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trigo(at)mail.telepac.pt
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 Posted: Mon Dec 26, 2016 3:33 am Post subject: A couple of questions about Fuses |
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Good morning ‘letric Bob and all ‘letric experts.
I hope you had a Merry Christmas!
In my fused power busses I decided to use LED blade fuses, those whose LED gets illuminated when the fuse trips.
It happens that I have a couple of circuits which require a 1A fuse, but there are no 1A LED blade fuses. Therefore, in these circuits, either I use a normal (non-illuminated) 1A fuse, or I go for a 2A illuminated fuse.
What happens if I decide for the latter? Do I get less protection? Is that particular device more prone to be fried?
Now, for the second question, in a broader perspective: I have heard dozens of times that a fuse (or other protection device) is there to protect the wire and not the device at its end.
However, the size (in A) of the fuse is determined by the device in use (you read in the installation manuals that “to protect this device, you should use a 3A fuse or circuit breaker”).
I know that this depends on the power (W) that is used by the device when working, and that both the wire and the fuse must be sized accordingly. I also know that if we use a thinner wire, it will get very hot and eventually melt.
But what about using an higher A or a lesser A fuse? What are the physics involved?
Thanks
Carlos
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art(at)zemon.name
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| Posted: Mon Dec 26, 2016 6:17 am Post subject: A couple of questions about Fuses |
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Hi Carlos,
Take a look at chapter 10, circuit protection, in The AeroElectric Connection. And here are a few articles: http://www.aeroelectric.com/articles.html#Circuit Protection
In building my plane, I am following the advice to size the fuse for the wire, except in a couple of cases where I only need a couple of amps and did not feel like installing tiny wire. In those cases, I went ahead and used an undersized fuse, even though the wire is capable of carrying more current.
Cheers,
-- Art Z.
On Mon, Dec 26, 2016 at 5:31 AM, Carlos Trigo <trigo(at)mail.telepac.pt (trigo(at)mail.telepac.pt)> wrote:
| Quote: |
Good morning ‘letric Bob and all ‘letric experts.
I hope you had a Merry Christmas!
In my fused power busses I decided to use LED blade fuses, those whose LED gets illuminated when the fuse trips.
It happens that I have a couple of circuits which require a 1A fuse, but there are no 1A LED blade fuses. Therefore, in these circuits, either I use a normal (non-illuminated) 1A fuse, or I go for a 2A illuminated fuse.
What happens if I decide for the latter? Do I get less protection? Is that particular device more prone to be fried?
Now, for the second question, in a broader perspective: I have heard dozens of times that a fuse (or other protection device) is there to protect the wire and not the device at its end.
However, the size (in A) of the fuse is determined by the device in use (you read in the installation manuals that “to protect this device, you should use a 3A fuse or circuit breaker”).
I know that this depends on the power (W) that is used by the device when working, and that both the wire and the fuse must be sized accordingly. I also know that if we use a thinner wire, it will get very hot and eventually melt.
But what about using an higher A or a lesser A fuse? What are the physics involved?
Thanks
Carlos
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--
https://CheerfulCurmudgeon.com/"If I am not for myself, who is for me? And if I am only for myself, what am I? And if not now, when?" Hillel
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ceengland7(at)gmail.com
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| Posted: Mon Dec 26, 2016 6:29 am Post subject: A couple of questions about Fuses |
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Charlie
On Dec 26, 2016, at 6:40 AM, Carlos Trigo <trigo(at)mail.telepac.pt (trigo(at)mail.telepac.pt)> wrote: | Quote: |
Good morning ‘letric Bob and all ‘letric experts.
I hope you had a Merry Christmas!
In my fused power busses I decided to use LED blade fuses, those whose LED gets illuminated when the fuse trips.
It happens that I have a couple of circuits which require a 1A fuse, but there are no 1A LED blade fuses. Therefore, in these circuits, either I use a normal (non-illuminated) 1A fuse, or I go for a 2A illuminated fuse.
What happens if I decide for the latter? Do I get less protection? Is that particular device more prone to be fried?
Now, for the second question, in a broader perspective: I have heard dozens of times that a fuse (or other protection device) is there to protect the wire and not the device at its end.
However, the size (in A) of the fuse is determined by the device in use (you read in the installation manuals that “to protect this device, you should use a 3A fuse or circuit breaker”).
I know that this depends on the power (W) that is used by the device when working, and that both the wire and the fuse must be sized accordingly. I also know that if we use a thinner wire, it will get very hot and eventually melt.
But what about using an higher A or a lesser A fuse? What are the physics involved?
Thanks
Carlos
The question is, from *what* will a fuse protect the device?
Overvoltage protection should be done system-wide, so that's out. If the device draws too much cu rrent, it's already failed, so that leaves the wire as all that's left to protect.
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user9253
Joined: 28 Mar 2008
Posts: 1921
Location: Riley TWP Michigan
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| Posted: Mon Dec 26, 2016 6:35 am Post subject: Re: A couple of questions about Fuses |
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Fuses that glow when blown depend on the load to complete the circuit. If the load circuit happens to open up as a result of the malfunction, the fuse will not glow. While the above scenario is possible, it is not likely. If some electrical device does not work, check the glow-type fuse with a meter to be sure that it is not blown.
Yes, the larger the fuse, the less protection there is. By protection, I am talking about protection against smoke in the cockpit. As far as protection for the electrical device, it doesn't matter if the fuse is 1 amp or 2 amps or 5 amps. If the device shorts out, it will be fried regardless of the fuse size. A lager fuse size will not make any electrical device more prone to fail. Many devices have internal fuses to protect against internal short circuits.
22 AWG wire can be protected by a 3 amp fuse. But if a device only requires one half amp, then a 1 amp fuse will offer more protection against smoke if the device ever shorts out.
Bottom line is, you can use either a 1 amp or 2 amp fuse. A 1 amp will give more peace of mind. A 2 amp will work too.
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Joe Gores |
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yellowduckduo(at)gmail.co
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| Posted: Mon Dec 26, 2016 8:09 am Post subject: A couple of questions about Fuses |
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I would add that I have seen cases where the recommended small fuse
popped after many years. Replacing the fuse with a handy slightly larger
fuse also popped but this time it was accompanied by smoke from the
transponder. Neither fuse put the wire at any risk and likely the smoke
would have stopped by itself but I prefer to follow the manufacturers
fuse recommendation. OTOH I don't worry about going up to the
appropriate larger wire protection fuse in cases where it is convenient
to put multiple items on the same fuse.
Of larger concern to me are some of the very small gage cords that get
plugged into 15 amp circuits in houses.
Ken
On 26/12/2016 9:35 AM, user9253 wrote:
| Quote: |
Fuses that glow when blown depend on the load to complete the circuit. If the load circuit happens to open up as a result of the malfunction, the fuse will not glow. While the above scenario is possible, it is not likely. If some electrical device does not work, check the glow-type fuse with a meter to be sure that it is not blown.
Yes, the larger the fuse, the less protection there is. By protection, I am talking about protection against smoke in the cockpit. As far as protection for the electrical device, it doesn't matter if the fuse is 1 amp or 2 amps or 5 amps. If the device shorts out, it will be fried regardless of the fuse size. A lager fuse size will not make any electrical device more prone to fail. Many devices have internal fuses to protect against internal short circuits.
22 AWG wire can be protected by a 3 amp fuse. But if a device only requires one half amp, then a 1 amp fuse will offer more protection against smoke if the device ever shorts out.
Bottom line is, you can use either a 1 amp or 2 amp fuse. A 1 amp will give more peace of mind. A 2 amp will work too.
--------
Joe Gores
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nuckolls.bob(at)aeroelect
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| Posted: Tue Dec 27, 2016 6:25 pm Post subject: A couple of questions about Fuses |
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In my fused power busses I decided to use LED blade fuses, those whose LED gets illuminated when the fuse trips.
Are you anticipating replacement of fuses in flight?
It happens that I have a couple of circuits which require a 1A fuse, but there are no 1A LED blade fuses. Therefore, in these circuits, either I use a normal (non-illuminated) 1A fuse, or I go for a 2A illuminated fuse.
I used to sell 1 and 2A ATC fuses but
never with failure annunciation. If your
appliance is 'dead in the water', little
lights on the fuses are of marginal convenience.
What happens if I decide for the latter? Do I get less protection? Is that particular device more prone to be fried?
"Less" and "more" are hard to quantify.
What we do know is that legacy wire
protection philosophy is based not upon
wire size but the robustness of the
insulation. You can purchase 22AWG wires
with recommended maximum operating
temperatures all over the map . . . generally
no lower than 80C but some as high as
180-200C. In EVERY case, it is not the
wire at-risk for failure due to effects
of current flow . . . it's the INSULATION
that drives the numbers on max recommended
temperature rise.
Wrestling with the consequences for choosing
1A vs 5A protection on a 22AWG wire is
not particularly difficult.
See http://tinyurl.com/zrmkoz6
That image demonstrates that loading
a 22AWG wire at 20 amps does not produce
a free-air temperature rise that places
the Tefzel insulation at risk for failure.
Now, for the second question, in a broader perspective: I have heard dozens of times that a fuse (or other protection device) is there to protect the wire and not the device at its end.
Absolutely . . .
However, the size (in A) of the fuse is determined by the device in use (you read in the installation manuals that to protect this device, you should use a 3A fuse or circuit breaker).
People who write installation manuals are
almost never system integrators. Frequently
they are not talented writers either. The
installation manuals should speak to continuous
and intermittent current drain values. Intermittent
values should also be quantified with a typical
duration.
It's up to the system integrator to supply
a source of energy that is (1) not at risk
for tripping its feeder protection. (2)
sizing the wire such that its temperature rise
does not exceed ratings for the proposed
insulation. Allowances must be made for
areas where the wire may traverse an
elevated environmental temperature. Further
consideration is made for loss of cooling
the wire is bundled. (3) The final consideration
evaluates potential for loss of appliance
performance due to voltage drop when the run
of wire is particularly long. Lear got
bit in the cushy behind when they discovered
an unacceptable loss of performance for a pitch
trim system because motor current ran the
length of the aircraft four times!
While the wires were not over-heating, the
voltage drop under certain conditions produced
unsatisfactory trim performance. I got to design
a solid state fleet retrofit trim controller.
My boss was really happy with that one . . .
got a big raise. He paid for my flying lessons
accompanied by a generous flying budget from the
rental fleet at United Beechcraft.
I know that this depends on the power (W) that is used by the device when working, and that both the wire and the fuse must be sized accordingly. I also know that if we use a thinner wire, it will get very hot and eventually melt.
But what about using an higher A or a lesser A fuse? What are the physics involved?
The copper wire is never at-risk except conditions
where ship's batteries can drive a hard-fault
of hundreds if not thousands of amps. So in
the photo example cited above, I COULD
consider running a 20A appliance through
a 22AWG feeder protected with a 30A fuse . . .
as long as the feeder length wasn't so long
that voltage drop becomes an issue.
When selecting wires and protection, you're
never wrong with the "legacy pairs" of
22AWG/5A, 20AWG/7A, 18AWG/10A, etc. etc.
These are very conservative pairings.
An installation manual should read something
like, "Supply DC power at 11-15 volts with
a continuous demand of X.X amps and a 10%
duty cycle of Y.Y amps."
After that, it's up to the system integrator
to meet that requirement. I would never have
attempted to qualify a product that begged
for EXTERNAL overcurrent protection beyond
that normally supplied with the recommended
wire size. If you've got a 22AWG feeder to
a device where 3A protection is suggested,
5A protection is not out of line.
Bob . . .
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nuckolls.bob(at)aeroelect
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| Posted: Thu Dec 29, 2016 2:29 pm Post subject: A couple of questions about Fuses |
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Some time back I wrote:
| Quote: | When selecting wires and protection, you're
never wrong with the "legacy pairs" of
22AWG/5A, 20AWG/7A, 18AWG/10A, etc. etc.
These are very conservative pairings. |
I thought of a quality unique to fuses that
justifies a departure from the 'leggacy
pairing' cited above.
Fuses and Circuit breakers have one thing in
common . . . they operated based on the HEATING
effect of a current flowing in a component
having some resistance . . . I.e. a voltage
drop that liberates heat due to 'lost'
energy.
The similarity ends there.
Circuit breakers have bi-metal components
which are close cousins to thermostats. Heating
produces a change in shape that causes some
latching mechanism to release at a predictable
temperature rise.
On the other hand, the resistive current sensing
component of a fuse is solid metal alloy designed
to MELT when a certain temperature is reached.
Conversion from a SOLID to a LIQUID calls for
a phase change that exhibits a kind of plateau
that cannot be exceeded until a unique quantum
of heat has been delivered into the mass.
In water, the heat removed in the change of phase
from liquid to solid is called the 'heat of
fusion' and if memory serves, is about
80 cal/gm for water. This means that you can
have ice at 0C and water at 0C . . . but for
every gram of ice at 0C you have to pump
80 cal of heat into it just to convert to
liquid water.
Every material, including the melting link
in a fuse has a similar characteristic.
This is important in the specification of fuses
to a particular task. While a circuit breaker
is a simplified 'thermostat' . . . capable of
repeated operations with no change of calibration.
On the other hand a fuse can, if you will, be
'hammered'.
Let's say you have a 5A fuse loaded to 4A but
subjected to an 8A transient measured in tens of
milliseconds. On the bench you can subject
the fuse to this stress many times with no
failure. Such an event, say once per flight cycle
in an airplane, may never produced a nuisance trip . . .
or it may happen after tens or hundreds of flight
hours.
It doesn't happen very often, but ONE TIME may be
too many for individuals who suffer the event. An
excellent case in point . . .
http://tinyurl.com/msfmldj
Through a combination of events and poor design
choices, fuses in the accident aircraft were
subject to transient stresses that 'hammered'
at the edge for heat-of-fusion on first one
and then the second of two fuses . . . a failure
that insured simultaneous failure of both
ignition systems.
The point to be pondered here illustrates
the value of failure mode effects analysis in
deducing risks associated with the failure of
any single component. When unacceptable risks
are identified, changes to selection components
and system architecture will easily negate the
risk.
THE BEST prophylactics against risk for popping
a 'hammered' fuse is to design the system such that
loss of that system is no-big-deal. After all, there
a dozens of OTHER reasons the fuse might open. Be
prepared for ANY event.
Based on what we know about the relative fragility
of a 22AWG wire, a 5A, nuisance tripping fuse on
a 22AWG feeder can be comfortably replaced with
a 7A fuse.
As a general rule, the engineering data on fuses
suggests they not be continuously loaded to more
than 75% of rating at 25C.
http://tinyurl.com/zobgmdv
If situated in an environment routinely hotter than
25C, perhaps de-rating to 50% is in order. This
thinking supports the notion that a fuse under the
cowl driving a 22AWG feeder loaded to 4A continuously
might be 'protected' at 10A. It's not radical thinking
. . . they do this in cars all the time.
Bob . . . 
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