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chasb(at)satx.rr.com
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 Posted: Fri Jan 02, 2009 4:44 pm Post subject: Follow-up on Carling switch failures. . . |
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Bob,
While I can't disagree with your findings, if I remember the discussion correctly, most of the failed B&C S700 switches were on strobe systems. I think I kicked off the recent discussion with a failed switch on my strobe system. I have several other B&C S700 series switches, all are wired similar to the strobe switch, and none of them have failed or showed signs of heat stress. Why should the switches on a strobe system be more failure prone?
Charlie Brame
RV 6A N11CB
San Antonio
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| Quote: | Time: 06:31:17 PM PST US
From: "Robert L. Nuckolls, III" <nuckolls.bob(at)aeroelectric.com (nuckolls.bob(at)aeroelectric.com)>
Subject: Follow-up on Carling switch failures . . .
A few weeks ago a discussion here on the List explored
a "rash" of failures in Carling switches sold by a number
of sources including B&C as their S700 series devices.
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--------------------------------snip------------------------------
[quote]
I can now comfortably suggest that failures we've
seen (and perceived as a sudden up-tick in frequency)
have been happening in these products for years. Further,
the smoked switches were not a failure on the part of the
manufacturer to meet electrical specifications nor
upon users for failure to respect those ratings.
Bob . . .
----------------------------------------)
( . . . a long habit of not thinking )
( a thing wrong, gives it a superficial )
( appearance of being right . . . )
( )
( -Thomas Paine 1776- )
----------------------------------------
[b]
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rv-9a-online(at)telus.net
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| Posted: Fri Jan 02, 2009 5:50 pm Post subject: Follow-up on Carling switch failures. . . |
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Bob's working hypothesis is that vibration is loosening the rivets on the Carling switches. High current switches (strobe supplies, landing lights) then exhibit overtemperature, increased resistance and thermal runaway. Strobe supplies are probably the worst load in the aircraft because they are effectively constant-power devices: as the input voltage decreases, the current increases to compensate. This would be bad news if the switch is resistive.
I've had two strobe switch failures, one landing light switch (actually a wig-wag) failure, and one master switch failure (loose rivet but no charred terminals). I have also received brand-new Carling switches with loose terminals.
As an experiment, I replaced several Carling switches with Honeywell switches and have been running them, monitoring for failures. I proposed about 100 hours of testing before any conclusions could be made. This will probably take several more months. I have check them in the interim, with no problems.
If the experiment shows failure of the Honeywell switches, then the problem may be attributed to my installation.
If the experiment does not show failure of the Honeywell switches, then the problem may be attributed to the Carling switches sensitivity to vibration.
Bob has concluded that the Carling switches are fine, but should have vibration decoupling loops to minimize vibration stresses on the terminals. Good advice no matter who's switches are installed. Unfortunately, I don't agree that the problem is that my (our) installations are incorrect. Rather, I believe that the problem is that the Carling switches are unusually sensitive to vibration.
In my next project, I am planning to use different switches, plus decoupling loops. In addition, strobe and landing light loads will be isolated with automotive relays which are generally more robust.
Vern Little
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fvalarm(at)rapidnet.net
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| Posted: Fri Jan 02, 2009 6:36 pm Post subject: Follow-up on Carling switch failures. . . |
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Vern,
When you say "Carling switches with loose rivets". How loose? Do you mean wobbly terminals? I've checked all mine and most of the terminals can pivot around the rivet until it hits a stop molded into the switch. I wonder if this is tight enough. I would expect them to be tighter. I have a couple Mil-spec switches and I don't think they pivot like this but will check again next time I'm at the shop. Maybe this pivoting action is what is not compatible with a high vibration environment.
Bevan
From: owner-aeroelectric-list-server(at)matronics.com [mailto:owner-aeroelectric-list-server(at)matronics.com] On Behalf Of Vern Little
Sent: Friday, January 02, 2009 5:45 PM
To: aeroelectric-list(at)matronics.com
Subject: Re: AeroElectric-List: Follow-up on Carling switch failures. . .
Bob's working hypothesis is that vibration is loosening the rivets on the Carling switches. High current switches (strobe supplies, landing lights) then exhibit overtemperature, increased resistance and thermal runaway. Strobe supplies are probably the worst load in the aircraft because they are effectively constant-power devices: as the input voltage decreases, the current increases to compensate. This would be bad news if the switch is resistive.
I've had two strobe switch failures, one landing light switch (actually a wig-wag) failure, and one master switch failure (loose rivet but no charred terminals). I have also received brand-new Carling switches with loose terminals.
As an experiment, I replaced several Carling switches with Honeywell switches and have been running them, monitoring for failures. I proposed about 100 hours of testing before any conclusions could be made. This will probably take several more months. I have check them in the interim, with no problems.
If the experiment shows failure of the Honeywell switches, then the problem may be attributed to my installation.
If the experiment does not show failure of the Honeywell switches, then the problem may be attributed to the Carling switches sensitivity to vibration.
Bob has concluded that the Carling switches are fine, but should have vibration decoupling loops to minimize vibration stresses on the terminals. Good advice no matter who's switches are installed. Unfortunately, I don't agree that the problem is that my (our) installations are incorrect. Rather, I believe that the problem is that the Carling switches are unusually sensitive to vibration.
In my next project, I am planning to use different switches, plus decoupling loops. In addition, strobe and landing light loads will be isolated with automotive relays which are generally more robust.
Vern Little
---
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rv-9a-online(at)telus.net
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| Posted: Fri Jan 02, 2009 9:09 pm Post subject: Follow-up on Carling switch failures. . . |
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Hey Bevan.
I would consider a terminal that easily pivots to be loose. It's certainly not gas-tight, and may exhibit troubles later on.
In my opinion, I would watch these switches carefully, and don't put any more stress on the rivets (use a decoupling loop on the wire).
I had an avtech recommend soldering the tabs to the rivets, but since I don't know what this does to the temper of the internals, I would not do it.
Vern
[quote] ---
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fvalarm(at)rapidnet.net
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| Posted: Fri Jan 02, 2009 11:28 pm Post subject: Follow-up on Carling switch failures. . . |
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Yes, I am considering contacting the vendor to see if I can return them, and change to Milspec switches throughout. I don't really want them for a few bucks savings and if the vendor gets enough back, perhaps they will offer/recommend a better product.
Bevan
From: owner-aeroelectric-list-server(at)matronics.com [mailto:owner-aeroelectric-list-server(at)matronics.com] On Behalf Of Vern Little
Sent: Friday, January 02, 2009 9:04 PM
To: aeroelectric-list(at)matronics.com
Subject: Re: AeroElectric-List: Follow-up on Carling switch failures. . .
Hey Bevan.
I would consider a terminal that easily pivots to be loose. It's certainly not gas-tight, and may exhibit troubles later on.
In my opinion, I would watch these switches carefully, and don't put any more stress on the rivets (use a decoupling loop on the wire).
I had an avtech recommend soldering the tabs to the rivets, but since I don't know what this does to the temper of the internals, I would not do it.
Vern
[quote] ---
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