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nuckolls.bob(at)cox.net
Guest

Posted: Sat Sep 13, 2008 6:51 am Post subject: Toggle Switches with Fast-On Tabs

Let's lay out all the simple ideas behind the design

fabrication and operation of a toggle switch fitted with

fast-on tabs for the purpose of discovering a failure mode

and deducing a remedy. I'll refer the serious students to

the quick-n-dirty sketch at:



http://aeroelectric.com/Pictures/Switches/Toggle_Switch_with_Fast-On_Tabs.jpg



This cross-section will allow us to trace the path of

current flow through the switch as follows:



Electrons come in via Wire(B) and pass to the fast-on

terminal through wire grip(1) and then on to the fast-on

tab(C) through a high-pressure metal-to-metal

terminal/tab interface(2). Current must then pass

through a high-pressure, metal-to-metal joint(3) to

the contact(D). A low-pressure, metal-to-metal

contact/contact interface(4) carries current to

the teeter-totter(F) via another high-pressure,

metal-to-metal joint(5).



Current flows through the teeter-totter to a

low-pressure, metal-to-metal sliding joint(6)

at the top of the saddle(G) and then down to

a high-pressure, metal-to-metal joint at the

saddle to rivet interface(7), through the plastic

housing(A) to another high-pressure, metal-to-metal

staked joint( and thence on to the fast-on-tab.



 From the fast-on-tab, we find another high-pressure,

metal-to-metal joint at the terminal/tab interface(9)

and finally, another high-pressure, metal-to-metal

joint at the terminal's wire grip(10).



So if you count them up, there are TEN, conductor-

to-conductor joints that carry current through this

switch installation when the switch is closed.



By inspection we can deduce that the weakest links

in this conductor chain are at the low-pressure,

metal-to-metal, NON GAS TIGHT joints at (4) and (6).

Indeed, the first switch failure we considered gave

us physical evidence of a failure at (6) that produced

a slowly progressing failure of the switch. This

study was described in detail at:



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



Now we are pondering a demonstrated case of repeated

failures over a period of years in one aircraft. Here

are some of the hard data points:



(a) All of the switches involved are Carlingswitch

     toggles with fast-on tabs. These switches are

     an exceedingly mature design that dates back

     at least 50 years. IF the root cause of failure

     lies with the switches, then it's most likely

     a failure of process and not of design.



(b) All switches showed signs of heating on terminals

     that are also on "loose rivets" at (3) or (.

     The fast-on terminals also showed signs of

     over-heating in the form of discolored insulators

     over the wire-grips.



(c) While the majority of switch failures were used

     in circuits that carry substantial amounts of

     current (strobe and landing lights) the first

     failure reported was in a master switch that

     carries 1A of contactor current and field

     current of perhaps 4A max with an in-flight

     average current on the order of 1A.



(d) A photo offered at:



http://aeroelectric.com/Pictures/Switches/VL_Switch_Failure_2.jpg



     shows distinct signs of over-heating at the

     terminal's insulator but no overt signs of

     overheating in the metal parts under the terminal.



     I posited the hypothesis a few days ago that IF

     the source of heating came from within the switch

     and IF temperatures rose high enough to discolor

     the terminal's insulation, then temperatures on

     the metal parts under the terminal would be high

     enough to discolor their surfaces as well. For

     example, in the photo at:



http://www.aeroelectric.com/articles/Anatomy_of_a_Switch_Failure/b.jpg



     We see an overheated and loose rivet. We also see

     signs of discoloration due to accelerated corrosion

     of the fast-on tab adjacent to the rivet head.

     These were the overt signs visible from OUTSIDE a

     switch that was in serious trouble from heating

     effects INSIDE . . . the fast-on-terminal was not

     hot enough to distort the shape or color of the

     terminal's wire grip.



     Now, the loose rivet phenomenon is easily explained

     by a degradation of structural integrity of the

     switch's plastic housing(A).  Further, since the

     failed switches have obviously been running hot,

     it follows that the plastic has lost structural

     integrity due to heating . . . what is NOT obvious

     is whether the initial heat-source came from INSIDE

     or OUTSIDE the switch.



(e) We know that hundreds of thousands of switches using

     this design and process are flying on aircraft. IF

     there is a problem with the switches, then the BIG

     puzzle to be solved is why we find a suite of failures

     spanning years of switch production batches and many

     flight hours of the subject aircraft. The astute

     investigator is obligated to consider all features

     of the current path study cited above and either

     confirm or discard each of the TEN metal-to-metal

     connections as candidates for root cause of

     the failures.



Okay, this dissertation illustrates our of understanding

at the time of this writing.



It's not only useful but necessary to discount or

confirm the integrity of wire grip joints on the terminals

ESPECIALLY in light of localized heating observed

on the wire-grips of the terminals. This line of

investigation is further encouraged by analysis

of the probability of such concentration of switch

failures having root cause in design or construction of

the switches. This consideration alone is strong suggestion of

an ALTERNATIVE EFFECT COMMON TO ALL THE FAILURES.



On a related topic it has been suggested in the

pages of this forum that Fast-On terminals have

no "Gas Tight" qualities. For clarity let us agree

on the meaning of gas tight. In the dissertation above

I've used the terms high-pressure and metal-to-metal

to describe the interface between two conductors.

By high pressure, I'm speaking of conditions severe

enough to deform metal, i.e. upset its surface or

shape. Keep in mind that this kind of activity

implies pressures in the tens of thousands of pounds

per square inch. In the context of gripping the

strands of wire in the crimp of a terminal, the

term "gas tight" is very descriptive of the

design goal.



Consider the sketched cross-section of a fast-on

joint which I've posted at:



http://aeroelectric.com/Pictures/Terminals/Fast-On_Physics.jpg



We speak to this drawing during the weekend seminars

and point out that most individuals look at a Fast-On

terminal and incorrectly deduce that the spring

forces at the ends of the grips(A) provide an

enduring connection to the tab(B) at the flat interface

between terminal and tab at (2).



Consider that when you push the Fast-On terminal onto

a tab and pull it off, an examination of the area under

the tips of the grips at (1) will show bright lines

or scratches in the tab metal surface. Tiny? yes.

Pressure? Pushing the terminal onto the tab plows

furrows in the surface of the tab i.e. exerts pressures

in the tens of thousands of PSI. The pressures on the

back side of the interface at (2) are a tiny fraction

of those found on the front side.



Therefore, I suggest that not only are the interfaces

at (1) gas-tight (due to the intimate contact of terminal

and tab) the interface at (2) is not gas tight. While

(2) may contribute significantly to joint conductivity

when shiny and new, it's contribution ten years hence is

a small fraction of the total.



     Bob . . .

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

Posted: Sat Sep 13, 2008 10:03 am Post subject: Toggle Switches with Fast-On Tabs

Hi Bob:  One more variable:  The serious switch failures (the one you

analyzed, plus my two) have all been in strobe circuits.  We've had some

debate about this in the past, but perhaps the analysis of the I-V

characteristics of Whelen strobe supply would be in order.  I'm curious if

there is a combination of elements problem here with switches, terminals and

load.  



When my master switch failed (loose rivets), it manifested itself with

alternator overvoltage alarms.  The voltage regulator was boosting

alternator output to compensate for a voltage drop in the master switch.

Normally, you'd suspect a regulator, but it was the switch.  The ultimate

boilerplate fix was to use the new switch to control a relay that connected

the alternator output to the regulator directly (through appropriate

fuselink).



I'm wondering if a similar band-aid fix for the strobe supply is in order.

Perhaps use the switch to control a good automotive relay for this circuit. 



I don't have a setup for testing a strobe supply myself.  Perhaps Whelen has

some insight.



Vern



[quote] --

nuckolls.bob(at)cox.net
Guest

Posted: Sat Sep 13, 2008 10:33 am Post subject: Toggle Switches with Fast-On Tabs

At 10:58 AM 9/13/2008 -0700, you wrote:

 	  Quote: 	
		  

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



Hi Bob:  One more variable:  The serious switch failures (the one you

analyzed, plus my two) have all been in strobe circuits.  We've had some

debate about this in the past, but perhaps the analysis of the I-V

characteristics of Whelen strobe supply would be in order.  I'm curious if

there is a combination of elements problem here with switches, terminals and

load.



When my master switch failed (loose rivets), it manifested itself with

alternator overvoltage alarms.  The voltage regulator was boosting

alternator output to compensate for a voltage drop in the master switch.

Normally, you'd suspect a regulator, but it was the switch.  The ultimate

boilerplate fix was to use the new switch to control a relay that connected

the alternator output to the regulator directly (through appropriate

fuselink).


	


    Yes, that drops the current through the switch to some

    value on the order of 100 milliamps on the alternator side

    while leaving the contactor load of about 1A on the battery

    side. This raise some big red flags suggesting that ratings

    abuse is not root cause of the failures you've suffered.

 	  Quote: 	
		  I'm wondering if a similar band-aid fix for the strobe supply is in order.

Perhaps use the switch to control a good automotive relay for this circuit.



I don't have a setup for testing a strobe supply myself.  Perhaps Whelen has

some insight.


	


   Assuming that root cause turns out to be some form of

   ratings abuse, then yes . . . the fix is get back within

   the bounds of ratings.



   This could include things like buffer relays . . .



http://www.aeroelectric.com/PPS/Adobe_Architecture_Pdfs/Z32K.pdf



http://www.aeroelectric.com/PPS/Adobe_Architecture_Pdfs/Z22-23K.pdf



   . . . or other adjustment to architecture.



   But it's not clear to me that we're dealing with a ratings

   issue. You've had failures in the master switch . . . this is

   probably the most lightly loaded device on your switch panel!

   If we are looking at a ratings issue, I think it will be a

   first in my career of chasing the sources of smoke in airplanes.



   Bob . . .

Dan Reeves

Joined: 10 Jan 2006
Posts: 16

Posted: Sat Sep 13, 2008 11:20 am Post subject: Toggle Switches with Fast-On Tabs

Perhaps this was already discussed but is it possible that you have a short through the panel itself somewhere?  Kind of like if you lost your engine ground strap and hit the starter, the current finds a ground through throttle cables, etc and fries whatever is in it's path.  Maybe a cigarette lighter style recepticle shorting through the panel and through the switch housings mounted to the panel.  I wonder if something is vibrating and maybe loosly or intermittantly laying across the fast-on tabs.  Any chance of a parking brake cable or something like that making contact with something electrical when it's set or released.  Maybe all irrelevant,,,but just thoughts that came to mind while reading through the recent posts and not hearing of others experiencing similar switch failures.

  

 

--- On Sat, 9/13/08, Vernon Little <rv-9a-online(at)telus.net> wrote:



 [quote]From: Vernon Little <rv-9a-online(at)telus.net>

Subject: RE: Toggle Switches with Fast-On Tabs

To: aeroelectric-list(at)matronics.com

Date: Saturday, September 13, 2008, 1:58 PM



[quote]--> AeroElectric-List message posted by: "Vernon Little" <rv-9a-online(at)telus.net>  Hi Bob:  One more variable:  The serious switch failures (the one you analyzed, plus my two) have all been in strobe circuits.  We've had some debate about this in the past, but perhaps the analysis of the I-V characteristics of Whelen strobe supply would be in order.  I'm curious if there is a combination of elements problem here with switches, terminals and load.    When my master switch failed (loose rivets), it manifested itself with alternator overvoltage alarms.  The voltage regulator was boosting alternator output to compensate for a voltage drop in the master switch. Normally, you'd suspect a regulator, but it was the switch.  The ultimate boilerplate fix was to use the new switch to control a relay that connected the alternator output to the regulator directly (through appropriate fuselink).  I'm wondering if a similar band-aid fix for the strobe supply is in order. Perhaps use the switch to control a good automotive relay for this circuit.   I don't have a setup for testing a strobe supply myself.  Perhaps Whelen has some insight.  Vern  > --

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

Posted: Sat Sep 13, 2008 1:14 pm Post subject: Toggle Switches with Fast-On Tabs

Hi  Dan.  Not likely.  I checked for for shorts.  What I measured was  a 2 volt drop across the switch when operating (terminal to terminal).   About 12.5V in, 10.5V out.  This does not mean I don't have a wiring  problem downstream somewhere, but there is no evidence of it, the breaker is  intact and the strobes operate.  It also means that the switch is  dissipation about 14 Watts.  No wonder it's hot.

  

 Vern

 [quote]   

   --

jpx(at)Qenesis.com
Guest

Posted: Thu Sep 18, 2008 10:09 am Post subject: Toggle Switches with Fast-On Tabs

Any chance that unsupported wires are vibrating in flight and helping  

to loosen the rivets, which then heat up from poor contact ?



Jeff Page

Dream Aircraft Tundra #10



 	  Quote: 	
		   From: "Vernon Little" <rv-9a-online(at)telus.net>

 The most recent information I have from the new case documented in the

 previous message is that yes, indeed, the switch rivets are loose.  It

 looks like a common contributing factor.

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

Posted: Thu Sep 18, 2008 10:55 am Post subject: Toggle Switches with Fast-On Tabs

Hi Jeff.  This came up previously, and the answer is that they are all

supported properly.  It appears that the underlying cause may be a

combination of high inrush currents and "economy" switches that start the

thermal runaway cycle:  switch heating causes rivets to loosen; loose rivets

cause more switch heating.



With the exception of one switch, all of the problems have been in the

strobe and landing/taxi light circuits in multiple aircraft.



I'm hoping for more results from the list members who are checking their

switches for damage or overheating using the finger test.



Thanks, Vern



[quote] --

fvalarm(at)rapidnet.net
Guest

Posted: Thu Sep 18, 2008 12:06 pm Post subject: Toggle Switches with Fast-On Tabs

Vern,



I understand that your switches were "properly" supported.  Just wondering

if the wire "support" needs to be more substantial than usual if there are

large gauge wires involved, such as would be expected for landing light

circuits?  My thinking is that the larger the wire, the more momentum it has

when vibrating due to it's mass.



Secondly, maybe I've missed something here, but concerning the "loose"

rivets.  It is thought that the rivets are loose because of overheating, or

vibration (with a heavy wire attached), or poor manufacturing process or

what?



Bevan

 



--

klehman(at)albedo.net
Guest

Posted: Thu Sep 18, 2008 12:12 pm Post subject: Toggle Switches with Fast-On Tabs

All my Carlings seem fine and run cold. 2+ years, 220 hours, AMP PIDG 

connectors, Aeroflash strobes, 35 watt wigwag lights.

Ken



snip

 	  Quote: 	
		   I'm hoping for more results from the list members who are checking their

 switches for damage or overheating using the finger test.

 

 Thanks, Vern

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

Posted: Thu Sep 18, 2008 12:59 pm Post subject: Toggle Switches with Fast-On Tabs

Good datapoint, thanks.  



Vern



[quote] --

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

Posted: Thu Sep 18, 2008 12:59 pm Post subject: Toggle Switches with Fast-On Tabs

Hey Bevan.  Based on the evidence, the Strobe switch is failing in multiple

aircraft, and the Wig-Wag/Ldg/Taxi lights have a couple of failures.  No

evidence in these aircraft of other switches with problems.  This tends to

rule out an installation problem.  The loose rivets are a both a cause and

an effect.  Some of the Carling switches have somewhat loose rivets when

new.  After the switch is fried, they are looser and the terminals are

discolored.



If vibration is the problem, there are many other places in the aircraft

that would show overheating (wig-wag flasher, terminal block at wing root

and so on). None do.



I think the obvious problem is the switches are being stressed by overload,

and these stresses are leading to looser rivets and further overheating.  I

have new/different switches now, but if I have a failure again, I'm

installing relays.



Vern



[quote] --

ftyoder(at)yoderbuilt.com
Guest

Posted: Thu Sep 18, 2008 3:59 pm Post subject: Toggle Switches with Fast-On Tabs

Is their a manufacturing date or any number on the switches that would let

you identify a bad batch of switches? Sounds like some switches come with

loose rivets and some don't have loose rivets. Is that correct?



The manufacture should be able to tell you if loose rivets are.acceptable.



Tim



---

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

Posted: Thu Sep 18, 2008 5:21 pm Post subject: Toggle Switches with Fast-On Tabs

If you look back in the thread, I identified the date code of my switches,

and the one that Bob wrote his article on.  No codes for the other failures

reported yet.  We have not seen any correlation, plus one switch was made by

another vendor altogether.



I returned a failed Carling master switch for failure analysis to the

vendor, but never heard anything.  It had loose rivets as well, but was not

a heavily loaded switch.  Since I never heard anything, I've gone public on

this list to get some collective wisdom working on the problem.



I'm convinced that loose rivets are a big problem.  I'm an electrical

engineer, not a mechanical engineer, but it's pretty obvious that the rivets

should not be loose.



Vern



 



[quote] --

mjpienaar(at)shaw.ca
Guest

Posted: Thu Sep 18, 2008 5:56 pm Post subject: Toggle Switches with Fast-On Tabs

Hi All,



Could somebody please explain in simple language what "high inrush currents" 

are.



Thanks



Mike



---

Dennis Glaeser

Joined: 10 Jan 2006
Posts: 53
Location: Rochester Hills, Michigan

Posted: Thu Sep 18, 2008 6:37 pm Post subject: Toggle Switches with Fast-On Tabs

 	  Quote: 	
		   Hi All,

 Could somebody please explain in simple language what "high inrush

currents" are.
	


 	  Quote: 	
		   Thanks

 Mike


	


When resistive items are cold, they have less resistance.  So when power is

first applied, the current draw is high - hence the name.  As things warm

up, resistance grows and the current drops to it's steady state value.



Dennis Glaeser

simon(at)synchronousdesig
Guest

Posted: Thu Sep 18, 2008 6:43 pm Post subject: Toggle Switches with Fast-On Tabs

Mike, a high inrush current is caused by a device that has an initial low

resistance.  This resistance then increases as a function of time.  I can

think of two simple devices that will do this to a switch



First is a capacitive load, or more accurately a capacitive-resistive load.

With this type of load, you are charging up a capacitor.  If the capacitor

is not charged up initially, then the only thing limiting the current is the

resistor.  If the resistance is small, it will result in a "high inrush

current."  As the capacitor charges up, the current decreases toward zero.



Second is a non-linear resistance such as an incandescent lamp.  Such a lamp

has a filament that is cold when it first passes current.  When cold, the

filament has low resistance and thus allows a "high inrush current."  As the

filament's temperature increases to produce light, its resistance increases,

and thus the current decreases.



There may be other simple examples and if so, anyone please feel free to

chime in.  I hope this helps.



Mike, it just so happens that when a switch makes or breaks is the worst

possible time for a high current or voltage to be occurring, as it takes its

toll on the contacts due to arcing.



Simon

Copyright C 2008

--

Joemotis(at)aol.com
Guest

Posted: Thu Sep 18, 2008 7:16 pm Post subject: Toggle Switches with Fast-On Tabs

Does anyone with these switches in question also have a laser thermometer  to shoot their switch bank with?

 Might be interesting although not very scientific.

 Most assuredly would find a hot one though.

  

 Joe Motis        Do not  archive



Looking for simple solutions to your real-life financial challenges? Check out WalletPop for the latest news and information, tips and calculators.

  [quote][b]

nuckolls.bob(at)cox.net
Guest

Posted: Fri Sep 19, 2008 5:38 am Post subject: Toggle Switches with Fast-On Tabs

At 08:18 PM 9/18/2008 -0400, you wrote:

 	  Quote: 	
		  

<Tom(at)costanzaandassociates.com>



Vern,



Don't you think that if it was an application problem that we'd be hearing

from people on a few orders of magnitude more than we have?  There must be

several hundred people using these switches in these circuits.  No??


	


   More like thousands. The sum total of years those switches were sold

   first by AEC and then B&C is about 15.

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


<ftyoder(at)yoderbuilt.com>



Is their a manufacturing date or any number on the switches that would let

you identify a bad batch of switches? Sounds like some switches come with

loose rivets and some don't have loose rivets. Is that correct?



The manufacture should be able to tell you if loose rivets are acceptable.



    Loose rivets are not acceptable. If you refer to the sketch

    at:



http://www.aeroelectric.com/Pictures/Switches/Toggle_Switch_with_Fast-On_Tabs.jpg



    The hollow rivets at (3) and ( are what hold the switch internal

    parts together -AND- provide a conductive path from part to part.

    As soon as the rivet's retaining force goes down, resistance

    goes up, heating goes up, and the device starts down the path

    to failure.

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




I understand that your switches were "properly" supported. Just wondering

if the wire "support" needs to be more substantial than usual if there are

large gauge wires involved, such as would be expected for landing light

circuits? My thinking is that the larger the wire, the more momentum it has

when vibrating due to it's mass.



Secondly, maybe I've missed something here, but concerning the "loose"

rivets. It is thought that the rivets are loose because of overheating, or

vibration (with a heavy wire attached), or poor manufacturing process or

what?



    Bevan, you may be on to something here . . . I'm embarrassed

    to have overlooked it up to now . . .



    Over the past week, I've received 5 corpses of dearly departed

    switches. All had loose rivets on the current carrying tabs. The

    OFF side tab was solidly retained by their rivets. Here's an

    exemplar photo set:



    These two pictures show the moving ON and OFF-side contacts.

    Noticeably free of signs of electrical stress.



http://www.aeroelectric.com/Pictures/Switches/Carling_Failures/BRA_01.jpg



http://www.aeroelectric.com/Pictures/Switches/Carling_Failures/BRA_02.jpg



    This picture shows the "saddle sores" that one would expect

    in a switch where the teeter-totter shaft was properly undersized . . .



(Ref 

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



    to sit in the saddle.



http://www.aeroelectric.com/Pictures/Switches/Carling_Failures/BRA_03.jpg

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



    The spring-rate of the coil spring inside the toggle was checked

    against a new switch and found to be the same within measurement

    tolerances. It takes right at a pound of force to push the plastic

    actuator post flush with the end of the hollow toggle shaft . . .



http://www.aeroelectric.com/Pictures/Switches/Carling_Failures/BRA_06.jpg



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



    This switch has loose rivets at the ON-side tab (left) and

    the center tab. OFF-side rivet was tight. Note signs of heating

    induced corrosion on the left side rivet.



http://www.aeroelectric.com/Pictures/Switches/Carling_Failures/BRA_04.jpg



http://www.aeroelectric.com/Pictures/Switches/Carling_Failures/BRA_09.jpg



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



    Here's a familiar picture . . .



http://www.aeroelectric.com/Pictures/Switches/Carling_Failures/BRA_08.JPG



    We've seen this on-side vs. off-side bending of the teeter-

    totter before. Notice the darker copper color on the right. This

    was the ON-side contact that was running warmer but the OFF-side

    was more severely deformed from flat.



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



    Here's a really cool picture . . .



http://www.aeroelectric.com/Pictures/Switches/Carling_Failures/BRA_05.jpg



    This switch was obviously powered at the center terminal. The

    teeter-totter was sufficiently distorted to cause teasing arc

    marks on BOTH the on and off extremes. I say "teasing" because

    this had to be going on for some time to make such strong marks

    before the failure progressed to the point where it was popping

    circuit protection.



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



    This picture shows perfectly good On and Off-side contacts

    in addition to the bright areas in the bottom of the

    saddle pocket. The ON-side contact is on the right with

    barely detectable signs of elevated temperature operation.



http://www.aeroelectric.com/Pictures/Switches/Carling_Failures/BRA_10.jpg

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



    This picture is shows the teeter-totter as removed and before

    the lubrication was wiped off.



http://www.aeroelectric.com/Pictures/Switches/Carling_Failures/BRA_11.jpg

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



     This picture shows the loose rivet at the ON-side tab

     but the tab itself is not showing signs of strong

     heating.



http://www.aeroelectric.com/Pictures/Switches/Carling_Failures/BRA_12.jpg



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



    Here's the OFF-side tab. Tight rivet, bright clean

    brass.



http://www.aeroelectric.com/Pictures/Switches/Carling_Failures/BRA_14.jpg



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



    The common feature of all the carcasses I received was

    distortion of the teeter-totter from flat to a bowed

    condition of various degrees. The teeter-totter metal

    had lost temper and bent with heating and operational

    hammering being the predominant stresses.



    Stationary and moving contacts of all failed switches

    showed only slight if any visible effects of heating.



    All switches had loose rivets at the connection tabs.

    (note that my sketched cross-section has the center

    rivet ( reversed. The head-formed-on-assembly is on

    the inside of the switch, not the outside).



    When dissecting a chain of failure events, one tries

    to deduce the first event . . . it's not unlike looking

    for the point of origin in a burned building. In this

    case, we're attempting to deduce the weakest (hence

    most likely to be overstressed) feature and then

    see if the damage patterns radiate out from that

    feature.



    In the case of the analysis published earlier this

    year, there were striking patterns laid down on the

    teeter-totter pivot that could easily be interpreted

    as a point of origin for chain of failures. However,

    Bevan's question about mechanical stresses to the

    tabs due to wire weight and bundling was an "eureka

    moment" of sorts.



    Consider the cross section sketch and photos of

    the riveted tabs and know that the weakest mechanical

    feature of these switches is the point where the hollow

    rivets are formed over to achieve retention forces

    that hold the parts together PLUS a gas-tight connection

    between the rivet and it's companion pieces and parts.



    While the failed switches were mostly concentrated

    in the ship's higher current systems, we've seen

    failures in the low current systems too (battery

    master). Nonetheless, Vern has experienced a rash

    of failures that spanned the full range of current

    carrying tasks.



    Consider the effects of a wire (heavier in the

    landing light and strobe systems) hanging off the

    back of the switch. Consider the effects of a wire

    bundle that is supported by the switch terminals

    with bundle-to-switch pigtails that are relatively

    short. There's a strong moment arm from the end of

    the tab that allows vibration of attached mass to

    put tension on the rolled over edges of the

    driven-head of the rivets.



    This makes more sense as a proximate root cause of

    the constellation of failures we've studied. Once

    gas-tightness of the rivet head is compromised,

    then corrosion goes up, resistance goes up, heating

    goes up, corrosion accelerates, etc. etc.



    I think I recall writing some words to the effect

    that pigtails that come off the backs of switches

    should have some substantial length before they

    drop into a wire bundle. The goal is two-fold:

    (1) service loop length to allow removal and

    replacement of the switch without disturbing other

    switches or the bundle and (2) a stress reliever

    that prevents mass of the wire bundle from adding

    to the vibrational stresses on the switch's tabs.

    I think I suggested a 2" service loop that offered

    free-slack in the leadwire between bundle and

    switch terminal.



    In the $high$ switches, the effects of mis-installation

    are reduced by the manner in which wire attach terminals

    are retained in the switch housing. See:



http://www.aeroelectric.com/Pictures/Switches/Toggle_Switch_with_Mold-Captured_Terminals.jpg



    Vern, take a look at your installation for the purpose

    of assessing probability that vibration in the

    wire bundle mass is being conducted to the rivet

    joints due to a short-coupled installation of the

    attach wires.



    If this new hypothesis proves plausible, then it

    may well explain the rash of failures noted in

    a product with an exceedingly rich history in

    the marketplace. This has been nagging at me since

    this thread began. Lots of mud was thrown against

    the wall about AC vs. DC, failure to observe ratings,

    high inrush due to nature of the loads, etc.



    Yet we were still dealing with a legacy product

    manufactured in the millions and used with

    success in aircraft since the 60s. There

    had to be a common thread that tied all of what

    we observed together in a rational assemblage

    of simple-ideas.



    Current working hypothesis: Loss of gas-tight

    integrity at the rolled head of the rivets anchors

    a chain of failures that manifests itself with

    signs of heating, distortion of the teeter-totter,

    internal shorting of teeter-totter to the frame and

    loss of continuity through the switch.



   I think we may be getting closer . . .



    Bob . . .

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

Posted: Fri Sep 19, 2008 9:47 am Post subject: Toggle Switches with Fast-On Tabs

Bob, thanks for your detailed response and analysis.



I'll take some photos of the wiring next week to see if I have overlooked a

potential source of vibration and unsupported wiring.  I'll also see if I

get photos from my friend's panel showing the same.



It may be that vibration per se is not the issue, but in combination with

suspect rivets and large currents, we have the recipe for problems.  I do

know that I've received new Carling switches with suspect rivets.  One

suggestion that I got was to solder the tabs to the rivets, but I am worried

that may screw up the temper of the materials.  Of note:  we have at least

one non-Carling switch failure as well.



One thing that I didn't pay too much attention to was stress on the

terminals from the wires.  Perhaps the terminals are putting torque or

bending preloads on the tabs that are stressing the tab/rivet joint.  In

fact, additional post-installation tie-wraps in the bundles tend to pull

wires off of their natural alignment and cause these stresses.



When I'm crawling under the panel with my camera, I will check the wire

grooming.



My bias on this (looking at my photos and my friend's) is that wire grooming

is at most a secondary contributing factor.  Given the wide variation in

wiring techniques in the OBAM community, if this was the cause, we'd have

serious trouble.



Maybe I'll blame it on mud-dauber wasps, that's popular!



V

[quote] --

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

Posted: Wed Sep 24, 2008 8:37 am Post subject: Toggle Switches with Fast-On Tabs

Update on my findings.



One question that I had was if vibration could be working the rivets loose

on the Carling switches.  Given that I believe the rivets to be suspect to

begin with-- even new switches have loose rivets-- vibration may be a

trigger for a cascade of overheating problems.



I took a photo of my switches from under the panel, seen here:

http://www.vx-aviation.com/rv-9a/photos/Electrical/IMG_0988_1.JPG.



This photo shows the black sheathed wiring bundled which brings the airframe

circuits to the panel, and the 2-3 inch pigtails to the switches.  



Based on my inspection, there may be a vibration coupling into the pigtails

from the main bundle.  Even though the bundle is secured every 8 inches or

so, I was able to move it by hand in the vertical (pitch) dimension. 



Given the uncertainties here, I will not add more tie-downs to this bundle

yet.  I want to see if the switch replacement has done the trick.  This may

take a year or so to complete the experiment.  If it solves the problem, we

will know that the superior construction of the switches is a fix.  If it

does not, I will add tie downs to the bundle and re-run the experiment.



The problem with doing both right now, is that we'll never know what the

root cause was.  I'm willing to sacrifice a few switches in the greater

search for knowledge.  At least this area is easy to get at!



Vern

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