Matronics Email Lists :: View topic - ANL current limiter location on rear battery installations

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

ANL current limiter location on rear battery installations

Matronics Email Lists Forum Index -> AeroElectric-List

View previous topic :: View next topic

Author Message

nuckolls.bob(at)aeroelect
Guest

Posted: Tue Jul 13, 2010 6:36 am Post subject: ANL current limiter location on rear battery installations

At 05:53 AM 7/13/2010, you wrote:

 	  Quote: 	
		  

<Tom(at)CostanzaAndAssociates.com>



Bob,



With respect to:

 > Small aircraft do not get circuit protection in

 > the battery feeders to the starter and distribution

 > busses.

Why does the battery feeder to the distribution buss not get protected?


	


   Because fat wires in dc electrical systems are

   exceedingly hard to burn, hence there is no

   value added in attempting to protect them. The

   legacy design goals for DC systems in most vehicles

   don't call for adding circuit protection to wires

   until they become so small compared to the potential

   energy sources (1000+ amps from a hard faulted battery)

   that wiring protection is called for.



   A fat wire in an airplane that's allowed to hat-dance

   up to a chunk of airframe will probably burn a hole

   in the airframe without opening the upstream

   circuit protection. I've seen a 8AWG wire protected with

   40A breaker burn an elevator cable in two without so

   much as generating bad smells,  blinking the lights

   or popping a breaker.



   Hence design goals and installation that suggest extra

   attention to the insulation and support of fat wires combined

   with a crew-controlled shut-off right at the battery

   i.e. battery contactor.



   Bob . . .

- The Matronics AeroElectric-List Email Forum -

Use the List Feature Navigator to browse the many List utilities available such as the Email Subscriptions page, Archive Search & Download, 7-Day Browse, Chat, FAQ, Photoshare, and much more:

http://www.matronics.com/Navigator?AeroElectric-List

tomcostanza

Joined: 19 Oct 2008
Posts: 49

Posted: Wed Jul 14, 2010 2:49 am Post subject: Re: ANL current limiter location on rear battery installat

Thanks for that Bob.  As often is the case, at least with me, the answer to one question leads to more questions.



 	  Quote: 	
		  Because fat wires in dc electrical systems are 

exceedingly hard to burn, hence there is no 

value added in attempting to protect them. 	




When you say "protect them", I'm not sure what you mean.  Not knowing any better, I might want to protect things near the high current path, or the battery itself, from melting.  Your thoughts?



 	  Quote: 	
		  Hence design goals and installation that suggest extra 

attention to the insulation and support of fat wires combined 

with a crew-controlled shut-off right at the battery 

i.e. battery contactor. 	




Got it.  But again, another question, this time about the wire supplying the main battery buss, for example in Z-11.  This is neither fused, nor does it go through the contactor.  So it isn't shut-off-able.  What's the worst case if this shorts to ground.  Will the wire just burn through as if it were a fuseable link?  What if someone used a heavier wire, anticipating a more robust e-bus?



Please pardon my ignorance, and thanks so much for your patience.

- The Matronics AeroElectric-List Email Forum -

Use the List Feature Navigator to browse the many List utilities available such as the Email Subscriptions page, Archive Search & Download, 7-Day Browse, Chat, FAQ, Photoshare, and much more:

http://www.matronics.com/Navigator?AeroElectric-List

_________________
Clear Skies,
Tom Costanza
-- in year 17 of a 3 year project

nuckolls.bob(at)aeroelect
Guest

Posted: Wed Jul 14, 2010 1:09 pm Post subject: ANL current limiter location on rear battery installations

At 05:50 AM 7/14/2010, you wrote:


<Tom(at)CostanzaAndAssociates.com>



Thanks for that Bob.  As often is the case, at least with me, the 

answer to one question leads to more questions.



BN: Because fat wires in dc electrical systems are

     exceedingly hard to burn, hence there is no

     value added in attempting to protect them.



When you say "protect them", I'm not sure what you mean.  Not knowing 

any better, I might want to protect things near the high current 

path, or the battery itself, from melting.  Your thoughts?



     The idea of adding "protection" to any wire goes

     toward risk reduction. Risk of expense for making

     easily avoided repairs, risk of experiencing a

     hazardous condition. When a wire is strung through

     the innards of a type certificated airplane, there's

     a standard waltz tune played called "Failure Mode

     Effects Analysis" or FMEA for short. This exercise

     considers all the potentials for increased risk and

     the return on investment for adding "protection" or

     risk reduction.



     The fat wires in the engine cranking path and major

     bus feeders were deduced to first be capable of

     very high reliability. Further, when installed with

     good materials and practice, they were very low risks

     to the airframe even if insulation integrity was lost.

     Finally, history has demonstrated that when insulation

     integrity of fat wires is lost, it's likely that the

     fault condition will be relatively "soft" . . . i.e.

     it will arc a lot over time releasing a lot of total

     energy . . . but at current levels too low to open

     the traditional protective devices. Hence the part

     23 rule:



------------

Sec. 23.1357   Circuit protective devices.



(a) Protective devices, such as fuses or circuit breakers, must be

installed in all electrical circuits other than--



    (1) Main circuits of starter motors used during starting only; and



    (2) Circuits in which no hazard is presented by their omission.

------------



     It was just such a fault that is believed to have

     brought down Swissair Flt 111 by setting some combustible

     insulation on fire after some not-designed-for-prime-time-

     insulation cracked and precipitated a "soft" arcing event.



     Rather than rewire thousands of airplanes with parsecs

     of better wire, the Dispensers of Great Wisdom decided

     that a new form of breaker was called for. Hence all

     our friends at Klixon, Eaton, et. als. rushed off to

     the lab to build little microprocessor based digital

     signal detectors into the breakers and train them to

     "listen" for soft faults.



     Now, if you'd like to protect your ship's battery wire

     for such events, I'm sure that somebody makes the breaker

     for the job . . . but it will be big, heavy, expensive,

     and probably run the lifetime of your airplane never

     being called upon to trip.



 > Hence design goals and installation that suggest extra

 > attention to the insulation and support of fat wires combined

 > with a crew-controlled shut-off right at the battery

 > i.e. battery contactor.



Got it.  But again, another question, this time about the wire 

supplying the main battery buss, for example in Z-11.  This is 

neither fused, nor does it go through the contactor.  So it isn't 

shut-off-able.  What's the worst case if this shorts to ground.  Will 

the wire just burn through as if it were a fuseable link?  What if 

someone used a heavier wire, anticipating a more robust e-bus?



     Check the definition of an E-Bus. Just how "robust"

     do you anticipate? See



http://www.aeroelectric.com/PPS/Adobe_Architecture_Pdfs/Z32K.pdf



     for recommendations concerning e-bus feeders FUSED

     at greater than say 10A. In other words, if your e-bus

     alternate feed path gets too big, then a crew-controlled,

     mini-contactor is called for. The battery bus is another

     critter. Since battery busses are located AT the battery,

     their feeders are short. See that (*) on the battery

     bus feeder wires? So yes, if that wire sees a fault

     hard enough to smoke it, the rule 23.1357(a)(2) suggests

     no hazard is presented if the wire is short because

     smoke-risk is low and the event will be short lived

     due to small gage of wire. At the same time, we want

     that path to be very robust (low parts count and well

     installed) so that reliability is high.



Please pardon my ignorance, and thanks so much for your patience.



TA: Bob; this question is similar to what I was asking a few days ago 

but I'm not sure we were on the same page, probably my fault. For 

example the Z8 I'm using does not show any protection device between 

the main bus and the contactor, in my case that is a 7'~8' run from 

the rear mounted battery to the Instrument panel where the main bus 

is located.  The E bus is protected from both ends with fuses in your 

drawing and I understand why but do not understand why there is no 

protection on the feeder to the main bus/fuse block



     BN: Because that should be a "fat" wire well installed.

     This same pathway falls under the FAR23 category cited

     above. Legacy design goals have supported the notion

     for many decades and hundreds of thousands of airplanes.



     You won't find this pathway fuse or breaker protected

     in any type certificated SE aircraft on a par with your

     grandpa's C-170.  The e-bus alternate feed wire is

     longer, smaller gage and capable of being fault-powered

     from either end. Further, we don't want a fault on the

     alternate feed path wire to open both the alternate and

     normal feed paths. Hence NO protection in the normal path

     and SHORT wires there too.



     Bob . . .

- The Matronics AeroElectric-List Email Forum -

Use the List Feature Navigator to browse the many List utilities available such as the Email Subscriptions page, Archive Search & Download, 7-Day Browse, Chat, FAQ, Photoshare, and much more:

http://www.matronics.com/Navigator?AeroElectric-List

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