When selecting a relay for an application, an important characteristic to consider is its life expectancy, which is usually specified in relay cycles. One cycle is defined as the action of opening and closing the relay. Most switching products provide both mechanical and electrical expected lifetimes.
The mechanical lifetime is the typical number of cycles that a relay can sustain under dry switching conditions (switched with no electrical load) and is based on metal fatigue within the bending armature or reed switch. This number tends to be several orders of magnitude above the electrical lifetime and is rarely reached in switching systems.
Switching active electrical signals, especially high-power signals, causes arcing across the relay contacts. This arcing produces pits on the contact surface and accelerates the contact wear. The electrical life specification is the number of switch cycles, under load, before the contact resistance rises above a certain value, or, in the case of reed relays, the reed switch contacts weld closes. This electrical life specification is often used as the cycle limit for a given relay; however, it cannot be used as a leading indicator for relay failure unless the load being switched in your test system matches the load that was used to determine the relay specification.
You can use relay count tracking to determine a more accurate life expectancy for each of your relays and their respective loads. In addition, if monitored, tracking can be used to anticipate relay failures over time. You can read this information from the hardware at any time using the NI-SWITCH driver and integrate it into your test code to supply operator warnings if relays exceed their electrical life expectancy.