Why relay contact haS two different ratings for AC VOLTAGE and DC VOLTAGE?

Answer

Relays rated for AC/DC use: the DC rating being lower than the AC, This is because DC can establish an arc across the contacts which cause the contacts to burn more,so they are rated lower.When I use relays on DC I will use double pole relays an put the contacts in series this gives a wider gap when the relay opens and helps exstinguish the arc quicker.Hope this helps.

AC IS +/- WHILE DC IS JUST A LEVEL AC120 MUST WITHSTAND THE PEAK VOLTAGE

Reply: How is putting the two sets contacts in series going to help?The same potential is still being broken at the first set of contacts.>-------------0___0--------0___0---------->Parallel would make more sense to me, since then the current would be split between the two sets of contacts.