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Circuit protection - Amps

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trigo(at)mail.telepac.pt
Guest

Posted: Fri Feb 26, 2010 7:16 am Post subject: Circuit protection - Amps

I had Whelen Nav lights in my RV-9A, in a circuit protected by a 7Amp PTC.  

I replaced those by new LED nav lights, which draw about 0.5A, and should need a protection device not bigger than 1A.  

   

If I leave the circuit with the same 7A protection, what are the implications?  

   

Carlos  

       [quote][b]

recapen(at)earthlink.net
Guest

Posted: Fri Feb 26, 2010 8:07 am Post subject: Circuit protection - Amps

The fuse is supposed to protect the wire.  Shouldn't be an issue.  You can downsize the fuse based on expected draw and any unexpected draw will pop the fuse.



I had similar Whelens which I replaced with LED units...kept the same fuse as I am carrying one of the old units (with both lenses) as a backup in case of hangar rash at a destination....I can easily swap out to my backup and the wiring is already set-up.



My .02



Ralph





--

dennis.glaeser(at)hp.com
Guest

Posted: Fri Feb 26, 2010 8:56 am Post subject: Circuit protection - Amps

From: Carlos Trigo (trigo(at)mail.telepac.pt)

Date: Fri Feb 26 - 7:16 AM

I had Whelen Nav lights in my RV-9A, in a circuit protected by a 7Amp PTC.

I replaced those by new LED nav lights, which draw about 0.5A, and should

need a protection device not bigger than 1A.

If I leave the circuit with the same 7A protection, what are the

implications?

Carlos

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

Did you replace the wires?  If not, then virtually no implications.

If you put in smaller wires, then you need to use a smaller protection device.

Protection is for the wires - so they can't overheat in case of a short.

Dennis

trigo(at)mail.telepac.pt
Guest

Posted: Fri Feb 26, 2010 10:28 am Post subject: Circuit protection - Amps

Thanks Dennis  

Just keep answering my questions in all forums (fora) in which we both lurk … J  

   

Carlos  

   

   

[quote] --

nuckolls.bob(at)aeroelect
Guest

Posted: Fri Feb 26, 2010 2:50 pm Post subject: Circuit protection - Amps

At 09:01 AM 2/26/2010, you wrote:

  	  Quote: 	
		  I had Whelen Nav lights in my RV-9A, in a circuit protected by a 7Amp PTC.

 I replaced those by new LED nav lights, which draw about 0.5A, and should need a protection device not bigger than 1A.

  

 If I leave the circuit with the same 7A protection, what are the implications? 	


  If you didn't downsize the wires, then none. The PTC

  is to protect wires.



  Bob . . .    [quote][b]

trigo(at)mail.telepac.pt
Guest

Posted: Sat Feb 27, 2010 12:46 pm Post subject: Circuit protection - Amps

Thanks Ralph  

I did the same as you and kept the old units (which I’ll probably have to toss when my bird will hit the weighing scales ... L), so I am maintaining the same protection.  

Carlos  

   

   

[quote] --

Eric M. Jones

Joined: 10 Jan 2006
Posts: 565
Location: Massachusetts

Posted: Sun Feb 28, 2010 7:05 am Post subject: Re: Circuit protection - Amps

Assuming the circuit protection is to protect the wire...(this could be argued but not now), let's examine some situations:



There is a specification called "Intrinsically Safe" which specifies that no circuit protection at all is needed if the circuit cannot exceed (depends on other factors...) about 0.2 Amperes. Google "intrisically-safe wiring" Determining the exact limit would make a great science project for an adventurous High School student. It involves --How many Joules does it takes to set fire to gasoline vapour?....Ah my misspent youth...!



But the resulting lesson here is that you can limit the current through the wire at the voltage source, perhaps the bus bar, with a suitable resistor and never see currents of >0.2A, for a variety of useful devices like LEDs, timers, clocks, sensors, indicators, memories and I suppose, some instruments.



Take note that intrinsically safe wiring requires special consideration when used in bundles....and that's usually an issue in aircraft wiring.



It might be true that switch-breakers are less reliable than switches. But in my airplane I will use switch-breakers instead of switches AND fuses OR breakers AND the associated wiring to connect everything together. 



Resettable solid state fuses also have a place, and they are perfect, if indeed, the wire is what you want to protect.



Modern auto fuses are worlds better than glass-tube AGC and similar contraptions. (Does Mercedes still use those awful ceramic fuses?), but it is a dictum of technological progress that the very best old technology appears just before it becomes completely extinct.



Fuses have had their day. You can't really make up a reason to have them at all.

_________________
Eric M. Jones
www.PerihelionDesign.com
113 Brentwood Drive
Southbridge, MA 01550
(508) 764-2072
emjones(at)charter.net

khorton01(at)rogers.com
Guest

Posted: Sun Feb 28, 2010 7:51 am Post subject: Circuit protection - Amps

On 2010-02-28, at 10:05 , Eric M. Jones wrote:



 	  Quote: 	
		   

 

 Fuses have had their day. You can't really make up a reason to have them at all.


	


Cost is a pretty important reason to many folks.  If a technology (e.g. fuses) does an perfectly adequate job, why pay more for some other technology (switch breakers) that also does a perfectly adequate job?



--

Kevin Horton

RV-8

Ottawa, Canada

http://www.kilohotel.com/rv8

mrspudandcompany(at)veriz
Guest

Posted: Sun Feb 28, 2010 8:24 am Post subject: Circuit protection - Amps

Fuses have had their day. You can't really make up a reason to have them at

all.



--------

Eric M. Jones

		Totally incorrect. . . .   There have been several reasons,

sited on this forum, for the use of fuses as opposed to breakers and other

circuit protection devices. Cost, weight, availability, ease of use, are

only a few of the reasons to use fuses.



		I do agree that they may not be for every application, but

for the vast majority of OBAM aircraft circuit protection applications, they

meet the needs.



		Roger

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nuckolls.bob(at)aeroelect
Guest

Posted: Sun Feb 28, 2010 10:54 am Post subject: Circuit protection - Amps

 	  Quote: 	
		  Modern auto fuses are worlds better than glass-tube AGC and similar 

contraptions. (Does Mercedes still use those awful ceramic fuses?), 

but it is a dictum of technological progress that the very best old 

technology appears just before it becomes completely extinct.


	


    Are you expecting fuses to become extinct? They have

    been around since long before Edison began to market

    electron-flow. The fuse catalogs are noteworthy for

    the absence of older products but still more noteworthy

    for the increase in numbers of new products.



    Not sure where "intrinsic safety" came from. While that

    too is a study in energy management under fault conditions,

    I'm not sure how it matters in conducting FMEA on our

    own design goals. Finally, whether Mercedes uses or does

    not use any product of any era is certainly not germane

    to our deliberations.



    But I cannot foresee how an elegantly simple, weak-link in

    the chain of electron flow will ever be totally replaced by

    panel-mounted thingys with handles on them.



 	  Quote: 	
		  Fuses have had their day. You can't really make up a reason to have 

them at all.


	


    "Make up a reason"? How about a low cost-of-ownership

    solution for meeting design goals with an attractive

    outcome of FMEA. It's a certainty that switch-breakers

    do not figure favorably in that equation . . . nor do

    solid state breakers. If cost, weight, panel space,

    and time to install are not components of your personal

    design goals, so be it. But to suggest that others on

    the List are somehow outdated because they do not embrace

    your goals borders on elitism . . . don't you think?



    Bob . . .

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