Arcing and switch life
Arcing is one of the most serious effects that reduce the life of a switch. During the time of an arc, as we have seen, very high temperatures can be reached, both in the air and on the metal of the contacts. The ionization of the air gives rise to a plasma, a quantity of completely ionized air that is a comparatively good conductor of electric current. The temperatures that are reached in this plasma can cause the metal of the contacts to vaporize and be carried from one contact to the other. This effect is very much more serious when the contacts carry DC, because the metal vapour will also be ionized, and the charged particles will always be carried in one direction. If, for example, the metal forms positive ions, then it will always be carried from the positive contact of the switch to the negative contact. This will result in the familiar effect of the positive contact developing a crater and the negative contact developing a mound. Since the surfaces of these parts of the contacts are rough, this greatly reduces the contact area, lessening the ability of the contacts to take their rated current. As usual, AC causes less trouble, because the arc is usually quenched within a half- cycle, and since the current direction will not always be the same at the time when the arc exists, the transfer of metal is not always in one direction.
Arcing is also a means by which contacts become contaminated, because in the plasma that exists during arcing any contaminant elements in the atmosphere become ionized, and are transferred to one of the contacts. By this means, contaminants that were present in the atmosphere become permanently embedded in the contacts, with detrimental effects on the contact resistance. Arcing can also cause severe oxidation of the metal of the contacts, and this will also raise contact resistance. Fortunately, the transfer of metal in arcing inhibits oxidation to some extent, because it causes a fresh set of metal surfaces to appear at each switching-off action.
Arcing, therefore, will greatly reduce the life of switch contacts, and it is an effect that cannot be completely eliminated. Arcing is almost imperceptible if the circuits that are being switched run at low voltage and contain no inductors, because a comparatively high voltage is needed to start an arc. For this reason, then, arcing is not a significant problem for switches that control low voltage, such as the 5 V or 9 V DC that is used as a supply for solid-state circuitry, with no appreciable inductance in the circuit. Even low-voltage circuits, however, will present arcing problems if they contain inductive components, and these include relays and electric motors as well as chokes. Circuits in which voltages above about 50 V are switched are the most susceptible to arcing problems, particularly if inductive components are present, and some consideration should be given to selecting suitably rated switches, and to arc suppression, if appropriate.