Arc Suppression
This is the set of measures that extend contact life by transferring the energy of the arc formed when a contact opens onto an auxiliary element. A reverse diode is used in DC circuits, an RC snubber in AC, and a varistor for transient overvoltages. Applied correctly, contact life increases many times over.
The aim is to offer the energy stored in the coil or in the line inductance a path other than the contact at the moment of switching. Once the energy has discharged by another route, no voltage arises between the contacts to start an arc.
- Reverse diode: the most effective and cheapest solution on DC coil loads. It clips the voltage peak at about 0.7 V, but it lengthens the release time.
- RC snubber: a resistor and capacitor connected in parallel with the contact terminals. Typically 100 nF is used together with 47-220 Ω; it works in both AC and DC.
- Varistor: becomes conductive above a certain voltage and clips the peak. A clamping voltage above the system peak voltage but below the contact withstand is chosen.
Placement matters: the element is mounted as close as possible to the load terminals, with short connections. A diode placed metres away from the coil loses part of its protection because of the inductance of the cable in between. When buying a relay board, check whether the suppression element is on the board.
Context in which this term is used: Installation, redundancy and field continuity