How to Derate a Relay based on Temperature, and load.
Relay Derating Rules
Notes:
1/ Warning: Do not derate coil voltage or current. Operating a relay at less than nominal coil rating can result in either switching failures or increased
switching times. The latter condition induces contact damage because of the longer arcing time, thus reducing relay reliability.
2/ Iderated = derated contact current carrying capacity
Irated = rated contact current
3/ If during switching, transient current surges exceed the derated contact current, the following applies, where:
t = period of time that transient current exceeds rated contact current (Irated)
Imax = maximum permitted surge current
Irated = rated contact current
 Mechanical Relay | Mechanical Relays 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; |
Altitude: With a decrease of atmospheric pressure, the spacing required to prevent flash over increases substantially. Small relays, 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.
A listing of Relay vendors; Relay Manufacturers
Derating Guidelines for Relays;
Relay Stress Ratio = Operating Contact Current / Rated Contact Current = 50%
Derating Recommendations for components; Guideline for Derating Electronic Devices
Derating devices is a common practice during system design. Note that at no time is a relay be used at 100% of its rated capability. At a minimum the relay is reduced to 75% of its rated load even at room temperature. As a Rule of Thumb always derate a relay to increase reliability.
