Contact Arc Suppression
All measures taken to suppress the electric arc that forms between relay contacts as they open. The arc transfers metal from the contact surface and, over time, causes erosion or welding; with inductive loads the arc energy rises markedly because it is fed by the magnetic energy stored in the coil.
At the moment the contacts open the gap is in the micrometre range, and the voltage ionises the air across this tiny gap, creating a conductive path. If the load is inductive the current cannot stop instantly; the coil raises the voltage to keep the current flowing and feeds the arc. As the arc burns, material is transferred from the contact surface.
The measures applied depend on the type of load:
- DC coil: a freewheeling (suppression) diode connected in parallel with the coil in reverse. It is the simplest and most effective solution.
- AC coil: an RC snubber across the contact or the coil. A diode cannot be used on AC.
- In both cases: derating the contact current according to the load type reduces arc energy from the outset.
In the field, arc suppression translates into service life. On a crane with frequent starts and stops, contact life without suppression is many times shorter than in a suppressed installation. The symptom first appears as intermittent operation, then as the contact staying closed (welding). During periodic maintenance, look for blackening and pitting on the contact surfaces.
Context where this term is used: Wireless Crane Remote Selection Guide