Contact Welding
The contacts of a relay or contactor melting at the surface under excess current and arcing and welding to each other as they cool. The result is that the contact does not open even though the command has been removed, meaning the movement does not stop. In lifting applications this is one of the most dangerous forms of electrical failure.
At the moment of closing, the inrush current of an inductive load can rise far above the continuous current; on inductive AC loads a current of roughly 10 times is assumed in the first instant. The contact surface melts locally at the first point of touch. At the moment of opening, the arc vaporises material and damages the surface. These two effects accumulate and lead to welding.
- Symptom: the movement continuing even though you have released the button, the contactor not dropping out audibly, excessive heating in the body.
- Prevention: derate correctly (do not use a contact rated only for AC-1 at full capacity on an inductive load), use a freewheeling diode on DC coils and an RC snubber on AC coils, and on large motors drive the contactor coil with the relay rather than the motor directly.
- When the fault occurs: isolate the machine at the main switch; do not carry on using a welded contact after cleaning it, replace the device.
This risk also explains why limit protection is wired in series into the power circuit rather than into the control side.
The context in which this term is used: Complete guide to IP protection classes