When an arc ignites between separating contacts, it will be sustained as
long as there is sufficient energy to feed it. As long as the arc exists,
material transfer will continue. In a direct current application, the arc can
be extinguished only by stretching it to such a length that its own
impedance causes it to extinguish, or by opening the circuit at some
other point. In many applications, though, the contact gap is wide enough
that the arc will extinguish before the contacts have fully opened. It is
for this reason that relays of a given contact rating will be rated for, say,
120 volts AC, but will have a considerably lower DC voltage rating—
usually 28 or 30 volts DC.