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Landing Light Pops Breaker

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Dennis Johnson

Joined: 10 Jan 2006
Posts: 89
Location: N. Calif.

Posted: Sun Aug 17, 2008 8:45 am Post subject: Landing Light Pops Breaker

The April issue of EAA's Sport Aviation magazine had a "technical"  column about an airplane that regularly popped the landing light circuit  breaker.  The author said he found several high resistance connections in  the circuit and solved the problem by cleaning those connections and making them  low resistance.  The current issue, August, has an excellent letter to  the editor that questions how a high resistance connection (presumably not at  the circuit breaker's terminals) could cause a high amperage condition that  would trip the breaker. 

  

 When I read the original article, I also didn't understand how a high  resistance connection could increase the amperage in a landing light circuit and  just figured that it was yet another example of bad advice in a Sport Aviation  "technical" article.  I enjoy the magazine, but I have found so much faulty  technical information in it that I don't pay much attention to their technical  articles.

  

 However, the author's response to the question reiterated  that a high resistance connection can increase the amperage enough to cause the  circuit breaker to trip.  

  

 I'm a beginner on things like this, but I've studied Bob's book and this  newsgroup since beginning my homebuilt project five years ago, and  cannot understand how the author could be right.  However, I've  learned from this list that some components try to consume constant power and  that reducing the voltage to them increases the current in order to keep  the power constant.  But I think a tungsten filament landing light is a  simple resistor (although very temperature sensitive).  I think if I put a  resistor in series with a landing light, the amps would go down, not up, and the  amount of light produced would decrease.  

  

 So what's the truth?  And why?

  

 Best,

 Dennis

 Lancair Legacy, 150 hours, did all my own wiring (also assembly, body-work,  paint, instrument panel, etc)   

    [quote][b]

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Joemotis(at)aol.com
Guest

Posted: Sun Aug 17, 2008 10:12 am Post subject: Landing Light Pops Breaker

That's easy, how about if the landing light is turned on and is  pulling maximum amps on a cold filament,

 meanwhile that high resistance connection upstream is starting to get hot  and arc and spark...Those values added together are greater than the   circuit breakers rating..

  

 Joe Motis

 Do not archive



Looking for a car that's sporty, fun and fits in your budget? Read reviews on AOL Autos.

  [quote][b]

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nuckolls.bob(at)cox.net
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Posted: Sun Aug 17, 2008 4:20 pm Post subject: Landing Light Pops Breaker

At 09:38 AM 8/17/2008 -0700, you wrote:

 	  Quote: 	
		  The April issue of EAA's Sport Aviation magazine had a "technical" column 

about an airplane that regularly popped the landing light circuit 

breaker.  The author said he found several high resistance connections in 

the circuit and solved the problem by cleaning those connections and 

making them low resistance.  The current issue, August, has an excellent 

letter to the editor that questions how a high resistance connection 

(presumably not at the circuit breaker's terminals) could cause a high 

amperage condition that would trip the breaker.



When I read the original article, I also didn't understand how a high 

resistance connection could increase the amperage in a landing light 

circuit and just figured that it was yet another example of bad advice in 

a Sport Aviation "technical" article.  I enjoy the magazine, but I have 

found so much faulty technical information in it that I don't pay much 

attention to their technical articles.


	


   Understand . . . and agree. I quit writing for S.A. some

   years ago after an article I reviewed for Jack Cox wound up

   getting published anyhow. See:



http://aeroelectric.com/articles/rules/review.html



   S.A. used to be the flagship publication of EAA and

   dedicated to the best we know how to do. Sadly, it

   now seems to be a cash cow for advertising sales

   and a venue where wanna-be's get published. When

   you have a high-volume, directed-market technical

   publication that won't even pay a rudimentary writer's

   fee or seek peer review of articles . . . well . . .

   what you see is what you get.

 	  Quote: 	
		  

However, the author's response to the question reiterated that a

high resistance connection can increase the amperage enough to

cause the circuit breaker to trip.




	


   Good for you! I don't know how many times I've seen

   words in ostensibly authoritative works that suggest

   "cleaning and re-tightening" junctions in order to

   cure a variety of ills . . . including the opening of

   breakers or fuses. See bottom of page 4 and top of

   5 on . . .



http://www.aeroelectric.com/articles/richter/response_2.pdf



   and upper left corner of page 2



http://www.aeroelectric.com/articles/Wired_for_Disaster.pdf



   These words are written by folks who's esteem in

   our society has been artificially elevated for

   reasons other than their understanding of physics

   and practical processes.



   Unless a high resistance junction is showing signs

   of impending failure due to heating . . . then its

   effects are limited simply to increasing path resistance.



   An increase in path resistance LOWERS overall power

   consumed by devices on that pathway . . . but indeed

   may concentrate power dissipation (heating) in a

   localized area (joints, worn contacts) that precipitate

   a failure at that location . . . but it certainly doesn't

   cause breakers to open. Unless . . .



   Here's a case where a switch failure DID first manifest

   itself by opening a breaker . . . when it was TURNED

   OFF. See:



http://www.aeroelectric.com/articles/Anatomy_of_a_Switch_Failure/Anatomy_of_a_Switch_Failure.html



   Of course, this was a double failure . . . the switch

   had not yet failed to conduct in the closed position

   . . .  but heating effects induced a second failure.

   A distortion of the internal parts that ultimately

   produced the short that tripped the breaker.



   We've studied on these pages how an increased path

   resistance contributes to instability of the voltage

   regulator on alternators where field supply current

   and bus voltage sense share the same pathway.



   But aside from this demonstrable effect, I'm aware

   of no other case where increased path resistance

   due to real or imagined join degradation will cause

   breakers or fuses to nuisance trip.



 	  Quote: 	
		  I'm a beginner on things like this, but I've studied Bob's book and this 

newsgroup since beginning my homebuilt project five years ago, and cannot 

understand how the author could be right.  However, I've learned from this 

list that some components try to consume constant power and that reducing 

the voltage to them increases the current in order to keep the power 

constant.  But I think a tungsten filament landing light is a simple 

resistor (although very temperature sensitive).  I think if I put a 

resistor in series with a landing light, the amps would go down, not up, 

and the amount of light produced would decrease.


	


   Correct. The resistance of the lamp will decrease as

   it gets dimmer . . . but not in inverse proportion

   to decrease in voltage . . . i.e. current draw

   STILL goes down, total power consumed still goes

   down, just not like it would if a simple resistive

   load were involved.

 	  Quote: 	
		  So what's the truth?  And why?


	


   You were pretty close to it when your skepticism

   based on what you understood raised some flags  . . .

   and you asked questions. With respect to the original

   article: if one assumes the author of the original

   article was not lying, then SOMETHING he did caused

   the problem to go away. He may have moved some

   wiring that cleared an intermittent short . . . but

   believed that cleaning the connections was what

   really did it. We'll never know since the "crime scene"

   was not properly processed and is now probably compromised

   to a degree that true root cause is no longer discoverable.

   But a goodly portion of 135,000 EAA members will join him

   in his erroneous beliefs.



   Bob . . .

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rv-9a-online(at)telus.net
Guest

Posted: Sun Aug 17, 2008 6:04 pm Post subject: Landing Light Pops Breaker

Bob, and others.

There are a class of devices that can cause increased current with increased

resistance in the power circuit.

Some devices use switching power supplies which are constant-power devices.

What this means is that if input voltage increases, the current drain

decreases and vice versa.  This negative resistance effect can cause

instability, and designers need to take it into consideration.



The best example I have is that my strobe power circuit had a faultly

(resistive) switch which caused current in the circuit to increase, and

tripped the breaker.  The strobe supply was attempting to make up for the

voltage drop in the switch by increasing it's drain current.  The fix was to

replace the switch and the burned terminals.  I had two switches fail in

similar manners in different within the first year of operation of my RV-9A.

One was returned to the vendor for failure analysis, but I never heard back.



I first experienced this type of problem when designing switch mode power

supplies for cyclotron equipment. Everything worked fine until battery

internal resistances increased, then voltage/current oscillations would

start.  Cheap fix was to put a fixed load on the input of the power supply

and waste a lot of energy to stabilize the load.  Not recommended for

aircraft!



Vern Little



--

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nuckolls.bob(at)cox.net
Guest

Posted: Sun Aug 17, 2008 6:42 pm Post subject: Landing Light Pops Breaker

At 07:02 PM 8/17/2008 -0700, you wrote:

 	  Quote: 	
		  

<rv-9a-online(at)telus.net>



Bob, and others.

There are a class of devices that can cause increased current with increased

resistance in the power circuit.

Some devices use switching power supplies which are constant-power devices.

What this means is that if input voltage increases, the current drain

decreases and vice versa.  This negative resistance effect can cause

instability, and designers need to take it into consideration.


	


   Sure . . . but let's put this into perspective. These are

   not unlike the oscillating voltage regulators we discussed

   earlier. But let's consider a system designed to run on 14v

   at 3A (42w input) and protected with a 5A breaker. If series

   resistance of a switch or other joint dropped say 2 volts,

   then the system would compensate and current would rise

   to 42/12 = 3.5A



   From the outside looking in, things would appear normal

   but the high resistance location would now be dissipating

   2 x 3.5 = 7 watts! This is probably 4 or 5 times more than

   the power it took to slowly toast the switch in the failure

   analysis I cited. The situation would not last long . . . and

   still wouldn't pop the breaker.



   Suppose we have a 28v 42w system that runs 1.5A. Hmmm . . .

   I suppose we could protect that line with a 2A breaker

   but those things are expensive . . . the 22AWG feeder

   is still happy at 5A breaker. But yeah, let's say a

   3A breaker. Now some high resistance condition that would

   push the 3A breaker has to drop 14v at 3A or 42 watts

   of dissipation! It's toasted in a hurry . . .



 	  Quote: 	
		  The best example I have is that my strobe power circuit had a faultly

(resistive) switch which caused current in the circuit to increase, and

tripped the breaker.  The strobe supply was attempting to make up for the

voltage drop in the switch by increasing it's drain current.  The fix was to

replace the switch and the burned terminals.  I had two switches fail in

similar manners in different within the first year of operation of my RV-9A.

One was returned to the vendor for failure analysis, but I never heard back.


	


   I recall that you sent an S700-2-10 switch back to B&C

   for loose terminals. In that case, voltage excursions caused

   by the loose terminals were probably upsetting the regulation

   stability so badly as to cause a true ov condition that tripped

   the system and opened the breaker. Breaker popping was a

   secondary effect and not caused directly by an increased load

   by a constant power device.



   Were you the one that sent me the strobe switch cited

   in the failure analysis I posted?



http://www.aeroelectric.com/articles/Anatomy_of_a_Switch_Failure/Anatomy_of_a_Switch_Failure.html



   Again, breaker popping was a secondary effect

   of the overheating of the switch that deformed the contact

   support teeter-totter. These switches are normally good

   for 10A or more in the closed state . . . but this switch

   obviously failed at the pivot bridge as indicated by

   heating patterns . . . this was happening with no more

   than 1/2 volt of drop internal to the switch. So if your

   strobe needed 5A average at 14v it would need only 5.2

   amps at the "reduced" voltage. Yet the 5A at 1/2 volt

   was 2.5 watts of heating on the switch's innards. Not

   enough to 'smoke' it but enough to heat the components

   to eventual failure but it took a number of hours but didn't

   cause smoke in the cockpit.

 	  Quote: 	
		  I first experienced this type of problem when designing switch mode power

supplies for cyclotron equipment. Everything worked fine until battery

internal resistances increased, then voltage/current oscillations would

start.  Cheap fix was to put a fixed load on the input of the power supply

and waste a lot of energy to stabilize the load.  Not recommended for

aircraft!


	


   That was a true negative resistance problem but those

   generally happen under conditions that are far below those

   needed to trip breakers that have any headroom at all.



   Your perception of the behavior of constant-power systems

   is accurate but increased resistance situations sufficient

   to put a switch at risk are too small to be a likely

   cause of breaker popping.



   Bob  . . .

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