For general-purpose relays, how does the contact current rating for switching DC compare to that of AC?

In the context of general-purpose relays, the contact current rating for switching DC is typically lower than that for AC. This difference arises from the distinct characteristics of alternating current (AC) and direct current (DC).

When AC flows through the contacts of a relay, it naturally crosses zero volts at every half cycle, allowing for self-extinction of any arc that may form. This zero-crossing feature helps prevent long-duration electrical arcing across the contacts, making it easier for the relay to switch off.

In contrast, DC does not have this zero-voltage point, which means if an arc is generated when the contacts open, it can persist longer and become more difficult to extinguish. This prolonged arcing can lead to more severe contact erosion and damage, necessitating lower current ratings for DC applications to ensure the reliable operation and longevity of the relay.

Thus, for general-purpose relays used with DC, the rated current is specified lower than that for AC, reflecting this higher risk associated with arcing.