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  #11  
Old 09-18-2018, 06:44 PM
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az_gila az_gila is offline
 
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Quote:
Originally Posted by Grumpy-Hodge View Post
.......

Gil pulled the reference I had in mind. Read specifically the next to last sentence in Gil's AC43-13 reference beginning with "Wires...".

Just like the house wiring rv7charlie mentioned, 12-2 Romex is usually found on 20A branches and 14-2 Romex is usually found on 15A branches. If a wiring fault occurs, the CB should trip and arrest the flow of current in the wiring before the wiring overheats, catches fire and cause potentially catastrophic damage. An aircraft CB may not be able to prevent many fault conditions in an Line Replaceable Unit (LRU) from continuing unless it is of the magnitude of the CB rating or much greater as in a short condition, but the aircraft wiring should be protected if sized correctly not only for the load but for the circuit protection as well.
That's were I read it differently, in English not FAA-ese.

As an example I could wire my G5 with #18 wire and still have a 5 A breaker on it.

It would meet the requirement you mention of protecting the wire. You can always use a lower value breaker to protect the wire.

I think your reading of the requirement and using the highest value breaker to protect a wire is incorrect. The wire is protected with the lower value breaker.

To answer the Garmin Q. - it says #22 wire and 5A breaker. I might be extrapolating but I read that as minimum value for the wire (bigger wire still lets the G5 work), with the breaker being a recommendation to protect the device. In normal certified aircraft practice, a dedicated wire would be used with no sharing of other devices. This sort of removes the house analogy.
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  #12  
Old 09-18-2018, 06:45 PM
Grumpy-Hodge Grumpy-Hodge is offline
 
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Default Smokin LRUs

LRU manufactures usually consider the possible internal fault conditions and apply main (at power entry) and distributed circuit protection devices. To rv7charlies point, a 7A CB wont protect a 2A fault inside an LRU. But the LRU designer hopefully did due diligence in the LRU circuit protection design.
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  #13  
Old 09-18-2018, 06:46 PM
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Quote:
Originally Posted by Grumpy-Hodge View Post
LRU manufactures usually consider the possible internal fault conditions and apply main (at power entry) and distributed circuit protection devices. To rv7charlies point, a 7A CB wont protect a 2A fault inside an LRU. But the LRU designer hopefully did due diligence in the LRU circuit protection design.
You mention the AC43.13 sentence starting with "Wires".

But this is the sentence before it -

A circuit breaker must always open before any component
downstream can overheat and generate smoke or fire.



I'll add in another certified example from Garmin - the GTR200

The IM calls for #20 power and ground wires, but a 7.5A breaker for 14v and 5A breaker for 28V systems. The #20 wire would melt/fail and the same current whether it is connected to 14 or 28 volts.
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Grumman Tiger N12GA - flying
La Cholla Airpark (57AZ) Tucson AZ

Last edited by az_gila : 09-18-2018 at 06:54 PM.
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  #14  
Old 09-18-2018, 07:05 PM
Grumpy-Hodge Grumpy-Hodge is offline
 
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Default AC43-13

Gil,
All I am saying is that the load, wire size, and circuit protection need to be compatible.
Use the smallest CB appropriate for the anticipated load. But also make sure the wire size is compatible with the CB. Especially if using an existing CB branch. I've seen it done. Wire was okay for the load but too small for the CB rating used.

If there is any part of the AC43-13 reference that is troubling to me is the one you just quoted..."A circuit breaker must always open..." A relatively low current component inside an LRU may fail, overheat, and smoke at a current level well below the normal operating current of the LRU as a whole. The CB can't protect against that of course.

rv7charlie noted bad requirements by the FAA. I agree with him in with respect to that particular sentence. The agreement with the ODA UMs of whom I am very familiar have focused on ensuring the wire design is correct. They recognize that there are impractical limits to the wording of that requirement.
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  #15  
Old 09-18-2018, 07:39 PM
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Quote:
Originally Posted by Grumpy-Hodge View Post
Gil,
All I am saying is that the load, wire size, and circuit protection need to be compatible.
Use the smallest CB appropriate for the anticipated load. But also make sure the wire size is compatible with the CB. Especially if using an existing CB branch. I've seen it done. Wire was okay for the load but too small for the CB rating used.

....
I totally agree... but it is not in concert with many over-simplified statements used -

Then size the circuit protection to protect the wire.

Bigger wire is OK (or put another way - a smaller CB is OK) but stick to the CB value recommended by the manufacturer, not the wire you use...

People keep quoting "the CB protects the wire" but that is only in the limiting case.

In general this should be pretty easy since you really should use a separate circuit for each device -rather than the "branch circuits" you mention.
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Last edited by az_gila : 09-18-2018 at 07:41 PM.
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  #16  
Old 09-18-2018, 08:17 PM
rv7charlie rv7charlie is offline
 
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Default

This subject might eventually rise to 'primer wars' levels, so I'll leave it with a point I hinted at earlier. Very few (if any) devices will be protected from damage by a circuit breaker or fuse. The reality is that if a protection device of the recommended rating blows, the damage is already done; the device has already failed.

I know Garmin's experimental guys post here; I wish they'd directly address what they mean when they spec a wire size and protection rating for their products. By that I mean what I mentioned earlier: are they spec'ing minimum, or maximum. Are they telling us what the device needs, or are they telling us that we are responsible for 'fusing' their device; something that should be built into the device itself.

Charlie
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  #17  
Old 09-18-2018, 08:59 PM
Grumpy-Hodge Grumpy-Hodge is offline
 
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Default Primer Wars

Nope. No primer wars here. I'm done.
Everyone has good points made and obviously very passionate about the hobby and industry which a good thing and very encouraging. Ciao!
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