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ANL current limiters

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jetfr8t(at)hotmail.com
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Posted: Sat May 20, 2006 2:02 pm Post subject: ANL current limiters

I understand that the ANL current limiter installed by the starter contactor 

protects the "B" lead from a large current draw from the battery.



My somewhat philosophical question is this:  Why not use one to protect the 

fat wire from the battery contactor to the main power distribution bus? Of 

course, the battery contactor can be manually opened, but there is 

recognition time and action required. Also, in the case of an accident that 

damages where that wire penetrates the firewall, there is no automatic 

protection for that wire.



Craig Mac Arthur



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jetfr8t(at)hotmail.com
Guest

Posted: Sun May 21, 2006 7:41 am Post subject: ANL current limiters

I found the answer to my own question. Bob answered it in another form a few 

years ago. It's in a FAQ file, but it took me a couple of sessions to find 

it. Here is the exchange:



Circuit protection for big wires



 	  Quote: 	
		  I'm building an RV-8 with an aft mounted battery. I note that the 

electrical system schematics

in Bob's Appendix Z don't show any circuit protection on the big wires going 
	


to the starter

contactor, and to the main fuse block. Both those wires will be quite long 

in my installation,

and I'm concerned about the lack of protection. I have visions about a major 

smoke in the

cockpit triggered by something chafing one of those cables, or a post crash 

fire triggered by

arcing from the broken, but still live cables. I'm seriously considering 

mounting the starter

contactor aft, beside the main contactor, next to the battery. That way the 

cable going to the

starter will only be live during a start. That still leaves the long cable 

going to the main fuse

block. Would it be practical to either make a large capacity fuselink, or 

install a large CB or fuse

in that line? I would rather deal with an inflight short by seeing 

everything go black, and then

selecting the essential bus alternate feed, than scrambling to throw the 

battery master in the

middle of a major smoke in the cockpit event. Comments?



A The interesting thing about FAT wires in an airplane is that while they 

carry the greatest

energy and potential for current flow, they represent the least hazard with 

respect to electrical

faults. It's very difficult to create a hard fault on one of these wires. 

Study the wire's pathway

through the airframe. What items of structure or loose systems might come to 

bear on one of

these wires with sufficient force to compromise its insulation and produce a 

hard short? If the

possiblity of such a scenario exists, it's easier to design out the 

possibility than to provide

fusible protection for the wire.

If a fault does develop, it's most likely a "soft" fault that causes some 

arcing (battery

wires rubbing the edge of a lightening hole is a good example) that simply 

burns away the area

it touches without bringing down the system. Over 200,000 airplanes have 

been built in this

country without fusible protection of the FAT wires. Like wing struts, 

propeller shafts, flight

controls . . . it's relatively easy to product very low probability of 

failure by design.

Another reader suggested that the battery contactor should be a hot-side 

control as

opposed to cold-side control. A little study will show that it doesn't make 

any difference which

side of the contactor is switched. A severe fault on the feedline downstream 

of the contactor

will load the battery to a point perhaps low enough to cause the contactor 

to open. When it

does, battery voltage comes back up and the contactor recloses. This sets up 

a scenario for

"buzzing" of the battery contactor which almost always results in a welding 

of the contactor.

It's really easy to protect he wire from faults to the degree that fuses 

and/or breakers are not

necessary.



 	  Quote: 	
		  >I have visions about a major smoke in the cockpit triggered by something 

>chafing one of

those cables, or a post crash fire triggered by arcing from the broken, but 
	


still live cables.

A If it's an impending crash that you're preparing for, ALL SWITCHES OFF is 

a good thing

to do before coming into contact with the earth.

 	  Quote: 	
		  I'm seriously considering mounting the starter contactor aft, beside the 

main contactor, next to

the battery. That way the cable going to the starter will only be live 
	


during a start.



A If you do this, you'll lose the advantage of tying alternator b-lead power 

feeds to the

starter contactor and avoid bringing the noisiest wire in the airplane into 

the cockpit for

attachment to the bus along with your audio system and radios. The power 

distribution

diagrams in Appendix Z have evolved over more than 12 years of refining 

ideas and

philosophies on owner built and maintained (OBAM) aircraft. I won't say that 

they're infallible

nor would I suggest that we won't deduce good reasons for changes in the 

future. Given the

rich database of history about fat wires in little airplanes and our 

understanding of the

designed in failure modes of certified airplanes (see Chapter 17 of the 

AeroElectric

Connection) I would counsel caution about major departures the philosophies 

presently

illustrated by those drawings. Bob . . .

------



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