Switches bite the dust

[Old Techo]

Near the end of our Oodna trip I found the van outside light stopped working on 12 volts. I didn't worry about it as the 240 volt one did work. I assumed of course, dud globe.

Today I checked it out and the globe was fine. I also noticed one of the inside 12 volt lights now didn't work. Hmmmmmm.... what was common? Broken wire somewhere?

Had me mystified for a while doing volts and earth wire checks with my Fluke. No fault found. Surely it could not be that 2 switches had failed?

There are 3 x 12 volt switches side by side. Maybe a wire broke effecting the last 2 in series?

The DC switches are high quality HPM mains type that you would have in your house on an architrave. I pulled one out to test and sure enough the switch was faulty. Two faulty switches....... not a chance

So I had a closer look and there was red dust around the switch including in where the rocker button pivots. I took the switch to the shed and blew it out with compressed air. Instant fix. Clearly dust particles had coated the contacts preventing them touching electrically. Same deal with the second faulty switch.

I was amazed that dust got up inside the wall of the van as these switches are about 1.5 metres above the floor

[inca222]

I have not heard of that before OT.Lucky you found it.Dry dust like that could be dangerous with a spark .Might be time to check all the switches including the 240v circuit. Ian

[Old Techo]

You are right about dust explosions but I don't think I should have called it dust in the typical household sense.

It was really ultra fine sand from the Oodna area red desert and silica won't ignite.

If any got into the collocated 240 volt light switches they probably instantly arced over and worked but at 12 volts the microscopic gap could not be jumped.

[bagmaker]

You know they are usually only DC rated to 10% of their typical amperage OT?

[Old Techo]

Bagman I thought the voltage de-rating was more important to reduce the arc drawn?

The switches installed by Roadmaster are marked 240V AC 10A and used at 12V and about 1.3A so both a substantial voltage and significant current de-rating.

[bagmaker]

Caveat- I'm no sparky
The de-rating is due to arcing not being cut-off in DC. In AC its cut-off 100 times a second so the switch contacts get to break apart quickly. With DC they have to do it using the spring force of the contacts arguing against the electrical arc. Eventually the arc burns the contacts.
1.3 amps is a bit more than 10%
I am pretty sure Clipsal have a switch of the standard size that is rated for higher DC loads, fits right in.
Alternately you can fit a little relay to take the load off the switch.
Perhaps a bit of a grizzle to the manufacturer as well, they should know all this and the results could be a fire hazard.
Edit- in reading your original post it seems that arcing -as you said= is not the cause so the above is mainly dribble
Perhaps a bit of expanding foam-in-a-can would seal it better? That red dust gets in some pretty fine places though.............

[Engel]

Nowadays if you use LED lamps and you would be a fool not to any 240 rated switch will work as the currents are so low. In the past switches used to have a wiping motion when closing but that would now have gone out the window.

One of the advantages of Andersen plugs as they wipe contacts when plugging in.

[PeterD]

In the past switches used to have a wiping motion when closing but that would now have gone out the window.

How long do you have to go back to find "in the past?" Back in the 50s we were taught not to use AC switches for DC in the DCA training school.

[Engel]

Round bakelite used in houses up to the 60's as I said in the past these switches had wiping motion. AC switches have been used on DC for low current devices and LED would be classed as low current devices.