سوختگی یا سایش کنتاکت‌های رله

A relay whose contacts have eroded, so it switches unreliably. Every time a relay opens under load it draws a small arc, and that arc slowly erodes the contact faces — so this is a wear-out failure of a part nobody treats as a wearing part.

Pitted contacts give high resistance rather than an open circuit, so the load runs weakly or intermittently and the voltage at it sags. That is worse than a clean failure because everything appears connected. Measure voltage drop across the relay's contacts while the load is drawing — a relay dropping more than a few tenths of a volt is finished. Substitution with an identical relay is the fastest test, and if the replacement erodes quickly the load is drawing above its rating.