Today's Message Index:
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1. 07:09 AM - Re: Re: Same rating for fuses and circuit breakers? (Robert L. Nuckolls, III)
2. 07:39 AM - Re: Re: Same rating for fuses and circuit breakers? (Robert L. Nuckolls, III)
3. 07:42 AM - Re: Re: Same rating for fuses and circuit breakers? (Robert L. Nuckolls, III)
4. 03:26 PM - Re: Re: Same rating for fuses and circuit breakers? (Robert L. Nuckolls, III)
Message 1
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Subject: | Re: Same rating for fuses and circuit breakers? |
>
>As for the CB protecting the wires and not the
>device...=C2 Well, I have heard that and read that for years.
>It does not protect the device ONLY because it
>does not function quick enough to prevent damage.
>But, as for protecting the wires...=C2 I feel that statement is a BUNK!=C2
=C2
>If it was only to protect the wires WHY doesn't
>ALL wires of the same size have the same size CB?
>That statement sure sounds like C.Y.A..
>The product manufacture did not want to go the
>extra distance - spelt - MONEY.=C2 To add protection to the product!
check out the narrative on wire and protection
sizing in AC4-13
Circuit protection and wire size can be fine
tuned but based mostly on the wire's insulation
temperature limits, voltage drop, and environment
through which the wire travels.
20 amps through a 22AWG wire in free air will not
raise the temperature above the limits for Tefzel
wire . . . yet we never see 22AWG operated and/or
protected at such extreme.
5A is the legacy rule of thumb limit for 22AWG wire
dating back to Mil-w-76 cotton over rubber insulated
wire. It's a good rule of thumb that frees the
system integrator from what could be a nightmare
should his boss task him with 'optimizing' every
strand of wire in every wire bundle.
The cost/benefit ratio of such a philosophy is
exceedingly poor. Having said that, there is
noting in the rule of thumb that prohibits
departure for calculated reasons . . . like those
manufacturers whose products feature some
electrical fragility that cries out for
external relief.
It would be interesting to see a system diagram
on a TC aircraft that departs from the 22/5,
20/7, 18/10, 14/15 protocol for 10 degree
temperature rise in free air.
If one wished to adopt some other policy for
temperature rise . . . say 21 degrees C,
they're certainly free to do so . . . and
the risks are low but so are the benefits.
Bob . . .
Message 2
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Subject: | Re: Same rating for fuses and circuit breakers? |
At 08:05 PM 10/25/2018, you wrote:
>Thanks Don,
>
>Ya know guys, the circuit is so simple it can be
>added to a CB.=C2 probably easier than a fuse.
>Good for the CB that is out of the standard field of view.
>
>Barry
But what is the cost/benefit ratio? 99.99% of all
breakers/fuses installed in all vehicles at the
factory go to the junkyard never having been
tasked to do their job.
For every fault that takes a system down by
opening the power feeder protection, there
are MANY more faults equally deleterious
to system operation that do not open the
feeder protection.
When circuit protection does operate in
a thoughtfully designed system, the thing
is broke. Resetting the breaker or replacing
the fuse will not bring it back.
The pilot's first clue that something is
amiss is when an expected event doesn't
occur on the airplane . . . but what value
is there in knowing the state of power
feeder protection? The reason for malfunction
is not relevant to the pilot's prime
directive: effect a return to earth
without bending airplanes or people.
Indicating fuses are cute but the feature
saves only seconds of diagnostic time which
is done on the ground.
Bob . . .
Message 3
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Subject: | Re: Same rating for fuses and circuit breakers? |
At 07:29 PM 10/25/2018, you wrote:
>Right You Are Charlie:
>
>Here is another thought?
>In the AC world there is a thing know as a Ground Fault Interrupt (GFI)
Yup, got 'em in airplanes too. After the Swiss Air 111
disaster there was a push to develop a breaker
with soft-fault detection built in. TI and others
rose to the occasion and you can buy such critters
today . . . if your wallet is fat enough.
Anybody work for Boeing or Airbus? Know anybody
who might fill us in on just how many such devices
have found their way onto there products? I'm
betting its VERY few if any.
Bob . . .
Message 4
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Subject: | Re: Same rating for fuses and circuit breakers? |
>
>I do recall years ago there were fuse holders
>for AC that would glow when the fuse blew.=C2 It
>was a simple circuit of a NE2 bulb and a series
>resistor.=C2 They were wired in series across the
>fuse.=C2 When the fuse blew the circuit would
>become active and the NE2 would glow.
>Is this the same idea?
Yup . . . they're still made
https://tinyurl.com/ych5z2u7
Just one of a family of devices called
'indicating' fuse holders.
Some of these critters have a little spring
loaded arm that pops up or a button that
pops out.
https://tinyurl.com/y8a9qkd5
These features have been used
to make contact with a 'detector strip'
of some type that will effect an alarm
when a blown fuse does its thing . . .
The idea isn't new . . . back in 1928 a
clever fellow got a patent on a system
for bridging an opaque cartridge fuse
with a transparent device. He correctly
reasoned that when the large fuse opens
in relative obscurity, the external device
would open as well and announce the
condition.
http://www.freepatentsonline.com/1868465.pdf
Wonder if he ever collected a royalty on
this . . .
Bob . . .
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