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  #11  
Old 08-13-2012, 05:02 AM
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Brantel Brantel is offline
 
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Quote:
Originally Posted by PerfTech View Post
...I have one of the ceramic so called smart power panels that should have shut down this circuit at 3.5 amps. I wish it had individual breakers and old school switches instead. Allan...
Allan,

Tons of electronic devices can smoke with way less current than 3.5 amps. Depending on how the device failed, it is very much possible that you got smoke out of it without ever reaching 3.5 amps.

Fuses and breakers are intended to protect the wire not the device.
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  #12  
Old 08-13-2012, 06:02 AM
John R. Graham John R. Graham is offline
 
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An analog volt meter itself shouldn't draw more than a few hundred microamps. A 3.5a circuit will carry probably in excess of a thousand times more current than the gauge needs. This thread has me thinking about my (admittedly still far off) panel design in a slightly new way.

- John
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  #13  
Old 08-13-2012, 10:15 AM
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Quote:
Originally Posted by Brantel View Post
Allan,

Tons of electronic devices can smoke with way less current than 3.5 amps. Depending on how the device failed, it is very much possible that you got smoke out of it without ever reaching 3.5 amps.

Fuses and breakers are intended to protect the wire not the device.
...The instruments are all wired from one lead like specified by Vans and apparently the 3.5 amp load in what they figure is needed collectively. No wires were compromised just the gauge internals. It just makes you a little nervous when you let the smoke out. Thanks all, Allan...
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  #14  
Old 08-13-2012, 10:27 AM
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Quote:
Originally Posted by Brantel View Post
Allan,

Tons of electronic devices can smoke with way less current than 3.5 amps. Depending on how the device failed, it is very much possible that you got smoke out of it without ever reaching 3.5 amps.

Fuses and breakers are intended to protect the wire not the device.
Well, not exactly.

Even if the wiring was 22g, a fuse/breaker of less than 5 amps (the approx. wire rating) would be quite acceptable and could protect a low power device as well as the actual wire.

I think the generic rule to use is never to have a fuse/breaker of a rating higher than the wire rating....

Does Van give a fuse rating (or even a current draw) for a set of his instruments? I couldn't find it on the data sheet.
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  #15  
Old 08-13-2012, 10:37 AM
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Quote:
Originally Posted by az_gila View Post
Well, not exactly.

Even if the wiring was 22g, a fuse/breaker of less than 5 amps (the approx. wire rating) would be quite acceptable and could protect a low power device as well as the actual wire.

I think the generic rule to use is never to have a fuse/breaker of a rating higher than the wire rating....

Does Van give a fuse rating (or even a current draw) for a set of his instruments? I couldn't find it on the data sheet.
Sure you can come up with a fuse that is rated low enough to barely drive the device but in most cases that is not practical.

I stand behind my comment...Fuses and breakers are intended to protect the wire, not the device. That's not to say you can't come up with a combination that achieves both though but it would take very specific knowledge of the internal design of the device and it's failure modes to guarantee you don't let the magic smoke out.
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---------------------------------------------------------------------
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Track N159SB (KK4LIF)
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Last edited by Brantel : 08-13-2012 at 10:51 AM.
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  #16  
Old 08-13-2012, 12:36 PM
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Default I had the same problem

http://www.vansairforce.com/communit...hlight=melting
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  #17  
Old 08-13-2012, 02:42 PM
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n82rb n82rb is offline
 
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a note about halon, be sure to read all you can about halon before putting it in your aircraft. There have been computer operators nearly killed when the auto halon system went off in the computer room. Don't get me wrong, a fire is really bad, and having a defense against it is a very good idea but just make sure you learn all about the extinguishing agent and the proper use before mounting it in the aircraft.

bob burns
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  #18  
Old 08-13-2012, 03:07 PM
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az_gila az_gila is offline
 
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Quote:
Originally Posted by Brantel View Post
Sure you can come up with a fuse that is rated low enough to barely drive the device but in most cases that is not practical.

I stand behind my comment...Fuses and breakers are intended to protect the wire, not the device. That's not to say you can't come up with a combination that achieves both though but it would take very specific knowledge of the internal design of the device and it's failure modes to guarantee you don't let the magic smoke out.
No, just a look at the device specification sheet or the sample wiring diagram for most avionics...

I never used the word "barely".... I stand by my comment too....

Just for an example - 1 amp fuse recommended for 24g (approx 4 amp) wire -

http://www.ps-engineering.com/docs/P...l%20Manual.pdf

A good avionics install manual will give a fuse rating, and usually a wire size greater than it's rating.

Protect the device first, and then the wire....
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Last edited by az_gila : 08-13-2012 at 03:13 PM.
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  #19  
Old 08-13-2012, 03:55 PM
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JonJay JonJay is offline
 
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Quote:
Originally Posted by az_gila View Post
Protect the device first, and then the wire....
That just does not work when you are cascading multiple devices off a single protection.

In the commercial construction world, we have standards like UL that could care less about protecting the device as long as when the device goes "boom", it does not cause a fire. You size the wire to the load and protection in kind. Many devices are hung off of one protection.

Not sure this exists in our experimental, aviation world.
So, if we are going to protect each device, we need an individual fuse or other protection sized for each component and should not cascade any devices off or one fuse or breaker. That means each and every post light, individually fused, as an example. I don't usually see this in aircraft.

Am I understanding this correctly?
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  #20  
Old 08-13-2012, 04:02 PM
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Brantel Brantel is offline
 
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Quote:
Originally Posted by az_gila View Post
No, just a look at the device specification sheet or the sample wiring diagram for most avionics...

I never used the word "barely".... I stand by my comment too....

Just for an example - 1 amp fuse recommended for 24g (approx 4 amp) wire -

http://www.ps-engineering.com/docs/P...l%20Manual.pdf

A good avionics install manual will give a fuse rating, and usually a wire size greater than it's rating.

Protect the device first, and then the wire....
There are tons of components in that device that will smoke at less than 1 amp.

Not trying to be argumentative, but everyone from high school electronics to MIT teach that the fuse or breaker is there to protect the wire not the device.

I just don't want to give someone the false impression that they can somehow guarantee that some component in one of their devices cannot smoke because they have the proper fuse and wire feeding power to it. It can and does happen all the time.....
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RV-10, #41942, N?????, Project Sold
---------------------------------------------------------------------
RV-7/TU, #72823, N159SB
Lyc. O-360 carbed, HARTZELL BA CS Prop, Dual P-MAGs, Dual Garmin G3X Touch
Track N159SB (KK4LIF)
Like EAA Chapter 1494 on Facebook
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