Contact resistance is the resistance across a closed set of contacts. This parameter is typically measured using a four-wire configuration with nominal voltage applied to the coil. Unless otherwise specified, a contact resistance specification is for new conditions.
For dry switches, the contact resistance is inversely related to the contact area, Electromechanical relays with large contacts have a lower contact resistance than a reed switch with relatively small contacts.
Life, or the number of operations for which a relay is expected to perform, is typically limited by mechanical operations and by the number of operations under a certain load until a specified resistance across closed contacts is reached. Without an end-of-life contact resistance specification, the life under load specification is meaningless. Dry switches typically have a life specified to a contact resistance less than 1 or 2W.
A relay may fail in one of several modes, including mechanical (actuation) failure and contact failure. A coil wire break, a glass seal fracture or reed fatigue in reed switches, or an armature mechanism failure in electromechanical relays can cause mechanical failure.
Contact failure can occur when excessive heat or a high current or voltage pulse causes the contacts to weld. Oxidation, excessive charring and pitting, or insulating deposits on the contacts can cause an open circuit.
Because actual contact life is important, much effort has gone into finding and choosing "good" contact materials. Gold, silver, and palladium have traditionally been used for their good electrical conductivity and relatively low susceptibility to oxidation. Increasingly, rhodium and ruthenium are being plated over more traditional contact materials to increase the contact life. Rhodium's structure is cubic, which is more stable than that of earlier contact plating. For even greater stability, the hexagonal structure of ruthenium is used.