Today's Message Index:
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1. 09:36 AM - Re: Question on Z-11, Main Buss Feed (Vern Little)
2. 07:13 PM - Re: Re: Polyfuses (Robert L. Nuckolls, III)
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| Subject: | Re: Question on Z-11, Main Buss Feed |
In many applications, this connection is short (<6") or even a copper
busbar between contactors. In this case, no additional protection is
required.
In other applications this connection is much longer and should have
primary protection. Depends on your configuration
Vern Little
----- Original Message -----
From: Don Morrisey
To: aeroelectric-list@matronics.com
Sent: Monday, March 16, 2009 8:43 AM
Subject: AeroElectric-List: Question on Z-11, Main Buss Feed
Howdy Listers,
I am using Z-11 as the basis for my wiring architecture. My question
is on the 6 AWG wire between the battery contactor and the main buss.
In my installation this wire will be running back through the firewall.
Z-11 shows no protection for this wire but does show an ANL 60 current
limiter between the starter contactor and the alternator.
A similar VFR elec drawing on the B&C website shows the 6 AWG wire
between the battery contactor and the main buss being protected with a
ANL 60 current limiter and also shows another ANL 60 current limiter
between the starter contactor and the alternator.
So for this type of setup are two ANL 60's required or one???
Thanks. Don...
www.donsbushcaddy.com
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At 11:01 AM 3/16/2009, you wrote:
>
>Jose,
>
>Would you bet your life on a floppy adaptation on your Lightspeed
>ignition? Perhaps yours is just cursory interest. I'm sure you can jam
>pennies in there if you like. There's no reason that would not work.
>
>Right now I am using Bob's proposed schematic which adds two in-line 30
>amp fuses in front of two 5 amp breakers. He also considered two relays
>in place of the 30 amp ATC's.
>
>I guess my deal is how am I going to get at those 30 amp in-line fuses
>and still keep my eye on flying the airplane. I suppose if you blow two
>30 amp fuses behind the two five amp breakers suggested by LSA, you're
>already in deep doo-doo.
>
>I wanted to ask Bob why the 30 amp fuse? Couldn't we get-a-way with
>something lighter, say 20 amp?
No . . . well . . . depends. When you compare the various
time-to-trip curves for over-current protection, there's
a often overlooked consideration - response time.
Magnetic breakers and electronic breakers can be exceedingly
fast. Most thermal devices are tailored to a design goal.
For example, you can get 5A fast blow fuses and 5A slow blow
fuses. Thermal breakers as a class of circuit protector
are generally VERY slow compared to a fuse of the same ratings.
So, hook a 5A breaker in series with a 5A fuse and hit
the feeder with a short. The fuse opens every time. Keep
increasing the fuse size and you'll find that it probably
takes a 20A fuse to out-muscle a 5A breaker. So for good
headroom in the design, we make the upstream protection MUCH
more robust than the downstream protection.
This is why ANL limiters are exceedingly robust. They are
intended to be part of a distribution system wherein
no single downstream feeder protection can trip an
upstream feeder protection . . . even if the upstream
feeder is running say 100% of system average. See:
http://aeroelectric.com/Mfgr_Data/Fuses_and_Current_Limiters/Bussman/ANL_Specs.pdf
Note that the "rating" for a current limiter
includes enough headroom that it can be loaded to
100% of rating and still stand off a downstream
breaker or fuse trip.
I'm working and accident right now wherein redundant
feeders to a common bus were protected with fuses
and not current limiters with an unfortunate outcome.
This is IMPORTANT. This is why my 5A crowbar breaker
feed is protected upstream with a fusible link.
The breaker will always open before the fusible link.
Bob . . .
>What I would like to see is a polyfuse mounted into an ATC fuse shell,
>preferably one of those ATC types that have the LED already built in to
>indicate a fault. By so doing, you could use the readily available ATC
>fuse blocks, thereby retaining the option to simply pull the Polyfuse
>out when you need to. For my first attempt, I was going to try a little
>surgery on an ATC fuse package and epoxy a Polyfuse to it..., but I have
>not yet taken the chance to play with this. My guess is that it may be
>only a matter of time before some Taiwanese manufacturer starts putting
>polyfuses in the ATC fuse format.
Read the discussions on my website about the downside for
having self resetting circuit protection. The Polyfuse is
not a drop-in replacement for fuses or breakers. The
design task I'm working right now goes to a very specific
application of the Polyfuse. Similarly, fusible links
are not to be used in place of breakers or fuses without
sifting all the simple ideas and making sure design
goals are being met.
Polyfuses ARE used in automobiles. Seat adjuster motors
and widow riser motors are commonly protected with Polyfuses.
But you can be sure that if there were good value in
using them everywhere, the car guys would have ditched
the fuse block a long time ago. They ARE NOT directly
interchangeable technologies.
Bob . . .
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