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Fuse rating on AWG 20 wire with AeroLED strobes?

 
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nuckolls.bob(at)aeroelect
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PostPosted: Fri Nov 30, 2018 1:22 pm    Post subject: Fuse rating on AWG 20 wire with AeroLED strobes? Reply with quote

At 07:59 PM 11/29/2018, you wrote:
Quote:
I found the Bussmann ATC fuse specs, looks like a 7.5A fuse should be fine for brief pulses up to 8.5A from the strobes, and should also be acceptable for protecting the wire notwithstanding the AC43.13 table that says to use 5A fuse for AWG 20. So I think I answered my own question with more research, but please advise if you see anything wrong with this approach.

20AWG is protected at 7A in most
TC aircraft. But keep in mind that
UNLIKE circuit breakers, fuses can
be "hammered" by short duration
transients that exceed the fuse rating.
"Hammering" contributed to the
sequence of events that brought
N811HB down . . .

https://tinyurl.com/msfmldj

There are no risks associated with
up-sizing the 20AWG protection to
the next level. I'd be quite comfortable
with 10A for your application.



Bob . . .


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nuckolls.bob(at)aeroelect
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PostPosted: Mon Dec 03, 2018 8:27 am    Post subject: Fuse rating on AWG 20 wire with AeroLED strobes? Reply with quote

At 10:40 PM 12/2/2018, you wrote:
Quote:
When you say 'unlike circuit breakers'...what do you mean? Would the CB trip more quickly than a fuse would blow, or would it be more tolerant of what you're calling 'hammering'?

Both devices operated based on temperature
rise caused by current generated heating.

Breakers are bi-metal 'thermostats' . . . they
trip at a calibrated temperature which is
NOT affected by previous excursions up to the
edge of tripping. A CB is good for thousands
of trip cycles with very little change in
performance.

Fuses are pieces of metal designed to heat up
and melt at a predictable temperature. A
characteristic that can be tailored to a
current flow through the 'sensing' metal.

But operating a fuse at its recommended
'limit' (75% of rating) produces SOME
heating. This means that a fuse under
normal load is already some way up the
I(squared)R trip ladder when it is presented
with transient 'overloads'. These little pushes
into trip territory can cause mini-melts to
occur in the fusible material which has the
effect of lowering the fuse's rating. Repeated
'hammering' will seriously compromise predictability
of that fuse to carry normal loads.

When N811HB lost both ignition systems on
base to the runway, the single fuse carrying
BOTH ignition systems had been loaded to
more than 75% on several occasions with
short excursions to over 100%. Compromised
fuse predictability combined with poor
choices in architecture to kill the
engine.

Quote:
And your last statement 'there are no risks associated with up-sizing the 20 AWG protection to the next level', do you mean using a 10A fuse with 20 AWG, or are you saying to increase wire gauge to 18?

'20AWG protection' is classically pegged
at 7A. Raising it to 10A or even 15A
in this situation will stand off the 'hammering'
effects. It is not a risky thing to do. It
will eliminate the possibility of nuisance
tripping.

Given that the lights do not represent a
critical flight system, you could leave the
7A fuse in place and see how it goes. If
you experience a nuisance trip, up-size the
fuse.

Let us know what you discover . . .



Bob . . .


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zwakie



Joined: 03 Aug 2009
Posts: 155

PostPosted: Mon Dec 03, 2018 1:34 pm    Post subject: Re: Fuse rating on AWG 20 wire with AeroLED strobes? Reply with quote

I did a little experiment on Saturday after having finalized the wiring on my strobes. Specs ask for 5A fuse, so I put a 5A fuse in and lighted up the strobes.

Having read this post on Friday night, I was expecting the fuse to blow, but I was very surprised to see that happen so quickly (literally well within 10 seconds).

A 7.5A fuse did hold while keeping the strobes on for more than 2-3 minutes, but to be safe I 'upped' the fuse to 10A and will 'up' the two short leads from fuse block to switch to the beefy wiring that a previous owner has put in to match the 10A fuse. I will also review the previously determined fuse-sizes for other circuits that (can possibly) carry transient loads.

This was an educational experiment.... Wink


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Marcel Zwakenberg
Europa XS TG || 912ULS || PH-SBR
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