I was looking through to buy a few relays and found the nominal DC contact voltages (~30v) are much smaller than the nominal AC contact voltages (250v). What is the reason for such a large difference?
When opening a contact on DC, the arc persists until the gap is large enough. With AC, because of the frequent drop to zero, the arc naturally extinguishes at smaller gaps than for comparable DC. Therefore you need a larger gap between contacts for high voltage relays, especially DC.