Electrical failure/switch failure

I've just experienced my second and possibly third switch failure in 95 hours.

On my first flight, my master switch failed... leading to overvoltage alarms. It was intermittent, and it took me a while to diagnose. B&C replaced the switch, and I sent them the failed switch for analysis.

Last week, my Strobe breaker kept popping when I switched the strobes off. Today, I went into diagnosis mode and found:
- Strobes did not work.
- Breaker was permanently open.
- Nylon insulation on the fast on terminals at switch were burnt black.
- Brass switch terminals were discolored.
- switch terminal rivets were loose (same failure as Master switch).

I tested the failed B&C (Carling) switch at home with a power supply in current-limit mode (5 amps) and it got quite warm. I measured the voltage across the terminals and got widely varying measurements as I wiggled the terminals.

Conclusion: switch failure.

Yesterday, I also had a flap system failure... I suspect a switch problem as well, but no conclusion yet.

Anyone else seen these failures with the B&C sourced Carling switches?

Vern Little

This switch?



I wouldn't install this switch on my lawnmower!

I googled up what you gave me and that is what popped up.

This is the page I saw:

http://www.sbw.org/switches/

Specifically, what switch are you using, Vern?

CJ

I'm using the Carling toggle switches with riveted terminals.

See this link

Vern

Vern, That LOOKS like a quality switch!



It comes at a good price of only five bucks, but there is no information as to it's amp rating.

Also, B&C usually has good quality stuff.

What is the Rating on the switch?

I bet B&C would want to know that the swich has been failing for you. Perhaps they have an answer.

Send them an email and tell them so. If you don't get satisfaction, I will post it on the aeroelectric list for you. I just HATE email based forums.

Lemme know. I was thinking of using those very switches!

CJ

Captain_John said:

Vern, That LOOKS like a quality switch!

It comes at a good price of only five bucks, but there is no information as to it's amp rating.

Also, B&C usually has good quality stuff.

What is the Rating on the switch?

I bet B&C would want to know that the swich has been failing for you. Perhaps they have an answer.

Send them an email and tell them so. If you don't get satisfaction, I will post it on the aeroelectric list for you. I just HATE email based forums.

Lemme know. I was thinking of using those very switches!

CJ

Click to expand...

Yes, I've been in touch with B&C to resolve. These switches are rated at 15A, 125 VAC, and should be good for the same current at 12VDC.

B&C did have a problem with Chinese manufactured Carling switches, but have pulled the supply of these. My switches are made in Mexico. Also, an avionics tech told me that I should solder the rivets to provide more mechanic al strength, but I still want to chase down the root cause.

Vern

Hmmmm,

I would not solder the rivets. Introducing heat is probably not going to anything good.

I have had the opportunity to tinker with the ones Van sells and they seem REALLY nice!

Maybe swapping them out would be the best option.

As far as the strobe breaker popping, is there any way you can ascertain if it was a ground fault or overcurrent causing the problem? Those are the only 2 reasons for a CB to trip.

CJ

WHOA Vern,

I see this is getting some serious press in the aeroelectric forum already!!!

Lemme read up on that one over there.

CJ

vlittle said:

. These switches are rated at 15A, 125 VAC, and should be good for the same current at 12VDC.

BVern

Click to expand...

You might be surprised.

D.C. current ratings are much lower than A.C.

Check, dont assume.

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


Mike

See this link Switch Ratings

To summarize: "In a nutshell, 125 VAC ratings equate favorably and
conservatively to 14 VDC ratings - as long as the switch has
a healthy "snap" action . . . all toggle switches and most rocker switches do."

I have no reason to doubt the information.

V

Interesting

Vern, interesting that we both cite the same work, but diferent parts of it.

In a prior paragraph, it says that D.C. ratings sometimes have to be de-rated by as much as 75%-----------what I was refering to.

I have bumped into this before, and I know from experiance that the D.C. capacity of a switch is less that the A.C.

Been there, done that.

I was not aware however of how much of the reduction is related th the TYPE of load--------as shown in the chart in the link.

Perhaps some electrical engineer type out there can add a bit of clarity here?? Stein??

My guess is that the answer is going to be something along the lines of "Well, you are both correct----but------"

Anyway, good luck in finding the problem, and keep us informed please.

Mike

The type of load just has to do mostly with how the switch is pounded on when opening or closing the connection. An inductive load, for example, doesn't do didly to the switch when you close it, but when you open the circuit it'll try it's best to maintain an arc across the contacts. For this, you'd want a very fast acting switch with a large airgap between contacts.

So just to take this one example, you could have a switch rated at 20VDC for a resistave load but in reality could slowly kill itself switching a 5 or 10 volt inductive load. So basically, the heavier the contacts are, the more current the switch can handle. The quicker the contacts move and the larger the air gap, the more inductive load it can SWITCH.

The "lamp" rating just has to do with how it's rated when you're switching a lamp. Tungsten has it's own unique characteristics as it heats up. The current is initially extremely high but very quickly drops as it heats.

Anyhow, I wouldn't solder the switch rivets. If the rivets are coming loose, either the switches are defective or you're killing them with very high currents/voltages causing the plastic case to deform, or something like that. All soldering could do is hide the problem.

Personally, I'd sit back, sip a beer and scratch my head for 20 minutes wondering why my electrical system keeps killing my switches. Most likely, the switches are just defective from a bad batch (loose terminals leading to high temperatures and arcing from what you've said) but I'd go through the exercise all the same just to convince myself I hadn't done anything dumb during a moment of "just being human"

just my $.02.

John, you are spot on.

jcoloccia said:

The type of load just has to do mostly with how the switch is pounded on when opening or closing the connection. An inductive load, for example, doesn't do didly to the switch when you close it, but when you open the circuit it'll try it's best to maintain an arc across the contacts. For this, you'd want a very fast acting switch with a large airgap between contacts.
..just my $.02.

Click to expand...

John is right, even the lowly mechanical switch needs to be correctly specified for the application (voltage, current, type of load, environmental conditions, sealing, vibration.. and so on). Each of these factors will affect the design, construction and materials used in the switch. Spring tension, contact size, shape, material, hardness, plating and wiping action are but a few of the factors that go into the design of a switch. Some, like the chart in the aeroelectric link (a Microswitch TP series) are quite flexible. Other switches are specified only for DC (AT series) and others for AC only (TS series). When you go through a catalog, you'll see pages and pages of identical looking switches with different part numbers. There is a reason for that. Different loads present vastly different conditions to the contacts, closing AND opening, and the switch manufacturer will take the indended load and power supply into account when designing the switch.

Will an AC specified switch work reliably on DC? The answer is 'maybe'. In the FAQ section on their site, Carlingswitch says as a 'rule of thumb' you can use an AC rated switch in DC applications up to 30V. They should know. But they also have switches that are only specified only for one or the other. Does the same rule apply? Don't know. One series of 125VAC toggle switches may work just fine at 12VDC, even if its not specified. The problems can start when the manufacturer changes the design of the switch, has it made in another factory (adios Mexico, hello China) or it goes obsolete and the supplier replaces it with another 125VAC unit thats 'just the same' except not with DC loads but thats 'OK because its not specified for DC'.

For my money, I want to know that the manufacturer of the switch agrees with what I want to use it for. The only way to be sure is to consult the datasheet for the exact switch and make sure that its specified and rated for the load you are switching. That doesn't mean that you need 30 different switches, there are lots that handle a wide range of load types and voltages. If you are not sure, a short email to an applications engineer at the mfr is usually all it takes to clear things up.

From all of this, you can probably guess that I would also recommend buying the highest quality switches you can afford and watch out for the overseas stuff. The Japanese (Omron, Aromat et al) generally make great stuff even if it not made right in Japan. Chinese stuff is getting hard to avoid but at least try to stick with a known brand. Avoid no-name 'will-fits' at all costs.

As for the loose rivets, John is right again. Soldering is out. The switch was not designed to have loose rivets so if they do get loose, you have an application or quality problem. Maybe vibration or just a crappy switch. Soldering not only masks the problem but can introduce brand new ones from contact misalignment or flux wicking up into the switch.

Here's a tech blurb from Omron. Its about their snap switches but the same principles apply. Of particular note is the variety of contact configurations, gaps, materials etc and the DC switching performance (from Excellent to Inferior, in their words) for the different part numbers. All in a series of switches that are identical in external appearance. Different load types are also described.

Switch Info

OTTO T9 Toggles are fully specified (and expensive). Good tech info on their site.

McGill aint bad either - Note AC/DC specs.

Hope some of this helps.
John

I'm an EE, but switch engineering is part mechanical design, part electrical.

The reason you can't translate AC ratings to DC is that AC currents all pass through zero at 120 times per second. Any arc across the contacts caused by switching loads off is thus automatically quenced.

If, however, you try to switch the same voltage, but the current is DC, then it's possible that the arc persists long enough to weld the contacts together.

At low voltages, however, the arcs have much less energy and will quench on their own... hence the rule of thumb that the 125VAC current rating is OK for 12VDC.

Switching inductive loads of any kind can really mess up a switch... hence the snubber diodes recommended on relay coils.

The load in question (strobe power supply) is not an inductive load, so it should not have damaged the switch unless it was already defective (loose tabs).

Any way, the problem is fixed and the strobes are running.

V