AC vs DC spst switch

I'm looking to add an spst switch, operated by key, to control 12vdc/2A power to my locked-away circuit.

I've seen "C rating 2A" but no mention of AC or DC, and "240vac/2a."

I was just told the AC switches cannot handle DC of similar current for long.

Can someone explain why DC doesn't work with AC switches? (no guesses, please)

A quick, simple, answer.

An AC power line has 60 full sine waves per second (in a 60Hz USA application). The sine wave crosses a "zero mark". When you open a switch, at the zero mark, arcing stops, and the switch opens, and remains open, cleanly.
A DC power line is constant. When you open a switch, there might be arcing, which may cause wear or might even keep the circuit energized via the arc.
So the rated life time for AC switch may be much less when used for a DC power application.

The the theory has validity, but only up to a point, 'cause rarely will a switch close at the zero cross.

Over time, arcing should happen in both cases, right?

The issue is how long it takes to extinguish an arc when the switch is opened. With AC this necessarily happens at the next zero crossing.

that makes sense. thank you

If it's rated for 240VAC, I wouldn't worry too much about low voltages (AC or DC).

But in general, it's good practice to "derate" components. If you're switching 2A, it's a good idea to get a switch rated for more than 2A... If you've got a capacitor with 12V across it, it's a good idea to use a capacitor rated for more than 12V, etc. It usually doesn't cost much more to give yourself some safety margin.

You can also consider the cost/consequences of a failure... If the airplane is going to crash when the switch fails, you'd better look for a DC rated switch and give yourself some safety margin... Same thing if you are going "into production"... You wouldn't want unhappy customers...

On the other hand if the switch fails after a couple of months (or a couple of years) and it's just an inconvenience, don't worry so much.

DC arcing also causes metal migration from one contact to the other. Eventually will build up to make a short. AC will migrate both ways.

Paul

Zip_Ferndale:
The the theory has validity, but only up to a point, 'cause rarely will a switch close at the zero cross.

Over time, arcing should happen in both cases, right?

Not really, when the switch closes no arc strikes till the contacts are a few microns apart, and < millisecond
later they are in firm contact.

On opening the arc is stuck immediately and if there's a lot of power available can grow to many mm
or even cm as the plasma in the arc is being sustained. Enough to start melting/pitting the contacts.

In practice the lower the DC voltage the less of a problem, the voltage as well as the power both affect the
stability of a DC arc. Switching 1A at 100V is a lot more demanding than 10A at 10V, both have the same
power available to hold an arc, but the voltage matters a lot too.

Get a switch that can handle 4 amps. Then you won't need to worry about it.

A big problem is breaking inductive loads with a dc source - then the arc is much more likely to sustain.

Hence a catch diode is usually fitted across the inductor in these circuits.

Allan