KB58 wrote:
Loads like lights are one thing, but inductive loads (radiator fan, etc) are a lot worse. When running, there's a magnetic field surrounding the device, and when the switch is opened, the magnetic field collapses. As it collapses through the coil, it briefly generates its own voltage, which can be 100s of volts on even 12V devices. That high voltage causes an arc across the switch contacts, wearing it out pretty fast. The solution is to add a diode across all inductive loads, though it still doesn't get around the switch needing to be able to deal with DC loads.
One day at work (aviation) I wondered what a mechanical relay would do at altitude. Cut the cover off an ordinary automotive relay, put it in the vacuum chamber, and ran 10 amps through it. We could switch it all day long without any drama - until it was taken to altitude. Sucked it down to the equivalent of 25,000 feet and opened the relay contacts exactly once, and an arc started that never extinguished! Within about 5 seconds the relay was completely destroyed. So, yeah, pick the right part for the conditions.
Yup. The voltage handling capability is reduced virtually linearly with respect to air pressure. Current rating goes down too but not as much. It is both the voltage and current that initiate and sustain the arc. Pikes Peak, which is about 1/3 the pressure of sea level would reduce the voltage capability of the switch. Not by 1/3 because switches are normally rated somewhere around 1000 meters to start with. You would have to check the mfgr's data sheet to find where they rated their switches for altitude.
Temperature too has an effect on the switch capability. As it is in a warmer environment, the air molecules are moving around lot more. It is easier to initiate and maintain an arc with less voltage.
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Chuck.
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