Arcing
This is the conductive plasma bridge formed by the ionisation of air between opening contacts. It erodes the contact surface by melting it, radiates broadband electromagnetic noise and, at sufficient energy, carries a risk of ignition. In DC circuits the arc is far more persistent than in AC because there is no natural zero crossing of the current.
As the contacts separate, the contact area shrinks and the current density and local temperature rise rapidly. The air in between becomes conductive and current keeps flowing even after the contacts have opened by several millimetres. On inductive loads the reverse voltage peak produced by the coil feeds the arc further.
- The DC difficulty: in alternating current the voltage passes through zero every half cycle, so the arc extinguishes there. In direct current there is no such moment; the arc is only broken when the contact gap has opened wide enough or has been cooled.
- Consequences: migration of contact material and crater formation, rising contact resistance, and at an advanced stage contact welding.
- Noise: an arc radiates over a wide frequency band; signal cables and radio receivers passing nearby are affected.
This is one of the reasons for keeping power contacts on a separate route from signal cables in crane remote control and DC driver installations. Arc suppression elements protect both the contact and the electronics next to it.
Context in which this term is used: Installation, redundancy and field continuity