker have such a high forward drop at low amperages?
>
>Thanks, John

Don't know about what is typical, but look at it this way. A breaker
trips because there is a bimetalic resistor that heats up when current
flows through it. If you have resistance you get heat and a voltage
drop. Stick in Ohm's law and you can probably get an answer to your
question.

BTW - Breakers and fuses are there for one thing only. They keep a
shorted wire from turning into a foam cutter. Components are usually
protected by an internal fusable device. Always size the breaker for
the wire.

[[email protected]]

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

> 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.

--
Paul Repacholi 1 Crescent Rd.,
+61 (08) 9257-1001 Kalamunda.
West Australia 6076
comp.os.vms,- The Older, Grumpier Slashdot
Raw, Cooked or Well-done, it's all half baked.
EPIC, The Architecture of the future, always has been, always will be.

[Bert Willing]

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.

--
Bert Willing

ASW20 "TW"


"Don Hammer" > a écrit dans le message de news:
...
>
>>The question is, what is the typical forward drop of a fuse? Also, why
>>does the breaker have such a high forward drop at low amperages?
>>
>>Thanks, John
>
> Don't know about what is typical, but look at it this way. A breaker
> trips because there is a bimetalic resistor that heats up when current
> flows through it. If you have resistance you get heat and a voltage
> drop. Stick in Ohm's law and you can probably get an answer to your
> question.
>
> BTW - Breakers and fuses are there for one thing only. They keep a
> shorted wire from turning into a foam cutter. Components are usually
> protected by an internal fusable device. Always size the breaker for
> the wire.

[John Giddy]

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.

[Bert Willing]

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.

[John Giddy]

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.

[David Kinsell]

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.

[David Kinsell]

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