How to Derate a Switch based on Temperature, and load.
| Current Derating Factor @ Application Ambient Temperature | ||||
| Military | Commercial | |||
| Load Type | 0oC to 85oC | Above 85oC | 0oC to 85oC | Above 85oC |
| Resistive |
75% of rated Resistive load |
60% of rated Resistive load |
75% of rated Resistive load |
60% of rated Resistive load |
|
Inductive / Motor |
75% of rated Inductive load |
60% of rated Inductive load |
40% of rated Resistive load |
30% of rated Resistive load |
|
Capacitive / Lamp |
75% of rated Capacitive load |
60% of rated Capacitive load |
25% of rated Resistive load |
20% of rated Resistive 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
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.
Derating Recommendations for components; Guideline for Derating Electronic Devives
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.
