Recommendations on operating relays
How to Derate a Relay based on Temperature, and load.
Relay Derating Rules
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 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.
Relay Failure modes.
Relays most commonly fail in the "stuck open" position where the mechanical switching element fails to close and the
relay fails to carry a current. Relays are less likely to unintentionally close or remain closed after the switching current
is released. For this reason, the reliability of relay circuits can be improved by using parallel redundancy. Unlike most
of the other electrical parts, relays (with the exception of solid state relays) contain a switching element that physically
moves to make electrical contact. This makes them less likely to follow a constant failure rate or traditional "bathtub"
curve profile. Instead, they are more prone to follow the failure rate curve for a mechanical part, with an increasing
failure rate with age. Except for special high voltage and high temperature applications, solid-state relays are
inherently more reliable and predictable for long life applications.
The two most common mechanical relay failure mechanisms are from contamination and mechanical wear.
A relay may experience early life failures due to mechanical wear of internal switching elements. The life of a relay is essentially determined by the life of its contacts. Degradation of contacts is caused from high in-rush currents, high-sustained currents, and from high voltage spikes. The source of high currents and voltages, in turn, are determined by the type of load. Inductive loads create the highest voltage and current spikes because they have lowest starting resistance compared to operating resistance. This is especially true for lamp filaments and motors, which is why derating is more severe for these types of loads. The life of a contact can be further degraded if contamination or pitting is present on the contact. Physical wear can also occur to other elements within the relay. Some relays contain springs to provide a mechanical resistance against electrical contact when a switching current is not applied. Springs will lose resiliency with time. Relays can also fail due to poor contact alignment and open coils.