Effects of Arcing
High voltages often are generated by breaking current to an inductor with a mechanical switch. They can, with time, cause pitting, corrosion, or material transfer of the switch contacts. In extreme cases, the contacts even can be welded together. The actual wear (or failure) of a mechanical switch is subject to many factors, including:
• Contact construction and the type of metal used
• Amount of contact bounce that typically occurs with the switching mechanism
• Steady-state and in-rush currents
• Whether ac or dc voltages are being switched by the mechanism
Effective transient suppression can significantly reduce the amount of energy dissipated during the operation of switch contacts. This reduction will result in a corresponding increase in switch life. In applications where relay contacts are acting as power-switching elements, the use of effective transient- suppression techniques will reduce the amount of maintenance (contact cleaning) required for the device.
The breakdown of a solid insulating material usually results in localized carbonization, which may be catastrophic, or may result in decreased dielectric strength at the arc-over point. The occurrence of additional transients often will cause a breakthrough at the weakened point in the insulating material, eventually resulting in catastrophic failure of the insulation. Similar problems can occur within the wind- ings of a transformer or coil. Arcing between the windings of an inductor often is caused by self-induced voltages with steep wavefronts that are distributed unevenly across the turns of the coil. Repetitive arcing between windings can cause eventual failure of the device.
Printed wiring board (PWB) arcing can result in system failure modes in ways outlined for insulating materials and coils. A breakdown induced by high voltage along the surface of a PWB can create a conductive path of carbonized insulation and vaporized metal from the printed wiring traces or compo- nent leads.
The greatest damage to equipment from insulation breakdown caused by transient disturbances generally occurs after the spike has passed. The follow-on steady-state current that can flow through fault paths created by a transient often cause the actual component damage and system failure.