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Fuse rating on AWG 20 wire with AeroLED strobes?

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nuckolls.bob(at)aeroelect
Guest

Posted: Fri Nov 30, 2018 1:22 pm Post subject: Fuse rating on AWG 20 wire with AeroLED strobes?

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
Guest

Posted: Mon Dec 03, 2018 8:27 am Post subject: Fuse rating on AWG 20 wire with AeroLED strobes?

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: 157

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

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....

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

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