Application note
AN0022Partially Welded Contacts
What a partial weld is, resulting auxiliary status, and how to detect and clear the fault.
Summary
A partial weld is a specific fault type in high-voltage DC contactors where only one side of the main contact fuses to its stationary terminal, and it occurs more frequently than a full weld. This application note details the mechanics of the condition, explaining how severe electrical or thermal stress, such as closing into a fault or an unmitigated capacitive inrush spike, concentrates energy on a single terminal. It describes how the resulting levered pivot reduces the contact gap and complicates auxiliary status interpretation, and it provides a diagnostic framework to detect the fault along with procedures to clear it. Pre-charging the DC circuit before the main contactor closes remains the primary defense against inrush-driven welding.
