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  #31  
Old 09-26-2017, 05:53 PM
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DanH DanH is offline
 
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Quote:
Originally Posted by johnbright View Post
This narrative explains the high order probability that even if you DID get your 6AWG feeder faulted to ground, it's most likely to be a soft fault that burns a hole in your airplane while doing little damage to the wire . . . and certainly far short of getting it to smoke and/or open a fuse/breaker.
... Bob Nuckolls ..."
Shorting a 6AWG to ground is "...far short of getting it to smoke and/or open a fuse breaker."

Interesting. I like the logic, but this may call for an experiment
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  #32  
Old 09-26-2017, 08:24 PM
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Quote:
Originally Posted by DanH View Post
Shorting a 6AWG to ground is "...far short of getting it to smoke and/or open a fuse breaker."

Interesting. I like the logic, but this may call for an experiment
Don't bother, I can vouch for this.
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  #33  
Old 09-26-2017, 08:32 PM
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Quote:
Originally Posted by Dbro172 View Post
Don't bother, I can vouch for this.
Do tell!
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  #34  
Old 09-26-2017, 10:45 PM
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Originally Posted by skylor View Post
Doesn't matter, the point I was trying to make is that in general, you can no longer go down the road to your favorite hardware store and buy one.

A decade ago you could. They are becoming obsolete.
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  #35  
Old 09-27-2017, 07:20 AM
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I like the ANL for what I am using it for, and the clamp force of the nut and stud on the fuse ends. For example, I have seen a lot of problems with power feeds on International trucks that use a standard size ATO 40 amp fuse to power the ECM, what I have seen is the clamp force and contact size of the holder on the fuse blades is inadequate for the amount of current, over time this contact area gets compromised and causes all kinds of problems, first its intermittent, (hard to find), and then an open circuit at the contact points. I like the overkill of the ANL for what it is powering.....for me, no electrons.....no fan.
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  #36  
Old 09-27-2017, 07:54 AM
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Previously I said:

Quote:
Originally Posted by DanH View Post
Shorting a 6AWG to ground is "...far short of getting it to smoke and/or open a fuse breaker."

Interesting. I like the logic, but this may call for an experiment
I have huge respect for Bob's work, and like many of you, consider his AEC manual an essential bookshelf reference. However, in retrospect I do find some fault in his logic regarding circuit protection on a main bus feed.

The cautionary tale involved a 40A feeder which came into shorted contact with an elevator cable. Eventually the cable parted, without blowing the fuse, the supposed lesson being whatever component a robust cable shorts against is likely to burn up (burn an open) before the fuse can blow. That is undoubtedly true when the victim is not robust. However, does it hold true if the victim is a motor mount, or the stainless firewall, or some other component with significant mass?

Now consider time vs fuse opening ("before it can blow"). Alex posted a link to some sample fuse specifications, where it states the expected open time is 1 second at 200% of rated load. We're talking about a battery-fed short, which can dump very large currents, so 200% or more can be expected. There is more than enough amps available to pop the fuse.

Assume we do insert circuit protection at the main bus feeder source, properly sized for the wire...say 60A max for a #6 wire. Now let's short that wire somewhere in the cockpit. if the short burns an immediate open in less than one second, great. We'll get a flash and a few sparks and maybe some dirty drawers, but smoke should be limited and it probably won't release enough energy to serve as an ignition source for fibers and plastics. This is what Bob speaks of when he describes why the certified airplanes don't have circuit protection on bus feeds.

Should the short not burn open, given a typical Nuckolls wire plan the pilot is expected (1) overcome his surprise, (2) close the essential bus switch, and (3) open the master switch. No argument; it should work. However, how fast does it work? Is it likely to be less than one second? That's all the time the above passive circuit protection would have required, with no pilot action at all.

(BTW, remember Bob's cautionary tale leading to his introduction of the essential bus concept? The big cockpit short didn't burn open.)

Good design is the art of compromise. The key compromise to be weighed here is the addition of another set of connections (and a few ounces), vs the probability of any practical value. I would caution that blanket judgments may not be appropriate. Allow an example.

Assume a VFR RV-8 with a rear battery. We all seem to agree about the value of circuit protection on the alternator B-lead. There isn't any reason why the main bus feed can't share the same fuse...no additional connections or components.

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Last edited by DanH : 09-27-2017 at 08:04 AM.
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  #37  
Old 09-27-2017, 08:05 AM
rv7charlie rv7charlie is offline
 
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Dan, you're protecting the wrong end of the wire.

edit: I should explain the logic. The alt B lead should be sized to easily handle the max current the alt can deliver. Therefore, it can't overheat & damage insulation from alternator power. The alternator is 'self limiting', and will only do what it can to supply demand, and won't be damaged by the load. So there's no need to protect the wire from the alternator, or protect the alternator itself.

However, the battery *can* cook that wire.

2nd edit: What does the start circuit look like, with that design? Do you mount the starter contactor in the tailcone, with the battery & master? If so, you now have 2 very long #2 wires the length of the plane, instead of one (with starter contactor on the firewall). If the start contactor is on the firewall, how do you fuse the run from master to firewall (it now carries start current)?

Last edited by rv7charlie : 09-27-2017 at 08:17 AM.
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  #38  
Old 09-27-2017, 09:28 AM
6 Gun 6 Gun is offline
 
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Question Alt

I had a problem on start up last year my GPS handheld that's mounted in panel SMOKED my batt is in back turned off my master and everything was still on on main buse still powered up with master off.Turned engine off everything on panel shut off,sit there puzzled as to what just happened always thought the alt had to have power from batt to produce power not so in this case it was sending power to buse and self exciting I had a 5 amp breaker to the b lead.Now I have a toggle in line with 5 amp breaker and it shuts off alt.Check to make sure if you shut off master with engine running main buse shuts off.Im sure I'm the only dummy that's wired buse wrong not wanting to run two long #2 wires to rear of plane and it was exciting enough on ground would have been really exciting in the air.
Bob
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  #39  
Old 09-27-2017, 09:56 AM
rv7charlie rv7charlie is offline
 
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B lead is the high current wire; you probably have the 5a breaker wired to the I terminal on an internally regulated alt.

What you described is common with IR alts.
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  #40  
Old 09-27-2017, 10:15 AM
6 Gun 6 Gun is offline
 
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That's correct Charlie it is a internally regulated alternator.
Bob
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