I've just come acros the following in the article "Interfacing Switches and Relays to the Real World in Real Time" on Maxim's website:

"Factors that affect the rated capacity of a switch [or relay] include the power factor, AC or DC voltage, voltage amplitude, type of load (lamp or motor, that is, whether the load is resistive, inductive, or capacitive), and the magnitude of current through the switch. Typically, the AC-voltage rating for a switch is specified at a given current level and power factor. Its operating voltage must be less than this AC-voltage rating.

"The type of load on a switch affects its rating drastically. A resistive load such as a tungsten heater is the least stressful. Inductive loads and motor loads both place additional stress on the switch as it opens and closes. The bouncing switch contacts can encounter inrush currents three to 10 times' [sic] greater than steady-state levels. Bounce occurs during both opening and closing, and the resultant arcing ["sparkle"] causes contact wear, higher resistance, and lower reliability.

"Lamp and capacitive loads represent the worst case. At the instant of switch closure, both of these loads subject the switch to a short circuit. A lamp's cold resistance is close to zero ohms, and a discharged capacitor is a short circuit at the instant voltage is applied! This state change can produce inrush currents up to a hundred times' [sic] the steady-state value. To compound the problem, switch contacts are bouncing during this short-circuit interval.[2] High current levels and bouncing contacts produce severe arcing ["sparkle"] across the switch contacts that causes contact erosion. What's worse is that repeated cycles of switch operation with severe arcing can create a short circuit by welding (fusing) ["melting"] the contacts together."