Switch Derating Guidelines

How to Derate a Switch based on Temperature, and load.

Mechanical Switches are derated based on their load and the operational temperature. Determine the type of load the switch will see, and than the temperature of operation. Example of Load types;
Inductive loads = relays, solenoids and motors
Resistive loads = resistors, incandescent lights, heaters
Capacitive loads = capacitors.
A listing of Switch vendors; Switch Manufacturers

Electrical erosion of the contacts occurs, on any switch, when an arc is drawn while breaking a circuit. This erosion normally tends to increase contact resistance, generate wear products by contamination of insulating surfaces, and reduce dielectric strength and insulation resistance. The amount of this erosion is a function of current, voltage, power, frequency, and speed of operation. The higher the current, the hotter the arc and the greater the erosion will be. The higher the voltage, the longer the arc will be maintained, resulting in greater erosion. In an inductive circuit the inductance acts as an energy storage device, which returns its energy to the circuit when the circuit is broken. The amount of erosion on an inductive circuit is in proportion to the amount of inductance.

Percent Rated Load vs Ambient Temperature

Altitude: With a decrease of atmospheric pressure, the spacing required to prevent flash over increases substantially. Small switches, because of their very close contact spacing, are partially susceptible to malfunction at high altitudes. Since arc-over occurs more readily at higher altitudes, contact life decreases substantially with operation at these altitudes. To compensate for increased arcing at high altitudes, users must derate the current ratings given by the manufacturer. To compensate for increased flashover, the user must derate voltage ratings.

Maximum voltage rating depends on the air gap, or contact separation. A gap of .005 inch to .008 inch will permit a 250 vac rating [at sea level]. A gap of .010 inch to .015 inch permits a 480 vac rating, and a gap of 0.020 inch to 0.070 inch. permits a 600 vac rating.

Derating Recommendations for components; Guideline for Derating Electronic Devices
Derating devices is a common practice during system design. Note from the table, that at no time is a switch used at 100% of its rated capability. At a minimum the switch is reduced to 75% of its rated load even at room temperature. As a Rule of Thumb always derate a switch to increase reliability and switch life.