Matronics Email Lists :: View topic - Fusible links vs fuses

Matronics Email Lists
Web Forum Interface to the Matronics Email Lists

Get Email Distribution Too! FAQ Search Memberlist Usergroups Register

Profile Log in to check your private messages Log in

Fusible links vs fuses

Matronics Email Lists Forum Index -> AeroElectric-List

View previous topic :: View next topic

Author Message

james(at)etravel.org
Guest

Posted: Fri Apr 02, 2010 12:35 pm Post subject: Fusible links vs fuses

Gents,



I've just about finished the wiring on my plane (a Jodel D150), but

I've lost sight of why I should have a fusible link between the master

switch and the main bus.  My electrical system is based on Z-11.



Now, given that the other side of the master switch has the 5A alt.

field breaker on, I can deduce that this fusible link is to protect

the other part of the wiring.  So, perhaps it's because it's a longish

run of cable to the master switch.  However, even if that's the case,

would a 5A fuse suffice, when presumably that's the maximum current

that can flow along that cable?



I've been ploughing through the Aeroelectric Connection again, to

brush up, and I've read the bits about fusible links.  If it's a case

of never-going-to-happen worst-case-scenario short protection, then I

guess it makes sense... but would a fuse do instead?



I'm playing catch up, as I've not worked on the plane for a couple of

years, so please forgive this dumb question (and my subsequent

ones!)...



Thanks in anticipation,



James

rckol

Joined: 14 Nov 2008
Posts: 36

Posted: Fri Apr 02, 2010 12:55 pm Post subject: Re: Fusible links vs fuses

James,



The fuseable link is to protect the wire between the bus and the breaker.  The breaker is there as part of the overvoltage protection circuit.  If you get an overvoltage event, you want the resetable breaker to trip, not the proposed fuse to blow, hence the sturdier fuseable link.

_________________
rck

N38CW

Joined: 16 Apr 2008
Posts: 57
Location: Winston-Salem, NC

Posted: Fri Apr 02, 2010 4:23 pm Post subject: Re: Fusible links vs fuses

 	  rckol wrote: 	
		  James,



The fuseable link is to protect the wire between the bus and the breaker.  The breaker is there as part of the overvoltage protection circuit.  If you get an overvoltage event, you want the resetable breaker to trip, not the proposed fuse to blow, hence the sturdier fuseable link. 	




Please correct me if I'm wrong, but don't you mean overcurrent instead of overvoltage?

_________________
Bill Settle
RV-8 Fuselage

rckol

Joined: 14 Nov 2008
Posts: 36

Posted: Fri Apr 02, 2010 7:14 pm Post subject: Re: Fusible links vs fuses

James,



The 5 amp breaker in the line controlling the regulator is for overvoltage protection, assuming you are using a B&C or PlanePower regulator for an externally regulated alternator or are using a PlanePower internally regulated alternator with built in OV protection (or an OVM-14 module).



Your overcurrent protection would be in the form of a big breaker or fuse (40+ amps) on the B lead (output) of the alternator.



Regards

_________________
rck

rckol

Joined: 14 Nov 2008
Posts: 36

Posted: Fri Apr 02, 2010 7:48 pm Post subject: Re: Fusible links vs fuses

As a follow up: the 5 amp breaker protects against overvoltage as part of the crowbar circuit.  If overvoltage is detected by the regulator, the breaker is shorted to ground, trips and interrupts the regulator control circuit.  



So the breaker is tripping due too much current running through it, but for the purpose of interrupting an overvoltage event.



This all assumes you have implemented this type of protection in your system.

_________________
rck

nuckolls.bob(at)aeroelect
Guest

Posted: Fri Apr 02, 2010 8:07 pm Post subject: Fusible links vs fuses

At 10:14 PM 4/2/2010, you wrote:

 	  Quote: 	
		  



James,



The 5 amp breaker in the line controlling the regulator is for 

overvoltage protection, assuming you are using a B&C or PlanePower 

regulator for an externally regulated alternator or are using a 

PlanePower internally regulated alternator with built in OV 

protection (or an OVM-14 module).



Your overcurrent protection would be in the form of a big breaker or 

fuse (40+ amps) on the B lead (output) of the alternator.


	


    Very close except that alternators don't require over-current

    protection like their older cousins, the generator.



    Alternators are magnetically limited in their ability

    to deliver current . . . so as the load on an alternator

    goes up, there comes a time about 10-20% over nameplate

    rating where the critter wont deliver any more and the

    output sags.



    Maximum output from the alternator happens when the

    machine is cold. On rare occasions (cold morning,

    jump start dead battery, battery is relatively

    new and will accept lots of recharge current)

    one can get a nuisance trip of the 60A breaker

    in an airplane fitted with a 60A alternator.

    This is because total ship's electrical loads

    plus battery recharge current will be what ever

    the alternator will deliver . . . which may be

    greater than the 60A breaker rating on the

    panel.



    This happened to me once . . . the second of

    only two times I've had a breaker open in flight.

    This is why we select b-lead protection well above the

    name-plate rating for the alternator so that the

    breaker doesn't nuisance trip. It's also why I

    call the 60A breaker on most Cessnas and Pipers

    the "breaker designed to nuisance trip".



    In any case, the b-lead breaker is to protect

    the rest of the system if you get shorted diodes

    in the alternator (very rare). The fusible link

    in most cares serves the same purpose.



    Bob . . .

james(at)etravel.org
Guest

Posted: Sun Apr 04, 2010 4:34 am Post subject: Fusible links vs fuses

Thanks for that rck.



So the fusible link is just intended to be a worst-case-scenario,

probably-won't-ever-blow fuse.  Makes sense as it reduces joints.



What Bob wrote about the alternator is interesting, and makes me think

of another question, which I'll post separately.



James



On Fri, Apr 2, 2010 at 9:55 PM, rckol <rckol(at)kaehlers.com> wrote:

 	  Quote: 	
		   



 James,



 The fuseable link is to protect the wire between the bus and the breaker.  The breaker is there as part of the overvoltage protection circuit.  If you get an overvoltage event, you want the resetable breaker to trip, not the proposed fuse to blow, hence the sturdier fuseable link.



 --------

 rck





 Read this topic online here:



 http://forums.matronics.com/viewtopic.php?p=292811#292811

nuckolls.bob(at)aeroelect
Guest

Posted: Sun Apr 04, 2010 5:54 am Post subject: Fusible links vs fuses

At 07:31 AM 4/4/2010, you wrote:

 	  Quote: 	
		  



Thanks for that rck.



So the fusible link is just intended to be a worst-case-scenario,

probably-won't-ever-blow fuse.


	


   Correct.



 	  Quote: 	
		     Makes sense as it reduces joints.


	


   That was the goal. Automobiles have



   See:



http://www.aeroelectric.com/articles/fuselink/fuselink.html



   There's a lot of discussion on the whys and wherefors

   for fusible links in the FAQ



http://www.aeroelectric.com/FAQ/AeroElectric-List/AeroElectric-List_FAQ.pdf



   Do a search on "fusible" to get a good review

   of their development, application and incorporation

   into AEC Z-figures.



   Bob . . .

rckol

Joined: 14 Nov 2008
Posts: 36

Posted: Sun Apr 04, 2010 6:24 am Post subject: Re: Fusible links vs fuses

Bob,



Am I correct in thinking that in the rare "shorted alternator diode" scenario, the alternator is providing a low resistance path to ground and the power source that the big breaker or fuse is interrupting is in fact the battery?

_________________
rck

james(at)etravel.org
Guest

Posted: Sun Apr 04, 2010 6:34 am Post subject: Fusible links vs fuses

Bob,



The FAQ is an awesome body of knowledge!  I started reading it from

top to bottom... about 20 pages in I noticed it's 310 pages!  After

that, Control-F became my friend!



I can't remember how I stumbled across your Aeroelectric Connection

book, and this list, but I'm extremely glad I did.  It's funny how we

spend all this time and money building our planes, knowing every part

and rationalising every decision... until we get to the electrics...

then we just do exactly what Cessna and Piper have done forever,

because "it must be fine".



It's great to have done an electrical system the AC way: to plan

properly, to question it all in the same way, and learn about why

something is or isn't appropriate.  It's also good to be able to

answer people's questions ("why don't you have breakers?" or "why

can't you change your fuses in flight?", etc) from a position of

knowing that, even if something turns out not to be perfect, at least

it was thought about in the first place.



James



On Sun, Apr 4, 2010 at 2:54 PM, Robert L. Nuckolls, III

<nuckolls.bob(at)aeroelectric.com> wrote:

 	  Quote: 	
		   

 <nuckolls.bob(at)aeroelectric.com>



 At 07:31 AM 4/4/2010, you wrote:

>

> 

>

> Thanks for that rck.

>

> So the fusible link is just intended to be a worst-case-scenario,

> probably-won't-ever-blow fuse.



  Correct.



>  Makes sense as it reduces joints.



  That was the goal. Automobiles have



  See:



 http://www.aeroelectric.com/articles/fuselink/fuselink.html



  There's a lot of discussion on the whys and wherefors

  for fusible links in the FAQ



 http://www.aeroelectric.com/FAQ/AeroElectric-List/AeroElectric-List_FAQ.pdf



  Do a search on "fusible" to get a good review

  of their development, application and incorporation

  into AEC Z-figures.



  Bob . . .

nuckolls.bob(at)aeroelect
Guest

Posted: Sun Apr 04, 2010 8:24 am Post subject: Fusible links vs fuses

 	  Quote: 	
		  Am I correct in thinking that in the rare "shorted alternator diode" 

scenario, the alternator is providing a low resistance path to 

ground and the power source that the big breaker or fuse is 

interrupting is in fact the battery?


	




    Absolutely. Compared to the alternator, the battery

    is the single most "dangerous" in terms of energy

    dumped during a fault . . . a new 35 a.h. RG battery

    can source over 2000 amps when presented with a short.



    So yes, the b-lead protection is not going to be

    tripped by the alternator but the battery when

    some (very rare) failure in the alternator presents

    the short.



    I've seen alternator stator leads burn before the

    ANL in the b-lead burned. So it may be that most

    alternators are pretty well "self limiting" in

    terms of effects on the rest of the system during

    and right after a fault. But without extensive

    testing/analysis, I wouldn't want to bet on it.



I forgot to ask when on the subject of ANL-40s, if there is a slightly

less "industrial" way of mounting an ANL-40.  The holder I have for it

looks as though it's from an electricl sub-station, and weighs about

50lbs (a slight exaggeration perhaps).



    They are pretty beastly . . . consider the miniature

    automotive equivalents to the ANL. Or, if you're

    doing b-lead protection on a 40A alternator, how about

    a 60A maxi-fuse. The ANL is not the be-all-end-all

    solution to the design goal.



    You can build your own holder for a mini-anl out

    of phenolic, delrin or other robust insulator material.

    This was discussed a few days ago here on the List.



Is it poor practice to mount the ANL-40 in some other way, perhaps

in-line to the B-lead close to the starter contactor?



    The goal is to place circuit protection as close

    as possible to the SOURCE of the energy likely to

    open the fuse. Note in all the Z-figures (at least

    I think I got it done on all of them) the wire on

    the battery side of the b-lead protection is marked

    with a (*). That symbol on the drawing suggests

    "make this wire as short as practical".



    Bob . . .

nuckolls.bob(at)aeroelect
Guest

Posted: Sun Apr 04, 2010 11:17 am Post subject: Fusible links vs fuses

At 09:33 AM 4/4/2010, you wrote:

 	  Quote: 	
		  



Bob,



The FAQ is an awesome body of knowledge!  I started reading it from

top to bottom... about 20 pages in I noticed it's 310 pages!  After

that, Control-F became my friend!


	


   Yeah, a reader on the List compiled that document some

   years ago . . . it needs updating and editing. But

   as you've discovered, the "find function" is very useful

   for sifting all the sand.



 	  Quote: 	
		  I can't remember how I stumbled across your Aeroelectric Connection

book, and this list, but I'm extremely glad I did.  It's funny how we

spend all this time and money building our planes, knowing every part

and rationalising every decision... until we get to the electrics...

then we just do exactly what Cessna and Piper have done forever,

because "it must be fine".


	


   And that's not wrong . . . but it does carve your airplane's

   electrical system into "legacy stone". Probably 90% of all

   OBAM aircraft are wired like Uncle's Walter, Duane, and

   Bill have been doing it for 60+ years and it works as

   advertised.

 	  Quote: 	
		  It's great to have done an electrical system the AC way: to plan

properly, to question it all in the same way, and learn about why

something is or isn't appropriate.  It's also good to be able to

answer people's questions ("why don't you have breakers?" or "why

can't you change your fuses in flight?", etc) from a position of

knowing that, even if something turns out not to be perfect, at least

it was thought about in the first place.


	


   Exactly! It would not yank my chain in the least if EVERYBODY

   wired their airplanes like a C-150 if I thought they understood

   how all the parts worked and made their decision from

   informed choice based not upon tradition or authoritative

   suggestion but from understanding.



   Thank you for the kind words. I'm pleased that you

   find the work informative and useful.



   Bob . . .

Display posts from previous:

	Matronics Email Lists Forum Index -> AeroElectric-List	All times are GMT - 8 Hours
Page 1 of 1

Jump to:

You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum
You cannot vote in polls in this forum
You cannot attach files in this forum
You can download files in this forum