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ANL current limiter location on rear battery installations

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nuckolls.bob(at)cox.net
Guest

Posted: Thu Nov 06, 2008 7:39 am Post subject: ANL current limiter location on rear battery installations

At 02:13 PM 11/4/2008 -0800, you wrote:

 	  Quote: 	
		  

 > > That interested me since in another context I had been wondering 

 about ANL location. I was

 > puzzling why in Z-24 the ANL was shown downstream of the internally 

 regulated alternator

 > isolation contactor. My thinking was that by having it close to the 

 alternator it would blow if

 > anything (including the isolation contactor) ran amok downstream. What 

 is the advantage of

 > having it at the main bus end ?

 > A The ANL limiter (fat fuse) is there to protect the alternator b-lead 

 wire . . . the source of

 > energy that places this wire at risk is NOT the alternator but the 

 battery. An alternator is

 > incapable of putting out enough current to open its own b-lead 

 protection while the battery is

 > capable of fat-wire faults approaching 1000 amps. So, while selecting 

 the SIZE of the protection

 > device is driven by alternator output capability, selecting LOCATION is 

 associated with the risk

 > source . . the BATTERY.

 > On some occasions, we have fat wires that can source a fault from 

 either end . . . in

 > which case, you might have a limiter at both ends of the same 

 conductor. However, I've never

 > encountered a situation like this for small aircraft. Bob . . .

The above is in the FAQ's for the list. I would like to clarify a bit 

further: If the battery is the current feeder in a frayed B-lead scenario, 

and the battery is in the tail (with the master contactor and eng power 

bus (FI auto)), then wouldn't the ANL current limiter be located back 

there as well? If so, between the battery and contactor, or between the 

contactor and main bus loads?


	


   Actually, a frayed b-lead is likely to arc a bit

   and "burn clear" like other battery connected fat

   wires in airplanes. The ANL is risk mitigation for

   shorted diodes . . . an exceedingly rare event

   in modern alternators . . . ESPECIALLY those

   supported by due diligence on the part of the

   designers/rebuilders.



   Small aircraft do not get circuit protection in

   the battery feeders to the starter and distribution

   busses. Only the alternator b-leads got the ANLs

   (or similar) to deal with the internally shorted

   alternator but that risk too has steadily diminished

   over the years.



   If you choose to include a limiter in the b-lead,

   then about any place in the b-lead wire is okay

   but best at the end away from the alternator.



   Bob . . .



        ----------------------------------------)

        ( . . .  a long habit of not thinking   )

        ( a thing wrong, gives it a superficial )

        ( appearance of being right . . .       )

        (                                       )

        (                  -Thomas Paine 1776-  )

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

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Beemer

Joined: 26 Aug 2006
Posts: 87
Location: Middle Georgia

Posted: Thu Nov 06, 2008 2:46 pm Post subject: ANL current limiter location on rear battery installations

Ahhh, I see. I took your stance to be that the ANL fuse is a much required

item in our OBAM systems. I now take you to mean them to be fine to add, but

not wholly necessary to safety. Correct?



So having no circuit protection in both/either the alternator and/or starter

circuit in a competently-designed system is ok? Hard to swallow, but that's

why I'm asking.



I have an ANL fuse and holder, but in the interest of fewer connections

(hence less to go wrong), I may forego it.



Bradley



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Beemer
KF2 (and now an M3!)
Suzuki G10 three-banger
Middle Georgia

tomcostanza

Joined: 19 Oct 2008
Posts: 49

Posted: Tue Jul 13, 2010 2:53 am Post subject: Re: ANL current limiter location on rear battery installat

Bob,



With respect to:



 	  Quote: 	
		  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?



Thanks,

-Tom

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

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