AeroElectric-List Digest Archive

Sat 09/13/08


Total Messages Posted: 14



Today's Message Index:
----------------------
 
     1. 07:51 AM - Toggle Switches with Fast-On Tabs (Robert L. Nuckolls, III)
     2. 08:04 AM - Re: Switch problem ***MAJOR UPDATE*** (Robert L. Nuckolls, III)
     3. 08:51 AM - Re: AeroElectric-List Digest: 11 Msgs - 09/12/08 (Scott Klemptner)
     4. 10:00 AM - Re: Re: AeroElectric-List Digest: 11 Msgs - 09/12/08 (Richard E. Tasker)
     5. 10:09 AM - Re: Re: AeroElectric-List Digest: 11 Msgs - 09/12/08 (Robert L. Nuckolls, III)
     6. 10:28 AM - Re: Re: AeroElectric-List Digest: 11 Msgs - 09/12/08 (Robert L. Nuckolls, III)
     7. 10:30 AM - Re: Re: AeroElectric-List Digest: 11 Msgs - 09/12/08 (ROGER & JEAN CURTIS)
     8. 10:39 AM - Re: Re: AeroElectric-List Digest: 11 Msgs - 09/12/08 (Vernon Little)
     9. 11:03 AM - Re: AC vs DC ratings???? (Robert L. Nuckolls, III)
    10. 11:03 AM - Re: Toggle Switches with Fast-On Tabs (Vernon Little)
    11. 11:16 AM - Switch Ratings versus Root Cause of Failure (Robert L. Nuckolls, III)
    12. 11:33 AM - Re: Toggle Switches with Fast-On Tabs (Robert L. Nuckolls, III)
    13. 12:20 PM - Re: Toggle Switches with Fast-On Tabs (Dan Reeves)
    14. 02:14 PM - Re: Toggle Switches with Fast-On Tabs (Vernon Little)
 
 
 


Message 1


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    Time: 07:51:02 AM PST US
    From: "Robert L. Nuckolls, III" <nuckolls.bob@cox.net>
    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(8) 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 (8). 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 . . .


    Message 2


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    Time: 08:04:01 AM PST US
    From: "Robert L. Nuckolls, III" <nuckolls.bob@cox.net>
    Subject: Switch problem ***MAJOR UPDATE***
    At 05:23 PM 9/12/2008 -0700, you wrote: ><rv-9a-online@telus.net> > >Bob: A couple of switches, the burnt fast-ons, and some sample terminations >on your way by airmail today. Hope you have time to add a learned second >opinion after you have a look. Great. I've published a working document that lists all the simple-ideas to be considered in this study of your switch problems. Taking a good look at joint integrity of the wire-grips is a good place to start. Bob . . .


    Message 3


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    Time: 08:51:35 AM PST US
    From: Scott Klemptner <bmwr606@yahoo.com>
    Subject: Re: AeroElectric-List Digest: 11 Msgs - 09/12/08
    Vern, you quoted Carling switch ratings, but if you read further down on the page, you find the following: "Types of Loads: An electric load is the amount of electric power delivered or required at any specific point or points on a system. The requirement originates at the energy consuming equipment of the consumers. More simply put, a load is the piece of equipment you turn on and off. Resistive loads primarily offer resistance to the flow of current. Examples of resistive loads include electric heaters, ranges, ovens, toasters, and irons. If the device is supposed to get hot and doesn't move, it's most likely a resistive load. Inductive loads are usually devices that move and normally include electric magnets, like an electric motor. Examples of inductive loads include such things as power drills, electric mixers, fans, sewing machines, and vacuum cleaners. Transformers also produce inductive loads. High Inrush loads draw a higher amount of current or amperage when first turned on, compared to the amount of current required to continue running. An example of a high inrush load is a light bulb, which may draw 20 or more times its normal operating current when first turned on. This is often referred to as lamp load. Other examples of loads that have high inrush are switching power supplies (capacitive load) and motors (inductive load). . . . . . L & T Ratings An "L" rating denotes the ability of a switch to handle the initial high inrush characteristics of a Tungsten Filament Lamp on AC voltage only. A "T" rating is the equivalent lamp load for DC. H Rating An "H" rating denotes a non-inductive resistive rating. Ratings listed in Carling Technologies' product information may appear with the symbol "H" or with the words "non-inductive" or "resistive". "H" ratings are typically required for switches used in commercial oven applications." Are you sure you are using the switches within their rated specs??????????? My experience using AC rated switches on DC circuits with high inrush currents (landing/taxi lights and strobes in particular) is failure after failure. Eric....this includes ROCKER SWITCHES TOO!.... Cessna rocker switches used in the 70's for example (AC rated "Mr Coffee Maker switches). My $.02 worth...... Scott A Klemptner bmwr606 on Yahoo IM What if the Hokey-Pokey IS what it's all about? ----- Original Message ---- From: AeroElectric-List Digest Server <aeroelectric-list@matronics.com> Sent: Saturday, September 13, 2008 1:55:46 AM ________________________________ Message 7 _____________________________________ Time: 09:42:41 AM PST US From: "Vernon Little" <rv-9a-online@telus.net> Subject: RE: AeroElectric-List: Re: Switch problem ***MAJOR UPDATE*** ... > Despite what Bob says, 115 VAC switches should never be used > for LV DC high-current applications. Do what the manufacturer > suggests. That's what's printed on the side of the switch > Bubela. Otherwise you're on your own. > ... > -------- > Eric M. Jones > www.PerihelionDesign.com ... Hi again Eric. Just to comment on what you said: "DC Rule of Thumb For those switches that list an AC voltage rating only, the "DC Rule of Thumb" can be applied for determining the switch's maximum DC current rating. This "rule" states the highest amperage on the switch should perform satisfactorily up to 30 volts DC. For example, a switch which is rated at 10A 250VAC; 15A 125VAC; 3/4HP 125-250VAC, will be likely to perform satisfactorily at 15 amps up to 30 volts DC (VDC)" http://www.carlingtech.com/products/switches/learn_more.asp?page=switches_amp-rating Therefore, all of my Carling switches are being used according to the manufacturers recommendations. The only gotcha is that the switches are not rated for operating below 0 degrees C. This is probably because the switches are not sealed from condensing moisture. This is not a problem where I live. Vern


    Message 4


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    Time: 10:00:44 AM PST US
    From: "Richard E. Tasker" <retasker@optonline.net>
    Subject: Re: AeroElectric-List Digest: 11 Msgs - 09/12/08
    A comment on the below comments: If you look at the schematic Vern posted, you would see that he is using NTC surge limiters on each light. These minimize startup surges caused by the very low cold filament resistance of the lamps. On the other hand, it depends on the time constants of the NTC surge limiters vs the time constants of the lamp filaments as to whether these are effective when in wig-wag mode. Dick Tasker Scott Klemptner wrote: > Vern, > > you quoted Carling switch ratings, but if you read further down on the > page, you find the following: > > "Types of Loads: > An *electric load* is the amount of electric power delivered or > required at any specific point or points on a system. The requirement > originates at the energy consuming equipment of the consumers. More > simply put, a load is the piece of equipment you turn on and off. > > *Resistive loads* primarily offer resistance to the flow of current. > Examples of resistive loads include electric heaters, ranges, ovens, > toasters, and irons. If the device is supposed to get hot and doesn't > move, it's most likely a resistive load. > > *Inductive loads* are usually devices that move and normally include > electric magnets, like an electric motor. Examples of inductive loads > include such things as power drills, electric mixers, fans, sewing > machines, and vacuum cleaners. Transformers also produce inductive loads. > > *High Inrush loads* draw a higher amount of current or amperage when > first turned on, compared to the amount of current required to > continue running. An example of a high inrush load is a light bulb, > which may draw 20 or more times its normal operating current when > first turned on. This is often referred to as lamp load. Other > examples of loads that have high inrush are switching power supplies > (capacitive load) and motors (inductive load). > > . > > . > > . > > . > > . > > L & T Ratings > > An "L" rating denotes the ability of a switch to handle the initial > high inrush characteristics of a Tungsten Filament Lamp on AC voltage > only. A "T" rating is the equivalent lamp load for DC. > > > H Rating > > An "H" rating denotes a non-inductive resistive rating. Ratings listed > in Carling Technologies' product information may appear with the > symbol "H" or with the words "non-inductive" or "resistive". "H" > ratings are typically required for switches used in commercial oven > applications." > > > Are you sure you are using the switches within their rated > specs??????????? > > My experience using AC rated switches on DC circuits with high inrush > currents (landing/taxi lights and strobes in particular) is failure > after failure. > > Eric....this includes ROCKER SWITCHES TOO!.... Cessna rocker switches > used in the 70's for example (AC rated "Mr Coffee Maker switches). > > My $.02 worth...... > > Scott A Klemptner > bmwr606 on Yahoo IM > > What if the Hokey-Pokey IS what it's all about? > > ----- Original Message ---- > From: AeroElectric-List Digest Server <aeroelectric-list@matronics.com> > To: AeroElectric-List Digest List <aeroelectric-list-digest@matronics.com> > Sent: Saturday, September 13, 2008 1:55:46 AM > > > ________________________________ Message 7 > _____________________________________ > > > Time: 09:42:41 AM PST US > From: "Vernon Little" <rv-9a-online@telus.net > <mailto:rv-9a-online@telus.net>> > Subject: RE: AeroElectric-List: Re: Switch problem ***MAJOR UPDATE*** > > > ... > > Despite what Bob says, 115 VAC switches should never be used > > for LV DC high-current applications. Do what the manufacturer > > suggests. That's what's printed on the side of the switch > > Bubela. Otherwise you're on your own. > > > ... > > -------- > > Eric M. Jones > > www.PerihelionDesign.com > ... > > Hi again Eric. Just to comment on what you said: > > "DC Rule of Thumb > For those switches that list an AC voltage rating only, the "DC Rule of > Thumb" can be applied for determining the switch's maximum DC current > rating. This "rule" states the highest amperage on the switch should > perform > satisfactorily up to 30 volts DC. For example, a switch which is rated at > 10A 250VAC; 15A 125VAC; 3/4HP 125-250VAC, will be likely to perform > satisfactorily at 15 amps up to 30 volts DC (VDC)" > http://www.carlingtech.com/products/switches/learn_more.asp?page=switches_am > <http://www.carlingtech.com/products/switches/learn_more.asp?page=switches_amp-rating%20>p-rating > <http://www.carlingtech.com/products/switches/learn_more.asp?page=switches_amp-rating%20> > Therefore, all of my Carling switches are being used according to the > manufacturers recommendations. The only gotcha is that the switches > are not > rated for operating below 0 degrees C. This is probably because the > switches are not sealed from condensing moisture. This is not a problem > where I live. > > Vern > > > * > > > * -- Please Note: No trees were destroyed in the sending of this message. We do concede, however, that a significant number of electrons may have been temporarily inconvenienced. --


    Message 5


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    Time: 10:09:43 AM PST US
    From: "Robert L. Nuckolls, III" <nuckolls.bob@cox.net>
    Subject: Re: AeroElectric-List Digest: 11 Msgs - 09/12/08
    At 08:48 AM 9/13/2008 -0700, you wrote: >Vern, > >you quoted Carling switch ratings, but if you read further down on the >page, you find the following: > >"Types of Loads: > An electric load is the amount of electric power delivered or required > at any specific point or points on a system. The requirement originates > at the energy consuming equipment of the consumers. More simply put, a > load is the piece of equipment you turn on and off. <snip> >Are you sure you are using the switches within their rated specs??????????? > >My experience using AC rated switches on DC circuits with high inrush >currents (landing/taxi lights and strobes in particular) is failure after >failure. > >Eric....this includes ROCKER SWITCHES TOO!.... Cessna rocker switches used >in the 70's for example (AC rated "Mr Coffee Maker switches). > >My $.02 worth...... > >Scott A Klemptner >bmwr606 on Yahoo IM Scott, Keep in mind that RATINGS have to do with SWITCH SERVICE LIFE generally given in the tens of thousands of cycles with various conditions. Our use of switches in light aircraft is exceedingly light-duty in terms of service life. For example, a master switch (one of those cited in the constellation of failures) operates ONCE PER FLIGHT-CYCLE and carries very small currents compared to the switch's RATINGS. Scott, please review the arguments offered in the piece at: http://www.aeroelectric.com/articles/Switch_Ratings.pdf and explain where you find discrepancies of fact or logic. I'll suggest that had Vern sat in his airplane one afternoon and put 1000 cycles on any of the failed switches, he would NOT have experienced a failure. Yet, over a period of many flight hours and a mere handful of operations he experienced failures that speak to long term effects of ENVIRONMENT that include DEFICIENCIES of process either in fabrication of the switch as described here . . . http://www.aeroelectric.com/articles/Anatomy_of_a_Switch_Failure/Anatomy_of_a_Switch_Failure.html or installation of the switch as hypothesized in my earlier treatise published this morning. The old AC vs. DC as a critical driving force in switch life is a myth. Further, it is clear that the study before us has nothing to do with switch life in terms of ratings, operating cycles, or application. Root cause of these failures will be loss of conduction integrity in one or more of the ten metal-to-metal joints that make up the switch's current path. Bob . . .


    Message 6


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    Time: 10:28:30 AM PST US
    From: "Robert L. Nuckolls, III" <nuckolls.bob@cox.net>
    Subject: Re: AeroElectric-List Digest: 11 Msgs - 09/12/08
    At 12:52 PM 9/13/2008 -0400, you wrote: ><retasker@optonline.net> > >A comment on the below comments: > >If you look at the schematic Vern posted, you would see that he is using >NTC surge limiters on each light. These minimize startup surges caused by >the very low cold filament resistance of the lamps. > >On the other hand, it depends on the time constants of the NTC surge >limiters vs the time constants of the lamp filaments as to whether these >are effective when in wig-wag mode. > >Dick Tasker A couple of years ago, the effectiveness/value of surge limiters in Wig-Wag systems was discussed here on the List. I went to the workbench and took this trace from the operating current of a 55W halogen lamp flashed at about 2 flashes per second. http://www.aeroelectric.com/Pictures/Curves/Wig_Wag_Currents.jpg Note that the first time the lamp gets power we see the characteristic cold-filament inrush current. However, on subsequent turn-ons, the lamp does not have enough time to cool such that the 7x inrush experience is repeated. Bob . . .


    Message 7


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    Time: 10:30:25 AM PST US
    From: "ROGER & JEAN CURTIS" <mrspudandcompany@verizon.net>
    Subject: Re: AeroElectric-List Digest: 11 Msgs - 09/12/08
    Vern, you quoted Carling switch ratings, but if you read further down on the page, you find the following: "Types of Loads: An electric load is the amount of electric power delivered or required at any specific point or points on a system. The requirement originates at the energy consuming equipment of the consumers. More simply put, a load is the piece of equipment you turn on and off. Resistive loads primarily offer resistance to the flow of current. Examples of resistive loads include electric heaters, ranges, ovens, toasters, and irons. If the device is supposed to get hot and doesn't move, it's most likely a resistive load. Inductive loads are usually devices that move and normally include electric magnets, like an electric motor. Examples of inductive loads include such things as power drills, electric mixers, fans, sewing machines, and vacuum cleaners. Transformers also produce inductive loads. High Inrush loads draw a higher amount of current or amperage when first turned on, compared to the amount of current required to continue running. An example of a high inrush load is a light bulb, which may draw 20 or more times its normal operating current when first turned on. This is often referred to as lamp load. Other examples of loads that have high inrush are switching power supplies (capacitive load) and motors (inductive load). . . . . . L & T Ratings An "L" rating denotes the ability of a switch to handle the initial high inrush characteristics of a Tungsten Filament Lamp on AC voltage only. A "T" rating is the equivalent lamp load for DC. H Rating An "H" rating denotes a non-inductive resistive rating. Ratings listed in Carling Technologies' product information may appear with the symbol "H" or with the words "non-inductive" or "resistive". "H" ratings are typically required for switches used in commercial oven applications." Are you sure you are using the switches within their rated specs??????????? My experience using AC rated switches on DC circuits with high inrush currents (landing/taxi lights and strobes in particular) is failure after failure. Eric....this includes ROCKER SWITCHES TOO!.... Cessna rocker switches used in the 70's for example (AC rated "Mr Coffee Maker switches). My $.02 worth...... Scott A Klemptner bmwr606 on Yahoo IM Since there is no reference to DC switch ratings above, does this mean that the DC switches are immune to different load types and L, T, & H ratings? What, may I ask, is the difference between a DC switch and an AC switch??? Roger


    Message 8


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    Time: 10:39:58 AM PST US
    From: "Vernon Little" <rv-9a-online@telus.net>
    Subject: Re: AeroElectric-List Digest: 11 Msgs - 09/12/08
    Hi Scott. The landing and taxi light circuits have inrush current limiters installed, for this very reason. The objective was to save wear and tear on the switches. Steady state draw is about 5 amps. As for the strobe supply, there was some debate on my theory of why this is the worst load in the a/c. My hypothesis is that strobe supplies have a negative voltage-current relationship (negative resistance). If you reduce the voltage to a strobe supply (or, in fact many switchmode power supplies), its current actually increases. Once a switch or terminal fails, this leads to thermal runaway and the results are what I have seen (twice). Not everyone agrees, but I've had this failure twice and Bob has documented another one-- all in the strobe circuits. Thanks for your feedback. -----Original Message----- From: owner-aeroelectric-list-server@matronics.com [mailto:owner-aeroelectric-list-server@matronics.com] On Behalf Of Scott Klemptner Sent: September 13, 2008 8:48 AM Subject: AeroElectric-List: Re: AeroElectric-List Digest: 11 Msgs - 09/12/08 Vern, you quoted Carling switch ratings, but if you read further down on the page, you find the following: "Types of Loads: An electric load is the amount of electric power delivered or required at any specific point or points on a system. The requirement originates at the energy consuming equipment of the consumers. More simply put, a load is the piece of equipment you turn on and off. Resistive loads primarily offer resistance to the flow of current. Examples of resistive loads include electric heaters, ranges, ovens, toasters, and irons. If the device is supposed to get hot and doesn't move, it's most likely a resistive load. Inductive loads are usually devices that move and normally include electric magnets, like an electric motor. Examples of inductive loads include such things as power drills, electric mixers, fans, sewing machines, and vacuum cleaners. Transformers also produce inductive loads. High Inrush loads draw a higher amount of current or amperage when first turned on, compared to the amount of current required to continue running. An example of a high inrush load is a light bulb, which may draw 20 or more times its normal operating current when first turned on. This is often referred to as lamp load. Other examples of loads that have high inrush are switching power supplies (capacitive load) and motors (inductive load). . . . . . L & T Ratings An "L" rating denotes the ability of a switch to handle the initial high inrush characteristics of a Tungsten Filament Lamp on AC voltage only. A "T" rating is the equivalent lamp load for DC. H Rating An "H" rating denotes a non-inductive resistive rating. Ratings listed in Carling Technologies' product information may appear with the symbol "H" or with the words "non-inductive" or "resistive". "H" ratings are typically required for switches used in commercial oven applications." Are you sure you are using the switches within their rated specs??????????? My experience using AC rated switches on DC circuits with high inrush currents (landing/taxi lights and strobes in particular) is failure after failure. Eric....this includes ROCKER SWITCHES TOO!.... Cessna rocker switches used in the 70's for example (AC rated "Mr Coffee Maker switches). My $.02 worth...... Scott A Klemptner bmwr606 on Yahoo IM What if the Hokey-Pokey IS what it's all about? ----- Original Message ---- From: AeroElectric-List Digest Server <aeroelectric-list@matronics.com> <aeroelectric-list-digest@matronics.com> Sent: Saturday, September 13, 2008 1:55:46 AM ________________________________ Message 7 _____________________________________ Time: 09:42:41 AM PST US From: "Vernon Little" <rv-9a-online@telus.net> Subject: RE: AeroElectric-List: Re: Switch problem ***MAJOR UPDATE*** ... > Despite what Bob says, 115 VAC switches should never be used > for LV DC high-current applications. Do what the manufacturer > suggests. That's what's printed on the side of the switch > Bubela. Otherwise you're on your own. > ... > -------- > Eric M. Jones > www.PerihelionDesign.com ... Hi again Eric. Just to comment on what you said: "DC Rule of Thumb For those switches that list an AC voltage rating only, the "DC Rule of Thumb" can be applied for determining the switch's maximum DC current rating. This "rule" states the highest amperage on the switch should perform satisfactorily up to 30 volts DC. For example, a switch which is rated at 10A 250VAC; 15A 125VAC; 3/4HP 125-250VAC, will be likely to perform satisfactorily at 15 amps up to 30 volts DC (VDC)" http://www.carlingtech.com/products/switches/learn_more.asp?page=switch es_am <http://www.carlingtech.com/products/switches/learn_more.asp?page=switc hes_a mp-rating%20> p-rating <http://www.carlingtech.com/products/switches/learn_more.asp?page=switc hes_a mp-rating%20> Therefore, all of my Carling switches are being used according to the manufacturers recommendations. The only gotcha is that the switches are not rated for operating below 0 degrees C. This is probably because the switches are not sealed from condensing moisture. This is not a problem where I live. Vern


    Message 9


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    Time: 11:03:16 AM PST US
    From: "Robert L. Nuckolls, III" <nuckolls.bob@cox.net>
    Subject: RE: AC vs DC ratings????
    At 01:28 PM 9/13/2008 -0400, you wrote: >Since there is no reference to DC switch ratings above, does this mean >that the DC switches are immune to different load types and L, T, & H ratings? >What, may I ask, is the difference between a DC switch and an AC switch??? Roger, check the article at: http://www.aeroelectric.com/articles/Switch_Ratings.pdf ALL switches have CAPABILITY in either DC or AC systems. Not all switches are RATED for all applications . . . especially if one limits their enlightenment to the tiny print on the side of the switch. There's not enough room to state the full constellation of a switch's capabilities in that little space. So what's a mother to do? Everyone prints ratings on the device that address the majority of applications where that switch would be used. Service in household and industrial AC line operated appliance. Does the lack of DC numbers mean that the device has no capability in DC operations? Not at all. But in spite of publication of engineering data there are individuals who cannot or choose not to interpret that data in useful ways. Such is the case for the author of the article in Van's Air Force newsletter that prompted the article cited above. Yes, there ARE switch designs that will exhibit optimized service life in DC or AC applications. However, the fact that any given switch is not optimized for one service or the other does not mean that it does not offer useful service in our airplanes. Because of a very low duty-cycle in light aircraft, ratings are only loosely tied to suitability to task. Failure of a switch in our airplanes is more likely to be related to environmental conditions as opposed to service stresses. Bob . . .


    Message 10


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    Time: 11:03:16 AM PST US
    From: "Vernon Little" <rv-9a-online@telus.net>
    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 > -----Original Message----- > From: owner-aeroelectric-list-server@matronics.com > [mailto:owner-aeroelectric-list-server@matronics.com] On > Behalf Of Robert L. Nuckolls, III > Sent: September 13, 2008 7:49 AM > To: aeroelectric-list@matronics.com > Subject: AeroElectric-List: Toggle Switches with Fast-On Tabs > > > > --> <nuckolls.bob@cox.net> > > 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_F > ast-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(8) 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_Failu > re/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 (8). > 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 . . . > > > > > >


    Message 11


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    Time: 11:16:02 AM PST US
    From: "Robert L. Nuckolls, III" <nuckolls.bob@cox.net>
    Subject: Switch Ratings versus Root Cause of Failure
    At 10:38 AM 9/13/2008 -0700, you wrote: >Hi Scott. The landing and taxi light circuits have inrush current >limiters installed, for this very reason. The objective was to save wear >and tear on the switches. Steady state draw is about 5 amps. >As for the strobe supply, there was some debate on my theory of why this >is the worst load in the a/c. My hypothesis is that strobe supplies have >a negative voltage-current relationship (negative resistance). If you >reduce the voltage to a strobe supply (or, in fact many switchmode power >supplies), its current actually increases. Once a switch or terminal >fails, this leads to thermal runaway and the results are what I have seen >(twice). > >Not everyone agrees, but I've had this failure twice and Bob has >documented another one-- all in the strobe circuits. It is true that the "negative resistance" or more appropriately "constant power" characteristics of switchmode power supplies may have contributed to the pace of failure. However, in terms of having abused the switch's ratings I'll refer the readers to the photo at: http://www.aeroelectric.com/articles/Anatomy_of_a_Switch_Failure/g.jpg This is the working contact in a failed strobe supply switch we examined a few years ago. Note that the contact - the major life-limiting component is almost pristine in comparison with the carnage going on elsewhere . . . http://www.aeroelectric.com/articles/Anatomy_of_a_Switch_Failure/e.jpg There's more to this failure analysis stuff than beating on the ratings drum. I'll have to sift through the gray matter but I don't think I have NEVER encountered a failure of a component aboard an aircraft wherein root cause was a failure to observe ratings. It's the FIRST question that should be asked but it's also the easiest concern to put to rest. Bob . . .


    Message 12


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    Time: 11:33:12 AM PST US
    From: "Robert L. Nuckolls, III" <nuckolls.bob@cox.net>
    Subject: Toggle Switches with Fast-On Tabs
    At 10:58 AM 9/13/2008 -0700, you wrote: ><rv-9a-online@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. >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 . . .


    Message 13


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    Time: 12:20:03 PM PST US
    From: Dan Reeves <n516dr@yahoo.com>
    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 engin e ground strap and hit the starter, the current finds a ground through thro ttle 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 chan ce of a parking brake cable or something like that making contact with some thing-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@telus.net> wrote: From: Vernon Little <rv-9a-online@telus.net> Subject: RE: AeroElectric-List: Toggle Switches with Fast-On Tabs <rv-9a-online@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 an d 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 > -----Original Message----- > From: owner-aeroelectric-list-server@matronics.com > [mailto:owner-aeroelectric-list-server@matronics.com] On > Behalf Of Robert L. Nuckolls, III > Sent: September 13, 2008 7:49 AM > To: aeroelectric-list@matronics.com > Subject: AeroElectric-List: Toggle Switches with Fast-On Tabs > > > III" > --> <nuckolls.bob@cox.net> > > 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_F > ast-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(8) 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_Failu > re/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 (8). > 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 . . . > > > > > > =0A=0A=0A


    Message 14


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    Time: 02:14:00 PM PST US
    From: "Vernon Little" <rv-9a-online@telus.net>
    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 -----Original Message----- From: owner-aeroelectric-list-server@matronics.com [mailto:owner-aeroelectric-list-server@matronics.com] On Behalf Of Dan Reeves Sent: September 13, 2008 12:17 PM Subject: RE: AeroElectric-List: 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@telus.net> wrote: From: Vernon Little <rv-9a-online@telus.net> Subject: RE: AeroElectric-List: Toggle Switches with Fast-On Tabs <rv-9a-online@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 > -----Original Message----- > From: owner-aeroelectric-list-server@matronics.com > [mailto:owner-aeroelectric-list-server@matronics.com] On > Behalf Of Robert L. Nuckolls, III > Sent: September 13, 2008 7:49 AM > To: aeroelectric-list@matronics.com > Subject: AeroElectric-List: Toggle Switches with Fast-On Tabs > > > III" > --> <nuckolls.bob@cox.net> > > 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_F > ast-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(8) 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_Failu > re/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 (8). > 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 . . . > > > > > > 3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3 D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D= 3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D 3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3 D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D= 3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D 3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3 D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D= 3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D 3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3 D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D= 3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D




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