Slide Switches AC/DC current rating question

[SebG]

For a project I'm working on I need a slide switch to select between two current paths. (Max 2.5 Amps DC)
Most switches I find are rated for 4 or 6 Amps at 125 VAC but only like 0.5A at DC.
Why is that? 

If I use such a switch in my project what kind of things would the the effect of passing high DC Current through the switch?
The contact look sufficient to conduct such a current but i don't understand why they have such a low DC rating and high AC rating.
The DC voltage maximum would be 20V and the switches would be positioned before current begins to flow, most of the time.

[mikeselectricstuff]

For a project I'm working on I need a slide switch to select between two current paths. (Max 2.5 Amps DC)
Most switches I find are rated for 4 or 6 Amps at 125 VAC but only like 0.5A at DC.
Why is that? 

If I use such a switch in my project what kind of things would the the effect of passing high DC Current through the switch?
The contact look sufficient to conduct such a current but i don't understand why they have such a low DC rating and high AC rating.
The DC voltage maximum would be 20V and the switches would be positioned before current begins to flow, most of the time.

The reason switches have lower ratings at DC, especially as voltages increase, is that DC arcs don't self-extinguish - with AC, any arc formed at opening  will stop at the zero-crossing and not restart. A DC arc will just keep burning until the contact seperation is too big to sustain it (which may be never).

[SebG]

This if very interesting and it explains a lot.  Thanks.